Best 300 JavaScript Coding Practice Questions
Q. Write a program in javascript. sum(2)(3); // Expected output is 5
function sum(x, y) { if (y !== undefined) { return x + y; } else { return function (y) { return x + y; }; } }Output
console.log(sum(2,3)); // Outputs 5 console.log(sum(2)(3)); // Outputs 5
Q. How to validate file size and extension before upload?
<!DOCTYPE html> <html> <head> <meta http-equiv="Content-type" content="text/html;charset=UTF-8" /> <title>Show File Data</title> <script type="text/javascript"> function showFileSize() { var input, file, extension; // (Can't use `typeof FileReader === "function"` because apparently // it comes back as "object" on some browsers. So just see if it's there // at all.) if (!window.FileReader) { bodyAppend("p", "The file API isn't supported on this browser yet."); return; } input = document.getElementById("fileinput"); if (!input) { bodyAppend("p", "Um, couldn't find the fileinput element."); } else if (!input.files) { bodyAppend( "p", "This browser doesn't seem to support the `files` property of file inputs." ); } else if (!input.files[0]) { bodyAppend("p", "Please select a file before clicking 'Load'"); } else { file = input.files[0]; extension = file.name.substring(file.name.lastIndexOf(".") + 1); bodyAppend( "p", "File Name: " + file.name + "<br/>File Size: " + file.size + " bytes <br/>File Extension: " + extension ); } } function bodyAppend(tagName, innerHTML) { var elm; elm = document.createElement(tagName); elm.innerHTML = innerHTML; document.body.appendChild(elm); } </script> </head> <body> <form action="#" onsubmit="return false;"> <input type="file" id="fileinput" /> <input type="button" id="btnLoad" value="Load" onclick="showFileSize();" /> </form> </body> </html>
Q. Create captcha using javascript?
<!DOCTYPE html> <html> <head> <title>JavaScript Captcha Example</title> </head> <script> var captcha; function generateCaptcha() { var a = Math.floor(Math.random() * 10); var b = Math.floor(Math.random() * 10); var c = Math.floor(Math.random() * 10); var d = Math.floor(Math.random() * 10); captcha = a.toString() + b.toString() + c.toString() + d.toString(); document.getElementById("captcha").value = captcha; } function check() { var input = document.getElementById("inputText").value; if (input == captcha) { alert("Valid Captcha"); } else { alert("Invalid Captcha"); } } </script> <body onload="generateCaptcha()"> <input type="text" id="captcha" disabled /><br /><br /> <input type="text" id="inputText" /><br /><br /> <button onclick="generateCaptcha()">Refresh</button> <button onclick="check()">Submit</button> </body> </html>
Q. Create a Stopwatch program in javascript?
<!DOCTYPE html> <html> <head> <title>Stopwatch Example</title> </head> <body> <form action="" method="post"> <h4>Simple stopwatch made in JavaScript</h4> <input type="button" onclick="startWatch()" value="START" /> <input type="button" onclick="stopWatch()" value="STOP" /> <input type="button" onclick="resetWatch()" value="ZERO" /> </form> <p id="res"> <span id="min">0</span> : <span id="sec">00</span> : <span id="msec">000</span> </p> <p> In this example Date() methods co-operate with timing function setInterval(). </p> <script type="text/javascript"> var timer = null; var min_txt = document.getElementById("min"); var min = Number(min_txt.innerHTML); var sec_txt = document.getElementById("sec"); var sec = Number(sec_txt.innerHTML); var msec_txt = document.getElementById("msec"); var msec = Number(msec_txt.innerHTML); function stopTimeMilliseconds(timer) { if (timer) { clearInterval(timer); return timer; } else return timer; } function startTimeMilliseconds() { var currDate = new Date(); return currDate.getTime(); } function getElapsedTimeMilliseconds(startMilliseconds) { if (startMilliseconds > 0) { var currDate = new Date(); elapsedMilliseconds = currDate.getTime() - startMilliseconds; return elapsedMilliseconds; } else { return (elapsedMilliseconds = 0); } } function startWatch() { // START TIMER timer = stopTimeMilliseconds(timer); var startMilliseconds = startTimeMilliseconds(); timer = setInterval(function () { var elapsedMilliseconds = getElapsedTimeMilliseconds( startMilliseconds ); if (msec < 10) { msec_txt.innerHTML = "00" + msec; } else if (msec < 100) { msec_txt.innerHTML = "0" + msec; } else { msec_txt.innerHTML = msec; } if (sec < 10) { sec_txt.innerHTML = "0" + sec; } else { sec_txt.innerHTML = sec; } min_txt.innerHTML = min; msec = elapsedMilliseconds; if (min >= 59 && sec >= 59 && msec > 900) { timer = stopTimeMilliseconds(timer); return true; } if (sec > 59) { sec = 0; min++; } if (msec > 999) { msec = 0; sec++; startWatch(); } }, 10); } function stopWatch() { // STOP TIMER timer = stopTimeMilliseconds(timer); return true; } function resetWatch() { // REZERO TIMER timer = stopTimeMilliseconds(timer); msec_txt.innerHTML = "000"; msec = 0; sec_txt.innerHTML = "00"; sec = 0; min_txt.innerHTML = "0"; min = 0; return true; } </script> </body> </html>
Q. Write a program to reverse a string?
function reverseString(str) { let stringRev = ""; for (let i = str.length; i >= 0; i--) { stringRev = stringRev + str.charAt(i); } return stringRev; } alert(reverseString("Pradeep")); // Output: peedarP
Q. How to check if object is empty or not in javaScript?
function isEmpty(obj) { return Object.keys(obj).length === 0; }
Q. JavaScript Regular Expression to validate Email
var pattern = /^\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}$/;
Q. Use RegEx to test password strength in JavaScript?
var newPassword = "Pq5*@a{J"; var regularExpression = new RegExp( "^(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])(?=.*[!@#$%^&*])(?=.{8,})" ); if (!regularExpression.test(newPassword)) { alert( "Password should contain atleast one number and one special character !" ); }
RegEx | Description |
---|---|
^ | The password string will start this way |
(?=.*[a-z]) | The string must contain at least 1 lowercase alphabetical character |
(?=.*[A-Z]) | The string must contain at least 1 uppercase alphabetical character |
(?=.*[0-9]) | The string must contain at least 1 numeric character |
(?=.[!@#$%^&]) | The string must contain at least one special character, but we are escaping reserved RegEx characters to avoid conflict |
(?=.{8,}) | The string must be eight characters or longer |
Q. How to compare objects ES6?
Example 01:
const matches = (obj, source) => Object.keys(source).every( (key) => obj.hasOwnProperty(key) && obj[key] === source[key] ); console.log( matches({ age: 25, hair: "long", beard: true }, { hair: "long", beard: true }) ); // true console.log( matches({ hair: "long", beard: true }, { age: 25, hair: "long", beard: true }) ); // false console.log( matches({ hair: "long", beard: true }, { age: 26, hair: "long", beard: true }) ); // false
Example 02:
const k1 = { fruit: "" }; const k2 = { fruit: "" }; // Using JavaScript JSON.stringify(k1) === JSON.stringify(k2); // true
Example 03:
const one = { fruit: "", energy: "255kJ", }; const two = { energy: "255kJ", fruit: "", }; // Using JavaScript JSON.stringify(one) === JSON.stringify(two); // false
Q. How to remove array element based on object property?
var myArray = [ { field: "id", operator: "eq" }, { field: "cStatus", operator: "eq" }, { field: "money", operator: "eq" }, ]; myArray = myArray.filter(function (obj) { return obj.field !== "money"; }); Console.log(myArray);
Output
myArray = [ {field: "id", operator: "eq"} {field: "cStatus", operator: "eq"} ]
Q. Predict the output of the following JavaScript code?
console.log(+"meow"); // Output: NaN
Q. Predict the output of the following JavaScript code?
var result; for (var i = 5; i > 0; i--) { result = result + i; } console.log(result); // Output: NaN
Q. Predict the output of the following JavaScript code?
var a = 1.2; console.log(typeof a); // Output: Number
Q. Predict the output of the following JavaScript code?
var x = 10; if (x) { let x = 4; } console.log(x); // Output: 10
Q. Predict the output of the following JavaScript code?
console.log(0.1 + 0.2 == 0.3); // Output: false
Q. Predict the output of the following JavaScript code?
console.log(1 + -"1" + 2); // Output: 2
Q. Predict the output of the following JavaScript code?
(function (x) { return (function (y) { console.log(x); })(10); })(20); // Output: 20
Q. Predict the output of the following JavaScript code?
var num = 20; var getNumber = function () { console.log(num); var num = 10; }; getNumber(); // Output: undefined
Q. Predict the output of the following JavaScript code?
function f1() { num = 10; } f1(); console.log("window.num: " + window.num); // output: 10
Q. Predict the output of the following JavaScript code?
console.log("(null + undefined): " + (null + undefined)); // Output: NaN
Q. Predict the output of the following JavaScript code?
(function () { var a = (b = 3); })(); console.log("value of a : " + a); // Output: undefined console.log("value of b : " + b); // Output: 3
Q. Predict the output of the following JavaScript code?
var y = 1; if (function f() {}) { y += typeof f; } console.log(y); // Output: 1Object
Q. Predict the output of the following JavaScript code?
var k = 1; if (1) { eval(function foo() {}); k += typeof foo; } console.log(k); // Output: 1undefined
Q. Predict the output of the following JavaScript code?
var k = 1; if (1) { function foo() {} k += typeof foo; } console.log(k); // Output: 1function
Q. Predict the output of the following JavaScript code?
console.log("(-1 / 0): " + -1 / 0); // Output: -Infinity console.log("(1 / 0): " + 1 / 0); // Output: Infinity console.log("(0 / 0): " + 0 / 0); // Output: NaN console.log("(0 / 1): " + 0 / 1); // Output: 0
Q. Predict the output of the following JavaScript code?
var a = 4; var b = "5"; var c = 6; console.log("(a + b): " + (a + b)); // Output: 45 console.log("(a - b): " + (a - b)); // Output: -1 console.log("(a * b): " + a * b); // Output: 20 console.log("(a / b): " + a / b); // Output: 0.8 console.log("(a % b): " + (a % b)); // Output: 4
Q. Predict the output of the following JavaScript code?
console.log("MAX : " + Math.max(10, 2, NaN)); // Output: NaN console.log("MAX : " + Math.max()); // Output: -Infinity
Q. Predict the output of the following JavaScript code?
(function () { var a = (b = 3); })(); console.log("a defined? " + (typeof a !== "undefined")); // Output: true console.log("b defined? " + (typeof b !== "undefined")); // Output: true
Q. Predict the output of the following JavaScript code?
var myObject = { foo: "bar", func: function () { var self = this; console.log("outer func: this.foo = " + this.foo); // Output: this.foo = bar console.log("outer func: self.foo = " + self.foo); // Output: self.foo = bar (function () { console.log("inner func: this.foo = " + this.foo); // Output: this.foo = function foo() {} console.log("inner func: self.foo = " + self.foo); // Output: self.foo = bar })(); }, }; myObject.func();
Q. Predict the output of the following JavaScript code?
console.log(0.1 + 0.2); // Output: 0.30000000000000004 console.log(0.1 + 0.2 == 0.3); // Output: false
Q. Predict the output of the following JavaScript code?
(function () { console.log(1); setTimeout(function () { console.log(2); }, 1000); setTimeout(function () { console.log(3); }, 0); console.log(4); })(); // Output: 1, 4, 3, 2
Q. Predict the output of the following JavaScript code?
var arr1 = "john".split(""); var arr2 = arr1.reverse(); var arr3 = "jones".split(""); arr2.push(arr3); console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1)); //array 1: length=5 last=j,o,n,e,s console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1)); //array 2: length=5 last=j,o,n,e,s
Q. Predict the output of the following JavaScript code?
console.log(1 + "2" + "2"); // Output: 122 console.log(1 + +"2" + "2"); // Output: 32 console.log(1 + -"1" + "2"); // Output: 02 console.log(+"1" + "1" + "2"); // Output: 112 console.log("A" - "B" + "2"); // Output: NaN2 console.log("A" - "B" + 2); // Output: NaN
Q. Predict the output of the following JavaScript code?
for (var i = 0; i < 5; i++) { setTimeout(function () { console.log(i); }, i * 1000); } // Output: 145, 5, 5, 5, 5, 5
Q. Predict the output of the following JavaScript code?
for (var i = 0; i < 5; i++) { (function (x) { setTimeout(function () { console.log(x); }, x * 1000); })(i); } //Output:- 0, 1, 2, 3, 4
Q. Predict the output of the following JavaScript code?
console.log("0 || 1 = " + (0 || 1)); // Output: 1 console.log("1 || 2 = " + (1 || 2)); // Output: 1 console.log("0 && 1 = " + (0 && 1)); // Output: 0 console.log("1 && 2 = " + (1 && 2)); // Output: 2
Q. Predict the output of the following JavaScript code?
console.log(false == "0"); // Output: true console.log(false === "0"); // Output: false
Q. Predict the output of the following JavaScript code?
var a = {}, b = { key: "b" }, c = { key: "c" }; a[b] = 123; a[c] = 456; console.log(a[b]); // Output: 456
Q. Predict the output of the following JavaScript code?
console.log( (function f(n) { return n > 1 ? n * f(n - 1) : n; })(10) ); // Output: 3628800
Q. Predict the output of the following JavaScript code?
(function (x) { return (function (y) { console.log(x); //1 })(2); })(1);
Q. Predict the output of the following JavaScript code?
var hero = { _name: "John Doe", getSecretIdentity: function () { return this._name; }, }; var stoleSecretIdentity = hero.getSecretIdentity; console.log(stoleSecretIdentity()); // Output: undefined console.log(hero.getSecretIdentity()); // Output: John Doe
Q. Predict the output of the following JavaScript code?
var length = 10; function fn() { console.log(this.length); } var obj = { length: 5, method: function (fn) { fn(); arguments[0](); }, }; obj.method(fn, 1); //Output: 10, 2
Q. Predict the output of the following JavaScript code?
(function () { try { throw new Error(); } catch (x) { var x = 1, y = 2; console.log(x); } console.log(x); console.log(y); })(); //Output: 1, undefined, 2
Q. Predict the output of the following JavaScript code?
var x = 21; var girl = function () { console.log(x); // Output: undefined var x = 20; }; girl();
Q. Predict the output of the following JavaScript code?
console.log(1 < 2 < 3); // Output: true console.log(3 > 2 > 1); // Output: false
Q. Predict the output of the following JavaScript code?
console.log(typeof typeof 1); // Output: string
Q. Predict the output of the following JavaScript code?
var b = 1; function outer() { var b = 2; function inner() { b++; var b = 3; console.log(b); //3 } inner(); } outer();
Q. Hoisting example in javascript?
x = 10; console.log(x); var x; // Output: 10
Q. Predict the output of the following JavaScript code?
const arr = [1, 2]; arr.push(3); // Output: 1, 2, 3
Q. Predict the output of the following JavaScript code?
var o = new F(); o.constructor === F;
Q. Predict the output of the following JavaScript code?
let sum = (a, b) => { a + b; }; console.log(sum(10, 20)); // Output: undefined; return keyword is missing
Q. Predict the output of the following JavaScript code?
var arr = ["javascript", "typescript", "es6"]; var searchValue = (value) => { return arr.filter((item) => { return item.indexOf(value) > -1; }); }; console.log(searchValue("script"));
Q. Write the program using fatarrow function?
var a = [1, 2, 3, 4]; function sumUsingFunction(acc, value) { return acc + value; } var sumOfArrayUsingFunc = a.reduce(sumUsingFunc);
Q. Write a program that prints the numbers from 1 to 15. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”?
for (var i = 1; i <= 15; i++) { if (i % 15 == 0) console.log("FizzBuzz"); else if (i % 3 == 0) console.log("Fizz"); else if (i % 5 == 0) console.log("Buzz"); else console.log(i); }
Output:
1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
Solution – 02
for (var i = 1; i <= 15; i++) { var f = i % 3 == 0, b = i % 5 == 0; console.log(f ? (b ? "FizzBuzz" : "Fizz") : b ? "Buzz" : i); }
Q. What will be the output of the following code?
var output = (function (x) { delete x; return x; })(0); console.log(output);
The code above will output 0
as output. delete
operator is used to delete a property from an object. Here x
is not an object it’s local variable. delete
operator doesn’t affect local variables.
Q. What will be the output of the following code?
var x = 1; var output = (function () { delete x; return x; })(); console.log(output);
The code above will output 1
as output. delete
operator is used to delete a property from an object. Here x
is not an object it’s global variable of type number
.
Q. What will be the output of the following code?
var x = { foo: 1 }; var output = (function () { delete x.foo; return x.foo; })(); console.log(output);
The code above will output undefined
as output. delete
operator is used to delete a property from an object. Here x
is an object which has foo as a property and from a self-invoking function, we are deleting the foo
property of object x
and after deletion, we are trying to reference deleted property foo
which result undefined
.
Q. What will be the output of the following code?
var Employee = { company: "xyz", }; var emp1 = Object.create(Employee); delete emp1.company; console.log(emp1.company);
The code above will output xyz
as output. Here emp1
object got company as prototype property. delete operator doesn’t delete prototype property.
emp1
object doesn’t have company as its own property. you can test it console.log(emp1.hasOwnProperty('company')); //output : false
However, we can delete company property directly from Employee
object using delete Employee.company
or we can also delete from emp1
object using __proto__
property delete emp1.__proto__.company
.
Q. What will be the output of the following code?
var trees = ["xyz", "xxxx", "test", "ryan", "apple"]; delete trees[3]; console.log(trees.length);
The code above will output 5
as output. When we used delete
operator for deleting an array element then, the array length is not affected by this. This holds even if you deleted all elements of an array using delete
operator.
So when delete operator removes an array element that deleted element is no longer present in the array. In place of value at deleted index undefined x 1
in chrome and undefined
is placed at the index. If you do console.log(trees)
output ["xyz", "xxxx", "test", undefined × 1, "apple"]
in Chrome and in Firefox ["xyz", "xxxx", "test", undefined, "apple"]
.
Q. What will be the output of the following code?
var bar = true; console.log(bar + 0); console.log(bar + "xyz"); console.log(bar + true); console.log(bar + false);
The code above will output 1, "truexyz", 2, 1
as output. Here’s a general guideline for the plus operator:
- Number + Number -> Addition
- Boolean + Number -> Addition
- Boolean + Boolean -> Addition
- Number + String -> Concatenation
- String + Boolean -> Concatenation
- String + String -> Concatenation
Q. What will be the output of the following code?
var z = 1, y = (z = typeof y); console.log(y);
The code above will print string "undefined"
as output. According to associativity rule operator with the same precedence are processed based on their associativity property of operator. Here associativity of the assignment operator is Right to Left
so first typeof y
will evaluate first which is string "undefined"
and assigned to z
and then y
would be assigned the value of z. The overall sequence will look like that:
var z; z = 1; var y; z = typeof y; y = z;
Q. What will be the output of the following code?
// NFE (Named Function Expression) var foo = function bar() { return 12; }; typeof bar();
The output will be Reference Error
. To fix the bug we can try to rewrite the code a little bit:
Sample 1
var bar = function () { return 12; }; typeof bar();
or
Sample 2
function bar() { return 12; } typeof bar();
The function definition can have only one reference variable as a function name, In sample 1 bar
is reference variable which is pointing to anonymous function
and in sample 2 we have function statement and bar
is the function name.
var foo = function bar() { // foo is visible here // bar is visible here console.log(typeof bar()); // Works here :) }; // foo is visible here // bar is undefined here
Q. What is the output of the following?
bar(); (function abc() { console.log("something"); })(); function bar() { console.log("bar got called"); }
The output will be :
bar got called something
Since the function is called first and defined during parse time the JS engine will try to find any possible parse time definitions and start the execution loop which will mean function is called first even if the definition is post another function.
Q. What will be the output of the following code?
var salary = "1000$"; (function () { console.log("Original salary was " + salary); var salary = "5000$"; console.log("My New Salary " + salary); })();
The code above will output: undefined, 5000$
because of hoisting. In the code presented above, you might be expecting salary
to retain it values from outer scope until the point that salary
was re-declared in the inner scope. But due to hoisting
salary value was undefined
instead. To understand it better have a look of the following code, here salary
variable is hoisted and declared at the top in function scope. When we print its value using console.log
the result is undefined
. Afterwards the variable is redeclared and the new value "5000$"
is assigned to it.
var salary = "1000$"; (function () { var salary = undefined; console.log("Original salary was " + salary); salary = "5000$"; console.log("My New Salary " + salary); })();
Q. What would be the output of the following code?
function User(name) { this.name = name || "JsGeeks"; } var person = (new User("xyz")["location"] = "USA"); console.log(person);
The output of above code would be "USA"
. Here new User("xyz")
creates a brand new object and created property location
on that and USA
has been assigned to object property location and that has been referenced by the person.
Let say new User("xyz")
created a object called foo
. The value "USA"
will be assigned to foo["location"]
, but according to ECMAScript Specification , pt 12.14.4 the assignment will itself return the rightmost value: in our case it’s "USA"
. Then it will be assigned to person.
To better understand What is going on here, try to execute this code in console, line by line:
function User(name) { this.name = name || "JsGeeks"; } var person; var foo = new User("xyz"); foo["location"] = "USA"; // the console will show you that the result of this is "USA"
Q. What would be the output of following code?
var strA = "hi there"; var strB = strA; strB = "bye there!"; console.log(strA);
The output will 'hi there'
because we’re dealing with strings here. Strings are passed by value, that is, copied.
Q. What would be the output of following code?
var objA = { prop1: 42 }; var objB = objA; objB.prop1 = 90; console.log(objA);
The output will {prop1: 90}
because we’re dealing with objects here. Objects are passed by reference, that is, objA
and objB
point to the same object in memory.
Q. What would be the output of following code?
var objA = { prop1: 42 }; var objB = objA; objB = {}; console.log(objA);
The output will {prop1: 42}
.
When we assign objA
to objB
, the objB
variable will point to the same object as the objB
variable.
However, when we reassign objB
to an empty object, we simply change where objB
variable references to. This doesn’t affect where objA
variable references to.
Q. What would be the output of following code?
var arrA = [0, 1, 2, 3, 4, 5]; var arrB = arrA; arrB[0] = 42; console.log(arrA);
The output will be [42,1,2,3,4,5]
.
Arrays are object in JavaScript and they are passed and assigned by reference. This is why both arrA
and arrB
point to the same array [0,1,2,3,4,5]
. That’s why changing the first element of the arrB
will also modify arrA
: it’s the same array in the memory.
Q. What would be the output of following code?
var arrA = [0, 1, 2, 3, 4, 5]; var arrB = arrA.slice(); arrB[0] = 42; console.log(arrA);
The output will be [0,1,2,3,4,5]
.
The slice
function copies all the elements of the array returning the new array. That’s why arrA
and arrB
reference two completely different arrays.
Q. What would be the output of following code?
var arrA = [ { prop1: "value of array A!!" }, { someProp: "also value of array A!" }, 3, 4, 5, ]; var arrB = arrA; arrB[0].prop1 = 42; console.log(arrA);
The output will be [{prop1: 42}, {someProp: "also value of array A!"}, 3,4,5]
.
Arrays are object in JS, so both varaibles arrA and arrB point to the same array. Changing arrB[0]
is the same as changing arrA[0]
Q. What would be the output of following code?
var arrA = [ { prop1: "value of array A!!" }, { someProp: "also value of array A!" }, 3, 4, 5, ]; var arrB = arrA.slice(); arrB[0].prop1 = 42; arrB[3] = 20; console.log(arrA);
The output will be [{prop1: 42}, {someProp: "also value of array A!"}, 3,4,5]
.
The slice
function copies all the elements of the array returning the new array. However, it doesn’t do deep copying. Instead it does shallow copying. You can imagine slice implemented like this:
function slice(arr) { var result = []; for (i = 0; i < arr.length; i++) { result.push(arr[i]); } return result; }
Look at the line with result.push(arr[i])
. If arr[i]
happens to be a number or string, it will be passed by value, in other words, copied. If arr[i]
is an object, it will be passed by reference.
In case of our array arr[0]
is an object {prop1: "value of array A!!"}
. Only the reference to this object will be copied. This effectively means that arrays arrA and arrB share first two elements.
This is why changing the property of arrB[0]
in arrB
will also change the arrA[0]
.
Q. console.log(employeeId);
- Some Value
- Undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 4) ReferenceError: employeeId is not defined
Q. What would be the output of following code?
console.log(employeeId); var employeeId = "19000";
- Some Value
- undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 2) undefined
Q. What would be the output of following code?
var employeeId = "1234abe"; (function () { console.log(employeeId); var employeeId = "122345"; })();
- ‘122345’
- undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 2) undefined
Q. What would be the output of following code?
var employeeId = "1234abe"; (function () { console.log(employeeId); var employeeId = "122345"; (function () { var employeeId = "abc1234"; })(); })();
- ‘122345’
- undefined
- ‘1234abe’
- ReferenceError: employeeId is not defined
Answer: 2) undefined
Q. What would be the output of following code?
(function () { console.log(typeof displayFunc); var displayFunc = function () { console.log("Hi I am inside displayFunc"); }; })();
- undefined
- function
- ‘Hi I am inside displayFunc’
- ReferenceError: displayFunc is not defined
Answer: 1) undefined
Q. What would be the output of following code?
var employeeId = "abc123"; function foo() { employeeId = "123bcd"; return; } foo(); console.log(employeeId);
- undefined
- ‘123bcd’
- ‘abc123’
- ReferenceError: employeeId is not defined
Answer: 2) ‘123bcd’
Q. What would be the output of following code?
var employeeId = "abc123"; function foo() { employeeId = "123bcd"; return; function employeeId() {} } foo(); console.log(employeeId);
- undefined
- ‘123bcd’
- ‘abc123’
- ReferenceError: employeeId is not defined
Answer: 3) ‘abc123’
Q. What would be the output of following code?
var employeeId = "abc123"; function foo() { employeeId(); return; function employeeId() { console.log(typeof employeeId); } } foo();
- undefined
- function
- string
- ReferenceError: employeeId is not defined
Answer: 2) ‘function’
Q. What would be the output of following code?
function foo() { employeeId(); var product = "Car"; return; function employeeId() { console.log(product); } } foo();
- undefined
- Type Error
- ‘Car’
- ReferenceError: product is not defined
Answer: 1) undefined
Q. What would be the output of following code?
(function foo() { bar(); function bar() { abc(); console.log(typeof abc); } function abc() { console.log(typeof bar); } })();
- undefined undefined
- Type Error
- function function
- ReferenceError: bar is not defined
Answer: 3) function function
Q. What would be the output of following code?
(function () { "use strict"; var person = { name: "John", }; person.salary = "10000$"; person["country"] = "USA"; Object.defineProperty(person, "phoneNo", { value: "8888888888", enumerable: true, }); console.log(Object.keys(person)); })();
- Type Error
- undefined
- [“name”, “salary”, “country”, “phoneNo”]
- [“name”, “salary”, “country”]
Answer: 3) [“name”, “salary”, “country”, “phoneNo”]
Q. What would be the output of following code?
(function () { "use strict"; var person = { name: "John", }; person.salary = "10000$"; person["country"] = "USA"; Object.defineProperty(person, "phoneNo", { value: "8888888888", enumerable: false, }); console.log(Object.keys(person)); })();
- Type Error
- undefined
- [“name”, “salary”, “country”, “phoneNo”]
- [“name”, “salary”, “country”]
Answer: 4) [“name”, “salary”, “country”]
Q. What would be the output of following code?
(function () { var objA = { foo: "foo", bar: "bar", }; var objB = { foo: "foo", bar: "bar", }; console.log(objA == objB); console.log(objA === objB); })();
- false true
- false false
- true false
- true true
Answer: 2) false false
Q. What would be the output of following code?
(function () { var objA = new Object({ foo: "foo" }); var objB = new Object({ foo: "foo" }); console.log(objA == objB); console.log(objA === objB); })();
- false true
- false false
- true false
- true true
Answer: 2) false false
Q. What would be the output of following code?
(function () { var objA = Object.create({ foo: "foo", }); var objB = Object.create({ foo: "foo", }); console.log(objA == objB); console.log(objA === objB); })();
- false true
- false false
- true false
- true true
Answer: 2) false false
Q. What would be the output of following code?
(function () { var objA = Object.create({ foo: "foo", }); var objB = Object.create(objA); console.log(objA == objB); console.log(objA === objB); })();
- false true
- false false
- true false
- true true
Answer: 2) false false
Q. What would be the output of following code?
(function () { var objA = Object.create({ foo: "foo", }); var objB = Object.create(objA); console.log(objA.toString() == objB.toString()); console.log(objA.toString() === objB.toString()); })();
- false true
- false false
- true false
- true true
Answer: 4) true true
Q. What would be the output of following code?
(function () { var objA = Object.create({ foo: "foo", }); var objB = objA; console.log(objA == objB); console.log(objA === objB); console.log(objA.toString() == objB.toString()); console.log(objA.toString() === objB.toString()); })();
- true true true false
- true false true true
- true true true true
- true true false false
Answer: 3) true true true true
Q. What would be the output of following code?
(function () { var objA = Object.create({ foo: "foo", }); var objB = objA; objB.foo = "bar"; console.log(objA.foo); console.log(objB.foo); })();
- foo bar
- bar bar
- foo foo
- bar foo
Answer: 2) bar bar
Q. What would be the output of following code?
(function () { var objA = Object.create({ foo: "foo", }); var objB = objA; objB.foo = "bar"; delete objA.foo; console.log(objA.foo); console.log(objB.foo); })();
- foo bar
- bar bar
- foo foo
- bar foo
Answer: 3) foo foo
Q. What would be the output of following code?
(function () { var objA = { foo: "foo", }; var objB = objA; objB.foo = "bar"; delete objA.foo; console.log(objA.foo); console.log(objB.foo); })();
- foo bar
- undefined undefined
- foo foo
- undefined bar
Answer: 2) undefined undefined
Q. What would be the output of following code?
(function () { var array = new Array("100"); console.log(array); console.log(array.length); })();
- undefined undefined
- [undefined × 100] 100
- [“100”] 1
- ReferenceError: array is not defined
Answer: 3) [“100”] 1
Q. What would be the output of following code?
(function () { var array1 = []; var array2 = new Array(100); var array3 = new Array(["1", 2, "3", 4, 5.6]); console.log(array1); console.log(array2); console.log(array3); console.log(array3.length); })();
- [] [] [Array[5]] 1
- [] [undefined × 100] Array[5] 1
- [] [] [‘1′,2,’3’,4,5.6] 5
- [] [] [Array[5]] 5
Answer: 1) [] [] [Array[5]] 1
Q. What would be the output of following code?
(function () { var array = new Array("a", "b", "c", "d", "e"); array[10] = "f"; delete array[10]; console.log(array.length); })();
- 11
- 5
- 6
- undefined
Answer: 1) 11
Q. What would be the output of following code?
(function () { var animal = ["cow", "horse"]; animal.push("cat"); animal.push("dog", "rat", "goat"); console.log(animal.length); })();
- 4
- 5
- 6
- undefined
Answer: 3) 6
Q. What would be the output of following code?
(function () { var animal = ["cow", "horse"]; animal.push("cat"); animal.unshift("dog", "rat", "goat"); console.log(animal); })();
- [ ‘dog’, ‘rat’, ‘goat’, ‘cow’, ‘horse’, ‘cat’ ]
- [ ‘cow’, ‘horse’, ‘cat’, ‘dog’, ‘rat’, ‘goat’ ]
- Type Error
- undefined
Answer: 1) [ ‘dog’, ‘rat’, ‘goat’, ‘cow’, ‘horse’, ‘cat’ ]
Q. What would be the output of following code?
(function () { var array = [1, 2, 3, 4, 5]; console.log(array.indexOf(2)); console.log([{ name: "John" }, { name: "John" }].indexOf({ name: "John" })); console.log([[1], [2], [3], [4]].indexOf([3])); console.log("abcdefgh".indexOf("e")); })();
- 1 -1 -1 4
- 1 0 -1 4
- 1 -1 -1 -1
- 1 undefined -1 4
Answer: 1) 1 -1 -1 4
Q. What would be the output of following code?
(function () { var array = [1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 6]; console.log(array.indexOf(2)); console.log(array.indexOf(2, 3)); console.log(array.indexOf(2, 10)); })();
- 1 -1 -1
- 1 6 -1
- 1 1 -1
- 1 undefined undefined
Answer: 2) 1 6 -1
Q. What would be the output of following code?
(function () { var numbers = [2, 3, 4, 8, 9, 11, 13, 12, 16]; var even = numbers.filter(function (element, index) { return element % 2 === 0; }); console.log(even); var containsDivisibleby3 = numbers.some(function (element, index) { return element % 3 === 0; }); console.log(containsDivisibleby3); })();
- [ 2, 4, 8, 12, 16 ] [ 0, 3, 0, 0, 9, 0, 12]
- [ 2, 4, 8, 12, 16 ] [ 3, 9, 12]
- [ 2, 4, 8, 12, 16 ] true
- [ 2, 4, 8, 12, 16 ] false
Answer: 3) [ 2, 4, 8, 12, 16 ] true
Q. What would be the output of following code?
(function () { var containers = [2, 0, false, "", "12", true]; var containers = containers.filter(Boolean); console.log(containers); var containers = containers.filter(Number); console.log(containers); var containers = containers.filter(String); console.log(containers); var containers = containers.filter(Object); console.log(containers); })();
- [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ]
- [false, true] [ 2 ] [’12’] [ ]
- [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true]
- [ 2, ’12’, true ] [ 2, ’12’, true, false ] [ 2, ’12’, true,false ] [ 2, ’12’, true,false]
Answer: 1) [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ]
Q. What would be the output of following code?
(function () { var list = ["foo", "bar", "john", "ritz"]; console.log(list.slice(1)); console.log(list.slice(1, 3)); console.log(list.slice()); console.log(list.slice(2, 2)); console.log(list); })();
- [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
- [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’,’ritz ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
- [ ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
- [ ‘foo’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
Answer: 1) [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
Q. What would be the output of following code?
(function () { var list = ["foo", "bar", "john"]; console.log(list.splice(1)); console.log(list.splice(1, 2)); console.log(list); })();
- [ ‘bar’, ‘john’ ] [] [ ‘foo’ ]
- [ ‘bar’, ‘john’ ] [] [ ‘bar’, ‘john’ ]
- [ ‘bar’, ‘john’ ] [ ‘bar’, ‘john’ ] [ ‘bar’, ‘john’ ]
- [ ‘bar’, ‘john’ ] [] []
Answer: 1. [ ‘bar’, ‘john’ ] [] [ ‘foo’ ]
Q. What would be the output of following code?
(function () { var arrayNumb = [2, 8, 15, 16, 23, 42]; arrayNumb.sort(); console.log(arrayNumb); })();
- [2, 8, 15, 16, 23, 42]
- [42, 23, 26, 15, 8, 2]
- [ 15, 16, 2, 23, 42, 8 ]
- [ 2, 8, 15, 16, 23, 42 ]
Answer: 3. [ 15, 16, 2, 23, 42, 8 ]
Q. What would be the output of following code?
function funcA() { console.log("funcA ", this); (function innerFuncA1() { console.log("innerFunc1", this); (function innerFunA11() { console.log("innerFunA11", this); })(); })(); } console.log(funcA());
- funcA Window {…} innerFunc1 Window {…} innerFunA11 Window {…}
- undefined
- Type Error
- ReferenceError: this is not defined
Answer: 1)
Q. What would be the output of following code?
var obj = { message: "Hello", innerMessage: !(function () { console.log(this.message); })(), }; console.log(obj.innerMessage);
- ReferenceError: this.message is not defined
- undefined
- Type Error
- undefined true
Answer: 4) undefined true
Q. What would be the output of following code?
var obj = { message: "Hello", innerMessage: function () { return this.message; }, }; console.log(obj.innerMessage());
- Hello
- undefined
- Type Error
- ReferenceError: this.message is not defined
Answer: 1) Hello
Q. What would be the output of following code?
var obj = { message: "Hello", innerMessage: function () { (function () { console.log(this.message); })(); }, }; console.log(obj.innerMessage());
- Type Error
- Hello
- undefined
- ReferenceError: this.message is not defined
Answer: 3) undefined
Q. What would be the output of following code?
var obj = { message: "Hello", innerMessage: function () { var self = this; (function () { console.log(self.message); })(); }, }; console.log(obj.innerMessage());
- Type Error
- ‘Hello’
- undefined
- ReferenceError: self.message is not defined
Answer: 2) ‘Hello’
Q. What would be the output of following code?
function myFunc() { console.log(this.message); } myFunc.message = "Hi John"; console.log(myFunc());
- Type Error
- ‘Hi John’
- undefined
- ReferenceError: this.message is not defined
Answer: 3) undefined
Q. What would be the output of following code?
function myFunc() { console.log(myFunc.message); } myFunc.message = "Hi John"; console.log(myFunc());
- Type Error
- ‘Hi John’
- undefined
- ReferenceError: this.message is not defined
Answer: 2) ‘Hi John’
Q. What would be the output of following code?
function myFunc() { myFunc.message = "Hi John"; console.log(myFunc.message); } console.log(myFunc());
- Type Error
- ‘Hi John’
- undefined
- ReferenceError: this.message is not defined
Answer: 2) ‘Hi John’
Q. What would be the output of following code?
function myFunc(param1, param2) { console.log(myFunc.length); } console.log(myFunc()); console.log(myFunc("a", "b")); console.log(myFunc("a", "b", "c", "d"));
- 2 2 2
- 0 2 4
- undefined
- ReferenceError
Answer: a) 2 2 2
Q. What would be the output of following code?
function myFunc() { console.log(arguments.length); } console.log(myFunc()); console.log(myFunc("a", "b")); console.log(myFunc("a", "b", "c", "d"));
- 2 2 2
- 0 2 4
- undefined
- ReferenceError
Answer: 2) 0 2 4
Q. What would be the output of following code?
function Person(name, age) { this.name = name || "John"; this.age = age || 24; this.displayName = function () { console.log(this.name); }; } Person.name = "John"; Person.displayName = function () { console.log(this.name); }; var person1 = new Person("John"); person1.displayName(); Person.displayName();
- John Person
- John John
- John undefined
- John John
Answer: 1) John Person
Q. What would be the output of following code?
function passWordMngr() { var password = "12345678"; this.userName = "John"; return { pwd: password, }; } // Block End var userInfo = passWordMngr(); console.log(userInfo.pwd); console.log(userInfo.userName);
- 12345678 Window
- 12345678 John
- 12345678 undefined
- undefined undefined
Answer: 3) 12345678 undefined
Q. What would be the output of following code?
var employeeId = "aq123"; function Employee() { this.employeeId = "bq1uy"; } console.log(Employee.employeeId);
- Reference Error
- aq123
- bq1uy
- undefined
Answer: 4) undefined
Q. What would be the output of following code?
var employeeId = "aq123"; function Employee() { this.employeeId = "bq1uy"; } console.log(new Employee().employeeId); Employee.prototype.employeeId = "kj182"; Employee.prototype.JobId = "1BJKSJ"; console.log(new Employee().JobId); console.log(new Employee().employeeId);
- bq1uy 1BJKSJ bq1uy undefined
- bq1uy 1BJKSJ bq1uy
- bq1uy 1BJKSJ kj182
- undefined 1BJKSJ kj182
Answer: 2) bq1uy 1BJKSJ bq1uy
Q. What would be the output of following code?
var employeeId = "aq123"; (function Employee() { try { throw "foo123"; } catch (employeeId) { console.log(employeeId); } console.log(employeeId); })();
- foo123 aq123
- foo123 foo123
- aq123 aq123
- foo123 undefined
Answer: 1) foo123 aq123
Q. What would be the output of following code?
(function () { var greet = "Hello World"; var toGreet = [].filter.call(greet, function (element, index) { return index > 5; }); console.log(toGreet); })();
- Hello World
- undefined
- World
- [ ‘W’, ‘o’, ‘r’, ‘l’, ‘d’ ]
Answer: 4) [ ‘W’, ‘o’, ‘r’, ‘l’, ‘d’ ]
Q. What would be the output of following code?
(function () { var fooAccount = { name: "John", amount: 4000, deductAmount: function (amount) { this.amount -= amount; return "Total amount left in account: " + this.amount; }, }; var barAccount = { name: "John", amount: 6000, }; var withdrawAmountBy = function (totalAmount) { return fooAccount.deductAmount.bind(barAccount, totalAmount); }; console.log(withdrawAmountBy(400)()); console.log(withdrawAmountBy(300)()); })();
- Total amount left in account: 5600 Total amount left in account: 5300
- undefined undefined
- Total amount left in account: 3600 Total amount left in account: 3300
- Total amount left in account: 5600 Total amount left in account: 5600
Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300
Q. What would be the output of following code?
(function () { var fooAccount = { name: "John", amount: 4000, deductAmount: function (amount) { this.amount -= amount; return this.amount; }, }; var barAccount = { name: "John", amount: 6000, }; var withdrawAmountBy = function (totalAmount) { return fooAccount.deductAmount.apply(barAccount, [totalAmount]); }; console.log(withdrawAmountBy(400)); console.log(withdrawAmountBy(300)); console.log(withdrawAmountBy(200)); })();
- 5600 5300 5100
- 3600 3300 3100
- 5600 3300 5100
- undefined undefined undefined
Answer: 1) 5600 5300 5100
Q. What would be the output of following code?
(function () { var fooAccount = { name: "John", amount: 6000, deductAmount: function (amount) { this.amount -= amount; return this.amount; }, }; var barAccount = { name: "John", amount: 4000, }; var withdrawAmountBy = function (totalAmount) { return fooAccount.deductAmount.call(barAccount, totalAmount); }; console.log(withdrawAmountBy(400)); console.log(withdrawAmountBy(300)); console.log(withdrawAmountBy(200)); })();
- 5600 5300 5100
- 3600 3300 3100
- 5600 3300 5100
- undefined undefined undefined
Answer: 2) 3600 3300 3100
Q. What would be the output of following code?
(function greetNewCustomer() { console.log("Hello " + this.name); }.bind({ name: "John", })());
- Hello John
- Reference Error
- Window
- undefined
Answer: 1) Hello John
Q. What would be the output of following code?
function getDataFromServer(apiUrl) { var name = "John"; return { then: function (fn) { fn(name); }, }; } getDataFromServer("www.google.com").then(function (name) { console.log(name); });
- John
- undefined
- Reference Error
- fn is not defined
Answer: 1) John
Q. What would be the output of following code?
(function () { var arrayNumb = [2, 8, 15, 16, 23, 42]; Array.prototype.sort = function (a, b) { return a - b; }; arrayNumb.sort(); console.log(arrayNumb); })(); (function () { var numberArray = [2, 8, 15, 16, 23, 42]; numberArray.sort(function (a, b) { if (a == b) { return 0; } else { return a < b ? -1 : 1; } }); console.log(numberArray); })(); (function () { var numberArray = [2, 8, 15, 16, 23, 42]; numberArray.sort(function (a, b) { return a - b; }); console.log(numberArray); })();
- [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]
- undefined undefined undefined
- [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2]
- Reference Error
Answer: 1) [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]
Q. What would be the output of following code?
(function () { function sayHello() { var name = "Hi John"; return; { fullName: name; } } console.log(sayHello().fullName); })();
- Hi John
- undefined
- Reference Error
- Uncaught TypeError: Cannot read property ‘fullName’ of undefined
Answer: 4) Uncaught TypeError: Cannot read property ‘fullName’ of undefined
Q. What would be the output of following code?
function getNumber() { return 2, 4, 5; } var numb = getNumber(); console.log(numb);
- 5
- undefined
- 2
- (2,4,5)
Answer: 1) 5
Q. What would be the output of following code?
function getNumber() { return; } var numb = getNumber(); console.log(numb);
- null
- undefined
- “”
- 0
Answer: 2) undefined
Q. What would be the output of following code?
function mul(x) { return function (y) { return [ x * y, function (z) { return x * y + z; }, ]; }; } console.log(mul(2)(3)[0]); console.log(mul(2)(3)[1](4));
- 6, 10
- undefined undefined
- Reference Error
- 10, 6
Answer: 1) 6, 10
Q. What would be the output of following code?
function mul(x) { return function (y) { return { result: x * y, sum: function (z) { return x * y + z; }, }; }; } console.log(mul(2)(3).result); console.log(mul(2)(3).sum(4));
- 6, 10
- undefined undefined
- Reference Error
- 10, 6
Answer: 1) 6, 10
Q. What would be the output of following code?
function mul(x) { return function (y) { return function (z) { return function (w) { return function (p) { return x * y * z * w * p; }; }; }; }; } console.log(mul(2)(3)(4)(5)(6));
- 720
- undefined
- Reference Error
- Type Error
Answer: 1) 720
Q. What is the value of foo
?
var foo = 10 + "20";
Answer: '1020'
, because of type coercion from Number to String
Q. How would you make this work?
add(2, 5); // 7 add(2)(5); // 7
Answer: A general solution for any number of parameters
"use strict"; let sum = (arr) => arr.reduce((a, b) => a + b); let addGenerator = (numArgs, prevArgs) => { return function () { let totalArgs = prevArgs.concat(Array.from(arguments)); if (totalArgs.length === numArgs) { return sum(totalArgs); } return addGenerator(numArgs, totalArgs); }; }; let add = addGenerator(2, []); add(2, 5); // 7 add(2)(5); // 7 add()(2, 5); // 7 add()(2)(5); // 7 add()()(2)(5); // 7
Q. What value is returned from the following statement?
"i'm a lasagna hog".split("").reverse().join("");
Answer: It’s actually a reverse method for a string – 'goh angasal a m\'i'
Q. What is the value of window.foo
?
window.foo || (window.foo = "bar");
Answer: Always 'bar'
Q. What is the outcome of the two alerts below?
var foo = "Hello"; (function () { var bar = " World"; alert(foo + bar); })(); alert(foo + bar);
Answer:
- First:
Hello World
- Second: Throws an exception,
ReferenceError: bar is not defined
Q. What is the value of foo.length
?
var foo = []; foo.push(1); foo.push(2);
Answer: .push
is mutable – 2
Q. What is the value of foo.x
?
var foo = { n: 1 }; var bar = foo; foo.x = foo = { n: 2 };
Answer: undefined
. Rather, bar.x
is {n: 2}
.
foo.x = foo = {n: 2}
is the same as foo.x = (foo = {n: 2})
. It is because a left term is first referenced and then a right term is evaluated when an assignment is performed in JavaScript. When foo.x
is referenced, it refers to an original object, {n: 1}
. So, when the result of the right term, {n: 2}
, is evaluated, it will assigned to the original object, which is at the moment referenced by bar
.
Q. What does the following code print?
console.log("one"); setTimeout(function () { console.log("two"); }, 0); console.log("three");
Answer: one
, three
and two
. It’s because console.log('two');
will be invoked in the next event loop.
Q. What would be the result of 1+2+’3′?
The output is going to be 33
. Since 1
and 2
are numeric values, the result of first two digits is going to be a numeric value 3
. The next digit is a string type value because of that the addition of numeric value 3
and string type value 3
is just going to be a concatenation value 33
.
Q. Write a script that returns the number of occurrences of character given a string as input?
function countCharacters(str) { return str .replace(/ /g, "") .toLowerCase() .split("") .reduce((arr, character) => { if (character in arr) { arr[character]++; } else { arr[character] = 1; } return arr; }, {}); } console.log(countCharacters("the brown fox jumps over the lazy dog"));
Q. What is the value of foo
?
var foo = 10 + "20";
Answer: '1020'
, because of type coercion from Number to String
Q. How would you make this work?
add(2, 5); // 7 add(2)(5); // 7
Answer: A general solution for any number of parameters
"use strict"; let sum = (arr) => arr.reduce((a, b) => a + b); let addGenerator = (numArgs, prevArgs) => { return function () { let totalArgs = prevArgs.concat(Array.from(arguments)); if (totalArgs.length === numArgs) { return sum(totalArgs); } return addGenerator(numArgs, totalArgs); }; }; let add = addGenerator(2, []); add(2, 5); // 7 add(2)(5); // 7 add()(2, 5); // 7 add()(2)(5); // 7 add()()(2)(5); // 7
Q. What value is returned from the following statement?
"i'm a lasagna hog".split("").reverse().join("");
Answer: It’s actually a reverse method for a string – 'goh angasal a m\'i'
Q. What is the value of window.foo
?
window.foo || (window.foo = "bar");
Answer: Always 'bar'
Q. What is the outcome of the two alerts below?
var foo = "Hello"; (function () { var bar = " World"; alert(foo + bar); })(); alert(foo + bar);
Answer:
- First:
Hello World
- Second: Throws an exception,
ReferenceError: bar is not defined
Q. What is the value of foo.length
?
var foo = []; foo.push(1); foo.push(2);
Answer: .push
is mutable – 2
Q. What is the value of foo.x
?
var foo = { n: 1 }; var bar = foo; foo.x = foo = { n: 2 };
Answer: undefined
. Rather, bar.x
is {n: 2}
.
foo.x = foo = {n: 2}
is the same as foo.x = (foo = {n: 2})
. It is because a left term is first referenced and then a right term is evaluated when an assignment is performed in JavaScript. When foo.x
is referenced, it refers to an original object, {n: 1}
. So, when the result of the right term, {n: 2}
, is evaluated, it will assigned to the original object, which is at the moment referenced by bar
.
Q. What does the following code print?
console.log("one"); setTimeout(function () { console.log("two"); }, 0); console.log("three");
Answer: one
, three
and two
. It’s because console.log('two');
will be invoked in the next event loop.
Q. For which value of x the results of the following statements are not the same?
// if( x <= 100 ) {...} if( !(x > 100) ) {...}
Answer: NaN <= 100
is false
and NaN > 100
is also false, so if the value of x
is NaN
, the statements are not the same.
The same holds true for any value of x that being converted to Number, returns NaN, e.g.: undefined
, [1,2,5]
, {a:22}
, etc.
Q. What is g value?
f = g = 0; (function () { try { f = function () { return f(); } && f(); } catch (e) { return g++ && f(); } finally { return ++g; } function f() { g += 5; return 0; } })();
Q. What will be the output?
function b(b) { return this.b && b(b); } b(b.bind(b));
Q. What will be the output?
c = (c) => { return this.c && c(c); }; c(c.bind(c));
Q. Predict the output of the following JavaScript code?
var g = 0; g = 1 && g++; console.log(g);
Q. Predict the output of the following JavaScript code?
!function(){}() function(){}() true && function(){}() (function(){})() function(){} !function(){}
Q. ***What will expression return?
var a = (b = true), c = (a) => a; (function a(a = (c(b).a = c = () => a)) { return a(); })();
Q. Predict the output of the following JavaScript code?
var a = true; (a = function () { return a; })();
Q. What will be the output?
var v = 0; try { throw (v = (function (c) { throw (v = function (a) { return v; }); })()); } catch (e) { console.log(e()()); }
Q. What will the following code output?
const arr = [10, 12, 15, 21]; for (var i = 0; i < arr.length; i++) { setTimeout(function () { console.log("Index: " + i + ", element: " + arr[i]); }, 3000); }
Q. What will be the output of the following code?
var output = (function (x) { delete x; return x; })(0); console.log(output);
Q. What will be the output of the following code?
var Employee = { company: "xyz", }; var emp1 = Object.create(Employee); delete emp1.company; console.log(emp1.company);
Q. ***Make this work:
duplicate([1, 2, 3, 4, 5]); // [1,2,3,4,5,1,2,3,4,5]
function duplicate(arr) { return arr.concat(arr); } duplicate([1, 2, 3, 4, 5]); // [1,2,3,4,5,1,2,3,4,5]
Q. Fix the bug using ES5 only?
var arr = [10, 32, 65, 2]; for (var i = 0; i < arr.length; i++) { setTimeout(function () { console.log("The index of this number is: " + i); }, 3000); }
For ES6, you can just replace var i
with let i
.
For ES5, you need to create a function scope like here:
var arr = [10, 32, 65, 2]; for (var i = 0; i < arr.length; i++) { setTimeout( (function (j) { return function () { console.log("The index of this number is: " + j); }; })(i), 3000 ); }
Q. What will be the output of the following code?
console.log(eval("10 + 10")); // 20 console.log(eval("5 + 5" + 10)); // 515 console.log(eval("5 + 5 + 5" + 10)); // 520 console.log(eval(10 + "5 + 5")); // 110 console.log(eval(10 + "5 + 5 + 5")); // 115
Q. What will be the output of the following code?
var x = 10; var y = 20; var a = eval("x * y") + "<br>"; var b = eval("2 + 2") + "<br>"; var c = eval("x + 30") + "<br>"; let result = a + b + c; console.log(result); // 200<br>4<br>40<br>
Q. What will be the output of the following code?
// Example 01: var prices = [12, 20, 18]; var newPriceArray = [...prices]; console.log(newPriceArray); // Example 02: var alphabets = ["A", ..."BCD", "E"]; console.log(alphabets); // Example 03: var prices = [12, 20, 18]; var maxPrice = Math.max(...prices); console.log(maxPrice); // Example 04: var max = Math.max(..."43210"); console.log(max); // Example 05: const fruits = ["apple", "orange"]; const vegetables = ["carrot", "potato"]; const result = ["bread", ...vegetables, "chicken", ...fruits]; console.log(result); // Example 06: const country = "USA"; console.log([...country]);
Q. Given and object and property path. Get value from property path
function getPropertyValue(TEMP_OBJECT, path) { return path.split('.').reduce((prev, key) => { return prev ? prev[key] : undefined; }, TEMP_OBJECT) } //Input : let srcObject = { 'system' : { 'database' : { '0' : { 'host' : '54.232.122', 'port' : 3306 }, '1' : { 'host' : '54.232.123', }, 'port' : 3307 '2' : { 'host' : '54.232.123', } } } }, path = "system.database.1.port"; //Output: 3307
Q. How to filter object from Arrays of Objects
let filteredArray = [{name: 'john'},{name: 'kelly'}].filter(value => value.name === 'kelly'); Filter method return Array of objects
Q. How to replace all the occurrences of string
str = str.replace(/test/g, "");
Q. Write a script that returns the number of occurrences of character given a string as input
function countCharacters(str) { return str .replace(/ /g, "") .toLowerCase() .split("") .reduce((p, c) => { if (c in p) { p[c]++; } else { p[c] = 1; } return p; }, {}); } console.log(countCharacters("the brown fox jumps over the lazy dog"));
Q. write a script that return the number of occurrences of a character in paragraph
function charCount(str, searchChar) { let count = 0; if (str) { let stripStr = str.replace(/ /g, "").toLowerCase(); //remove spaces and covert to lowercase for (let chr of stripStr) { if (chr === searchChar) { count++; } } } return count; } console.log(charCount("the brown fox jumps over the lazy dog", "o"));
Q. Recursive and non-recursive Factorial function
function recursiveFactorial(n) { if (n < 1) { throw Error("Value of N has to be greater then 1"); } if (n === 1) { return 1; } else { return n * recursiveFactorial(n - 1); } } console.log(recursiveFactorial(5)); function factorial(n) { if (n < 1) { throw Error("Value of N has to be greater then 1"); } if (n === 1) { return 1; } let result = 1; for (let i = 1; i <= n; i++) { result = result * i; } return result; } console.log(factorial(5));
Q. Recursive and non recursive fibonacci-sequence
// 1, 1, 2, 3, 5, 8, 13, 21, 34 function recursiveFibonacci(num) { if (num <= 1) { return 1; } else { return recursiveFibonacci(num - 1) + recursiveFibonacci(num - 2); } } console.log(recursiveFibonacci(8)); function fibonnaci(num) { let a = 1, b = 0, temp; while (num >= 0) { temp = a; a = a + b; b = temp; num--; } return b; } console.log(fibonnaci(7)); // Memoization fibonnaci function fibonnaci(num, memo = {}) { if (num in memo) { return memo[num]; } if (num <= 1) { return 1; } return (memo[num] = fibonnaci(num - 1, memo) + fibonnaci(num - 2, memo)); } console.log(fibonnaci(5)); // 8
Q. Random Number between min and max
// 5 to 7 let min = 5; let max = 7; console.log(min + Math.floor(Math.random() * (max - min + 1)));
Q. Get HTML form values as JSON object
// Use the array reduce function with form elements. const formToJSON = (elements) => [].reduce.call( elements, (data, element) => { data[element.name] = element.value; // Check if name and value exist on element // Check if it checkbox or radio button which can select multiple or single //check for multiple select options return data; }, {} ); // pass the elements to above method, to get values document.querySelector("HTML_FORM_CLASS").elements;
Q. Reverse the number
function reverse(num) { let result = 0; while (num != 0) { result = result * 10; result = result + (num % 10); num = Math.floor(num / 10); } return result; } console.log(reverse(12345));
Q. Remove Duplicate elements from Array
var arr = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8]; function removeDuplicate() { return ar.reduce((prev, current) => { //Cannot use includes of array, since it is not supported by many browser if (prev.indexOf(current) === -1) { prev.push(current); } return prev; }, []); } console.log(removeDuplicate(ar)); const removeDuplicates = (arr) => { let holder = {}; return arr.filter((el) => { if (!holder[el]) { holder[el] = true; return true; } return false; }); }; const arr = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8]; console.log(removeDuplicates(arr)); // ["1", "2", "3", "5", "8", "9"] // O(n) // Es6 console.log([...new Set(arr)]);
Q. Deep copy of object or clone of object
function deepExtend(out = {}) { for (let i = 1; i < arguments.length; i++) { let obj = arguments[i]; if (obj == null) // skip undefined and null [check with double equal not triple] continue; obj = Object(obj); for (let key in obj) { // avoid shadow hasownproperty of parent if (Object.prototype.hasOwnProperty.call(obj, key)) { if ( typeof obj[key] === "object" && !Array.isArray(obj[key]) && obj[key] != null ) out[key] = deepExtend(out[key], obj[key]); else out[key] = obj[key]; } } } return out; } //Alternative if there are no function let cloneObj = JSON.parse(JSON.stringify(obj)); console.log(deepExtend({}, { a: 1, b: { c: 2, d: 3 } }, { e: 4, b: { f: 1 } })); //output : { a: 1, b: {c: 2, d: 3, f: 1}, e: 4 }
Q. Sort ticket based on flying order.
"use strict"; function SortTickets(tickets) { this.tickets = tickets; // reverse the order of tickets this.reverseTickets = {}; for (let key in this.tickets) { this.reverseTickets[tickets[key]] = key; } // Get the starting point of ticket let orderedTivckets = [...this.getStartingPoint()]; // Get the ticket destination. let currentValue = orderedTickets[orderedTickets.length - 1]; while (currentValue) { currentValue = this.tickets[currentValue]; if (currentValue) { orderedTickets.push(currentValue); } } console.log(orderedTickets); } SortTickets.prototype.getStartingPoint = function () { for (let tick in this.tickets) { if (!(tick in this.reverseTickets)) { return [tick, this.tickets[tick]]; } } return null; }; new SortTickets({ Athens: "Rio", Barcelona: "Athens", London: "NYC", ND: "Lahore", NYC: "Barcelona", Rio: "ND", });
Q. Cuncurrent execute function based on input number
function concurrent(num) { this.queue = []; this.num = num; } concurrent.prototype.enqueue = function (value) { this.queue.push(value); }; concurrent.prototype.start = function () { this.runningCount = 0; while (this.queue.length > 0) { if (this.runningCount < this.num) { this.queue.pop().call(this, () => { this.runningCount--; let count = this.runningCount; if (count === 0) { this.start(); } }); this.runningCount++; } } }; let callback = (done) => { console.log("starting"); setTimeout(() => { console.log("stopped"); done(); }, 200); }; let c = new concurrent(2); c.enqueue(callback); c.enqueue(callback); c.enqueue(callback); c.enqueue(callback); c.enqueue(callback); c.enqueue(callback); c.start();
Q. Reversing an array
let a = [1, 2, 3, 4, 5]; //Approach 1: console.log(a.reverse()); //Approach 2: let reverse = a.reduce((prev, current) => { prev.unshift(current); return prev; }, []); console.log(reverse);
Q. Rotate 2D array
const transpose = (arr) => arr[0].map((col, i) => arr.map((row) => row[i])); console.log( transpose([ [1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], ]) );
Q. Get Column from 2D Array
const getColumn = (arr, n) => arr.map((x) => x[n]); const twoDimensionalArray = [ [1, 2, 3], [4, 5, 6], [7, 8, 9], ]; console.log(getColumn(twoDimensionalArray, 1)); //Result = [2,5,8]
Q. Get top N from array
function topN(arr, num) { let sorted = arr.sort((a, b) => a - b); return sorted.slice(sorted.length - num, sorted.length); } console.log(topN([1, 8, 3, 4, 5], 2)); // [5,8]
Q. Get query params from Object
function getQueryParams(obj) { let parms = ""; for (let key in obj) { if (obj.hasOwnProperty(key)) { if (parms.length > 0) { parms += "&"; } parms += encodeURI(`${key}=${obj[key]}`); } } return parms; } console.log( getQueryParams({ name: "Umesh", tel: "48289", add: "3333 emearld st", }) );
Q. Consecutive 1’s in binary
function consecutiveOne(num) { let binaryArray = num.toString(2); let maxOccurence = 0, occurence = 0; for (let val of binaryArray) { if (val === "1") { occurence += 1; maxOccurence = Math.max(maxOccurence, occurence); } else { occurence = 0; } } return maxOccurence; } //13 = 1101 = 2 //5 = 101 = 1 console.log(consecutiveOne(5)); //1
Q. Spiral travesal of matrix
var input = [ [1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16], ]; var spiralTraversal = function (matriks) { let result = []; var goAround = function (matrix) { if (matrix.length === 0) { return; } // right result = result.concat(matrix.shift()); // down for (var j = 0; j < matrix.length - 1; j++) { result.push(matrix[j].pop()); } // bottom result = result.concat(matrix.pop().reverse()); // up for (var k = matrix.length - 1; k > 0; k--) { result.push(matrix[k].shift()); } return goAround(matrix); }; goAround(matriks); return result; }; console.log(spiralTraversal(input)); // [1, 2, 3, 4, 8, 12, 16, 15, 14, 13, 9, 5, 6, 7, 11, 10]
Q. Merge Sorted array and sort it.
function mergeSortedArray(arr1, arr2) { return [...new Set(arr1.concat(arr2))].sort((a, b) => a - b); } console.log(mergeSortedArray([1, 2, 3, 4, 5, 6], [0, 3, 4, 7])); // [0, 1, 2, 3, 4, 5, 6, 7]
Q. Anagram of words
const alphabetize = (word) => word.split("").sort().join(""); function groupAnagram(wordsArr) { return wordsArr.reduce((p, c) => { const sortedWord = alphabetize(c); if (sortedWord in p) { p[sortedWord].push(c); } else { p[sortedWord] = [c]; } return p; }, {}); } console.log( groupAnagram(["map", "art", "how", "rat", "tar", "who", "pam", "shoop"]) ); // result : { // amp: ["map", "pam"], // art: ["art", "rat", "tar"], // hoops: ["shoop"], // how: ["how", "who"] // }
Q. Print the largest (maximum) hourglass sum found in 2d array.
// if arr 6 X 6 then iterate it till 4 X 4 [reduce by two] // if arr 8 X 8 then iterate it till 6 X 6 [reduce by two] function main(arr) { let maxScore = -999; let len = arr.length; for (let i = 0; i < len - 2; i++) { for (let j = 0; j < len - 2; j++) { let total = arr[i][j] + arr[i][j + 1] + arr[i][j + 2] + arr[i + 1][j + 1] + arr[i + 2][j] + arr[i + 2][j + 1] + arr[i + 2][j + 2]; maxScore = Math.max(maxScore, total); } } console.log(maxScore); }
Q. Transform array of object to array
let data = [ { vid: "aaa", san: 12 }, { vid: "aaa", san: 18 }, { vid: "aaa", san: 2 }, { vid: "bbb", san: 33 }, { vid: "bbb", san: 44 }, { vid: "aaa", san: 100 }, ]; let newData = data.reduce((acc, item) => { acc[item.vid] = acc[item.vid] || { vid: item.vid, san: [] }; acc[item.vid]["san"].push(item.san); return acc; }, {}); console.log(Object.keys(newData).map((key) => newData[key])); // Result // [[object Object] { // san: [12, 18, 2, 100], // vid: "aaa" // }, [object Object] { // san: [33, 44], // vid: "bbb" // }]
Q. Create a private variable or private method in object
let obj = (function () { function getPrivateFunction() { console.log("this is private function"); } let p = "You are accessing private variable"; return { getPrivateProperty: () => { console.log(p); }, callPrivateFunction: getPrivateFunction, }; })(); obj.getPrivateValue(); // You are accessing private variable console.log("p" in obj); // false obj.callPrivateFunction(); // this is private function
Q. Flatten only Array not objects
function flatten(arr, result = []) { arr.forEach((val) => { if (Array.isArray(val)) { flatten(val, result); } else { result.push(val); } }); return result; } let input = [1, { a: [2, [3]] }, 4, [5, [6]], [[7, ["hi"]], 8, 9], 10]; console.log(flatten(input)); // [1, { a: [2, [3]]}, 4, 5, 6, 7, "hi", 8, 9, 10] function flattenIterative(out) { // iteratively let result = out; while (result.some(Array.isArray)) { result = [].concat.apply([], result); } return result; } var list1 = [ [0, 1], [2, 3], [4, 5], ]; console.log(flattenIterative(list1)); // [0, 1, 2, 3, 4, 5] function flattenIterative1(current) { let result = []; while (current.length) { let firstValue = current.shift(); if (Array.isArray(firstValue)) { current = firstValue.concat(current); } else { result.push(firstValue); } } return result; } let input = [1, { a: [2, [3]] }, 4, [5, [6]], [[7, ["hi"]], 8, 9], 10]; console.log(flattenIterative1(input)); var list2 = [0, [1, [2, [3, [4, [5]]]]]]; console.log(flattenIterative1(list2)); // [0, 1, 2, 3, 4, 5]
Q. Find max difference between two number in Array
function maxDifference(arr) { let maxDiff = 0; for (let i = 0; i < arr.length; i++) { for (let j = i + 1; j < arr.length; j++) { let diff = Math.abs(arr[i] - arr[j]); maxDiff = Math.max(maxDiff, diff); } } return maxDiff; } console.log(maxDifference([1, 2, 4])); // [1 - 4 ] = 3
Q. swap two number in ES6 [destructing]
let a = 10, b = 5; [a, b] = [b, a];
Q. Panagram ? it means all the 26 letters of alphabet are there
function panagram(input) { if (input == null) { // Check for null and undefined return false; } if (input.length < 26) { // if length is less then 26 then it is not return false; } input = input.replace(/ /g, "").toLowerCase().split(""); let obj = input.reduce((prev, current) => { if (!(current in prev)) { prev[current] = current; } return prev; }, {}); console.log(Object.keys(obj).length === 26 ? "panagram" : "not pangram"); } processData("We promptly judged antique ivory buckles for the next prize"); // pangram processData("We promptly judged antique ivory buckles for the prize"); // Not Pangram
Q. Given two identical DOM trees (not the same one), and a node from one of them find the node in the other one.
function indexOf(arrLike, target) { return Array.prototype.indexOf.call(arrLike, target); } // Given a node and a tree, extract the nodes path function getPath(root, target) { var current = target; var path = []; while (current !== root) { let parentNode = current.parentNode; path.unshift(indexOf(parentNode.childNodes, current)); current = parentNode; } return path; } // Given a tree and a path, let's locate a node function locateNodeFromPath(node, path) { return path.reduce((root, index) => root.childNodes[index], node); } const rootA = document.querySelector("#root-a"); const rootB = document.querySelector("#root-b"); const target = rootA.querySelector(".person__age"); console.log(locateNodeFromPath(rootB, getPath(rootA, target)));
Q. Convert a number into a Roman Numeral
function romanize(num) { let lookup = { M: 1000, CM: 900, D: 500, CD: 400, C: 100, XC: 90, L: 50, XL: 40, X: 10, IX: 9, V: 5, IV: 4, I: 1, }, roman = ""; for (let i in lookup) { while (num >= lookup[i]) { roman += i; num -= lookup[i]; } } return roman; } console.log(romanize(3)); // III
Q. check if parenthesis is malformed or not
function matchParenthesis(str) { let obj = { "{": "}", "(": ")", "[": "]" }; let result = []; for (let s of str) { if (s === "{" || s === "(" || s === "[") { // All opening brackets result.push(s); } else { if (result.length > 0) { let lastValue = result.pop(); //pop the last value and compare with key if (obj[lastValue] !== s) { // if it is not same then it is not formated properly return false; } } else { return false; // empty array, there is nothing to pop. so it is not formated properly } } } return result.length === 0; } console.log(matchParenthesis("}{{}}"), matchParenthesis("{{[]}}")); // false - true
Q. Create Custom Event Emitter class
class EventEmitter { constructor() { this.holder = {}; } on(eventName, fn) { if (eventName && typeof fn === "function") { this.holder[eventName] = this.holder[eventName] || []; this.holder[eventName].push(fn); } } emit(eventName, ...args) { let eventColl = this.holder[eventName]; if (eventColl) { eventColl.forEach((callback) => callback(args)); } } } let e = new EventEmitter(); e.on("callme", function (args) { console.log(`you called me ${args}`); }); e.on("callme", function (args) { console.log(`testing`); }); e.emit("callme", ["a", "b"], { firstName: "umesh", lastName: "gohil" });
Q. Max value from an array
const arr = [-2, -3, 4, 3, 2, 1]; Math.max(...arr); // Fastest Math.max.apply(Math, arr); // Slow
Q. DOM methods
https://github.com/nefe/You-Dont-Need-jQuery var el = document.querySelector('div'); el.childNodes; // get the list of child nodes of el el.firstChild; // get the first child node of el el.lastChild; // get the last child node of el el.parentNode; // get the parent node of el el.previousSibling; // get the previous sibling of el el.nextSibling; // get the next sibling of el el.textContent; // get the text content of el el.innerHTML; // get the inner html of el document.createElement('div') // create dom element document.creatTextNode('Hello world'); // create text node document.createDocumentFragment(); el.appendChild(); //append child to el; el.insertBefore(); // insert child before el; el.parentNode.replaceChild(NEW_NODE, REPLACE_ME) // replace the node el.removechild(); // remove the child node Array.from(NODES) // convert nodelist to regular array el.classList[contains | add | remove | replace] // class of el el.dataset.<camelCaseName> // data-count is dataset.count, data-index-number is dataset.indexNumber el.setAttribute | el.getAttribute | el.removeAttribute // attributes of el el.style // get the style of el
Q. search function called after 500 ms
<input type="text" class="search" />; let timer = null; function searchOptions(value) { clearTimeout(timer); timer = setTimeout(() => { console.log(`value is - ${value}`); }, 500); } let search = document.querySelector(".search"); search.addEventListener("keyup", function () { searchOptions(this.value); });
Q. Move all zero’s to end
const moveZeroToEnd = (arr) => { for (let i = 0, j = 0; j < arr.length; j++) { if (arr[j] !== 0) { if (i < j) { [arr[i], arr[j]] = [arr[j], arr[i]]; // swap i and j } i++; } } return arr; }; console.log(moveZeroToEnd([1, 8, 2, 0, 0, 0, 3, 4, 0, 5, 0])); // [1, 8, 2, 3, 4, 5, 0, 0, 0, 0, 0]
Q. Decode message in matrix [diagional down right, diagional up right]
const decodeMessage = (mat) => { // check if matrix is null or empty if (mat == null || mat.length === 0) { return ""; } let x = mat.length - 1; let y = mat[0].length - 1; let message = ""; let decode = (mat, i = 0, j = 0, direction = "DOWN") => { message += mat[i][j]; if (i === x) { direction = "UP"; } if (direction === "DOWN") { i++; } else { i--; } if (j === y) { return; } j++; decode(mat, i, j, direction); }; decode(mat); return message; }; let mat = [ ["I", "B", "C", "A", "L", "K", "A"], ["D", "R", "F", "C", "A", "E", "A"], ["G", "H", "O", "E", "L", "A", "D"], ["G", "H", "O", "E", "L", "A", "D"], ]; console.log(decodeMessage(mat)); //IROELEA
Q. find a pair in array, whose sum is equal to given number.
const hasPairSum = (arr, sum) => { if (arr == null && arr.length < 2) { return false; } let left = 0; let right = arr.length - 1; let result = false; while (left < right && !result) { let pairSum = arr[left] + arr[right]; if (pairSum < sum) { left++; } else if (pairSum > sum) { right--; } else { result = true; } } return result; }; console.log(hasPairSum([1, 2, 4, 5], 8)); // null console.log(hasPairSum([1, 2, 4, 4], 8)); // [2,3] const hasPairSum = (arr, sum) => { let difference = {}; let hasPair = false; arr.forEach((item) => { let diff = sum - item; if (!difference[diff]) { difference[item] = true; } else { hasPair = true; } }); return hasPair; }; console.log(hasPairSum([6, 4, 3, 8], 8)); // NOTE: if array is not sorted then subtract the value with sum and store in difference // then see if that value exist in difference then return true.
Q. Binary Search [Array should be sorted]
function binarySearch(arr, val) { let startIndex = 0, stopIndex = arr.length - 1, middleIndex = Math.floor((startIndex + stopIndex) / 2); while (arr[middleIndex] !== val && startIndex < stopIndex) { if (val < arr[middleIndex]) { stopIndex = middleIndex - 1; } else if (val > arr[middleIndex]) { startIndex = middleIndex + 1; } middleIndex = Math.floor((startIndex + stopIndex) / 2); } return arr[middleIndex] === val ? middleIndex : -1; } console.log(binarySearch([-1, 10, 22, 35, 48, 56, 67], 22)); console.log(binarySearch([-1, 10, 22, 35, 48, 56, 67], 27));
Q. Pascal triangle.
function pascalTriangle(n) { let last = [1], triangle = [last]; for (let i = 0; i < n; i++) { const ls = [0].concat(last), //[0,1] // [0,1,1] rs = last.concat([0]); //[1,0] // [1,1,0] last = rs.map((r, i) => ls[i] + r); //[1, 1] // [1,2,1] triangle = triangle.concat([last]); // [[1], [1,1]] // [1], [1, 1], [1, 2, 1] } return triangle; } console.log(pascalTriangle(2));
Q. Explain the code below. How many times the createVal function is called?
function createVal() { return Math.random(); } function fun(val = createVal()) { console.log(val); } fun(); fun(5);
createVal()
function will execute only once.
Output
0.2162050091554224 VM298:6 5
Q. What is the output?
function sayHi() { console.log(name); console.log(age); var name = "Lydia"; let age = 21; } sayHi();
- A:
Lydia
andundefined
- B:
Lydia
andReferenceError
- C:
ReferenceError
and21
- D:
undefined
andReferenceError
Answer: D
Within the function, we first declare the name
variable with the var
keyword. This means that the variable gets hoisted (memory space is set up during the creation phase) with the default value of undefined
, until we actually get to the line where we define the variable. We haven’t defined the variable yet on the line where we try to log the name
variable, so it still holds the value of undefined
.
Variables with the let
keyword (and const
) are hoisted, but unlike var
, don’t get initialized. They are not accessible before the line we declare (initialize) them. This is called the “temporal dead zone”. When we try to access the variables before they are declared, JavaScript throws a ReferenceError
.
Q. What is the output?
for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1); } for (let i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1); }
- A:
0 1 2
and0 1 2
- B:
0 1 2
and3 3 3
- C:
3 3 3
and0 1 2
Answer: C
Because of the event queue in JavaScript, the setTimeout
callback function is called after the loop has been executed. Since the variable i
in the first loop was declared using the var
keyword, this value was global. During the loop, we incremented the value of i
by 1
each time, using the unary operator ++
. By the time the setTimeout
callback function was invoked, i
was equal to 3
in the first example.
In the second loop, the variable i
was declared using the let
keyword: variables declared with the let
(and const
) keyword are block-scoped (a block is anything between { }
). During each iteration, i
will have a new value, and each value is scoped inside the loop.
Q. What is the output?
const shape = { radius: 10, diameter() { return this.radius * 2; }, perimeter: () => 2 * Math.PI * this.radius, }; console.log(shape.diameter()); console.log(shape.perimeter());
- A:
20
and62.83185307179586
- B:
20
andNaN
- C:
20
and63
- D:
NaN
and63
Answer: B
Note that the value of diameter
is a regular function, whereas the value of perimeter
is an arrow function.
With arrow functions, the this
keyword refers to its current surrounding scope, unlike regular functions! This means that when we call perimeter
, it doesn’t refer to the shape object, but to its surrounding scope (window for example).
There is no value radius
on that object, which returns undefined
.
Q. What is the output?
+true; !"Lydia";
- A:
1
andfalse
- B:
false
andNaN
- C:
false
andfalse
Answer: A
The unary plus tries to convert an operand to a number. true
is 1
, and false
is 0
.
The string 'Lydia'
is a truthy value. What we’re actually asking, is “is this truthy value falsy?”. This returns false
.
Q. Which one is true?
const bird = { size: "small", }; const mouse = { name: "Mickey", small: true, };
- A:
mouse.bird.size
is not valid - B:
mouse[bird.size]
is not valid - C:
mouse[bird["size"]]
is not valid - D: All of them are valid
Answer: A
In JavaScript, all object keys are strings (unless it’s a Symbol). Even though we might not type them as strings, they are always converted into strings under the hood.
JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket [
and keeps going until it finds the closing bracket ]
. Only then, it will evaluate the statement.
mouse[bird.size]
: First it evaluates bird.size
, which is "small"
. mouse["small"]
returns true
However, with dot notation, this doesn’t happen. mouse
does not have a key called bird
, which means that mouse.bird
is undefined
. Then, we ask for the size
using dot notation: mouse.bird.size
. Since mouse.bird
is undefined
, we’re actually asking undefined.size
. This isn’t valid, and will throw an error similar to Cannot read property "size" of undefined
.
Q. What is the output?
let c = { greeting: "Hey!" }; let d; d = c; c.greeting = "Hello"; console.log(d.greeting);
- A:
Hello
- B:
Hey!
- C:
undefined
- D:
ReferenceError
- E:
TypeError
Answer: A
In JavaScript, all objects interact by reference when setting them equal to each other.
First, variable c
holds a value to an object. Later, we assign d
with the same reference that c
has to the object.
When you change one object, you change all of them.
Q. What is the output?
let a = 3; let b = new Number(3); let c = 3; console.log(a == b); console.log(a === b); console.log(b === c);
- A:
true
false
true
- B:
false
false
true
- C:
true
false
false
- D:
false
true
true
Answer: C
new Number()
is a built-in function constructor. Although it looks like a number, it’s not really a number: it has a bunch of extra features and is an object.
When we use the ==
operator, it only checks whether it has the same value. They both have the value of 3
, so it returns true
.
However, when we use the ===
operator, both value and type should be the same. It’s not: new Number()
is not a number, it’s an object. Both return false.
Q. What is the output?
class Chameleon { static colorChange(newColor) { this.newColor = newColor; return this.newColor; } constructor({ newColor = "green" } = {}) { this.newColor = newColor; } } const freddie = new Chameleon({ newColor: "purple" }); console.log(freddie.colorChange("orange"));
- A:
orange
- B:
purple
- C:
green
- D:
TypeError
Answer: D
The colorChange
function is static. Static methods are designed to live only on the constructor in which they are created, and cannot be passed down to any children. Since freddie
is a child, the function is not passed down, and not available on the freddie
instance: a TypeError
is thrown.
Q. What is the output?
let greeting; greetign = {}; // Typo! console.log(greetign);
- A:
{}
- B:
ReferenceError: greetign is not defined
- C:
undefined
Answer: A
It logs the object, because we just created an empty object on the global object! When we mistyped greeting
as greetign
, the JS interpreter actually saw this as global.greetign = {}
(or window.greetign = {}
in a browser).
In order to avoid this, we can use "use strict"
. This makes sure that you have declared a variable before setting it equal to anything.
Q. What happens when we do this?
function bark() { console.log("Woof!"); } bark.animal = "dog";
- A: Nothing, this is totally fine!
- B:
SyntaxError
. You cannot add properties to a function this way. - C:
"Woof"
gets logged. - D:
ReferenceError
Answer: A
This is possible in JavaScript, because functions are objects! (Everything besides primitive types are objects)
A function is a special type of object. The code you write yourself isn’t the actual function. The function is an object with properties. This property is invocable.
Q. What is the output?
function Person(firstName, lastName) { this.firstName = firstName; this.lastName = lastName; } const member = new Person("Lydia", "Hallie"); Person.getFullName = function () { return `${this.firstName} ${this.lastName}`; }; console.log(member.getFullName());
- A:
TypeError
- B:
SyntaxError
- C:
Lydia Hallie
- D:
undefined
undefined
Answer: A
You can’t add properties to a constructor like you can with regular objects. If you want to add a feature to all objects at once, you have to use the prototype instead. So in this case,
Person.prototype.getFullName = function () { return `${this.firstName} ${this.lastName}`; };
would have made member.getFullName()
work. Why is this beneficial? Say that we added this method to the constructor itself. Maybe not every Person
instance needed this method. This would waste a lot of memory space, since they would still have that property, which takes of memory space for each instance. Instead, if we only add it to the prototype, we just have it at one spot in memory, yet they all have access to it!
Q. What is the output?
function Person(firstName, lastName) { this.firstName = firstName; this.lastName = lastName; } const lydia = new Person("Lydia", "Hallie"); const sarah = Person("Sarah", "Smith"); console.log(lydia); console.log(sarah);
- A:
Person {firstName: "Lydia", lastName: "Hallie"}
andundefined
- B:
Person {firstName: "Lydia", lastName: "Hallie"}
andPerson {firstName: "Sarah", lastName: "Smith"}
- C:
Person {firstName: "Lydia", lastName: "Hallie"}
and{}
- D:
Person {firstName: "Lydia", lastName: "Hallie"}
andReferenceError
Answer: A
For sarah
, we didn’t use the new
keyword. When using new
, it refers to the new empty object we create. However, if you don’t add new
it refers to the global object!
We said that this.firstName
equals "Sarah"
and this.lastName
equals "Smith"
. What we actually did, is defining global.firstName = 'Sarah'
and global.lastName = 'Smith'
. sarah
itself is left undefined
, since we don’t return a value from the Person
function.
Q. What are the three phases of event propagation?
- A: Target > Capturing > Bubbling
- B: Bubbling > Target > Capturing
- C: Target > Bubbling > Capturing
- D: Capturing > Target > Bubbling
Answer: D
During the capturing phase, the event goes through the ancestor elements down to the target element. It then reaches the target element, and bubbling begins.
Q. All object have prototypes.
- A: true
- B: false
Answer: B
All objects have prototypes, except for the base object. The base object is the object created by the user, or an object that is created using the new
keyword. The base object has access to some methods and properties, such as .toString
. This is the reason why you can use built-in JavaScript methods! All of such methods are available on the prototype. Although JavaScript can’t find it directly on your object, it goes down the prototype chain and finds it there, which makes it accessible for you.
Q. What is the output?
function sum(a, b) { return a + b; } sum(1, "2");
- A:
NaN
- B:
TypeError
- C:
"12"
- D:
3
Answer: C
JavaScript is a dynamically typed language: we don’t specify what types certain variables are. Values can automatically be converted into another type without you knowing, which is called implicit type coercion. Coercion is converting from one type into another.
In this example, JavaScript converts the number 1
into a string, in order for the function to make sense and return a value. During the addition of a numeric type (1
) and a string type ('2'
), the number is treated as a string. We can concatenate strings like "Hello" + "World"
, so What is happening here is "1" + "2"
which returns "12"
.
Q. What is the output?
let number = 0; console.log(number++); console.log(++number); console.log(number);
- A:
1
1
2
- B:
1
2
2
- C:
0
2
2
- D:
0
1
2
Answer: C
The postfix unary operator ++
:
- Returns the value (this returns
0
) - Increments the value (number is now
1
)
The prefix unary operator ++
:
- Increments the value (number is now
2
) - Returns the value (this returns
2
)
This returns 0 2 2
.
Q. What is the output?
function getPersonInfo(one, two, three) { console.log(one); console.log(two); console.log(three); } const person = "Lydia"; const age = 21; getPersonInfo`${person} is ${age} years old`;
- A:
"Lydia"
21
["", " is ", " years old"]
- B:
["", " is ", " years old"]
"Lydia"
21
- C:
"Lydia"
["", " is ", " years old"]
21
Answer: B
If you use tagged template literals, the value of the first argument is always an array of the string values. The remaining arguments get the values of the passed expressions!
Q. What is the output?
function checkAge(data) { if (data === { age: 18 }) { console.log("You are an adult!"); } else if (data == { age: 18 }) { console.log("You are still an adult."); } else { console.log(`Hmm.. You don't have an age I guess`); } } checkAge({ age: 18 });
- A:
You are an adult!
- B:
You are still an adult.
- C:
Hmm.. You don't have an age I guess
Answer: C
When testing equality, primitives are compared by their value, while objects are compared by their reference. JavaScript checks if the objects have a reference to the same location in memory.
The two objects that we are comparing don’t have that: the object we passed as a parameter refers to a different location in memory than the object we used in order to check equality.
This is why both { age: 18 } === { age: 18 }
and { age: 18 } == { age: 18 }
return false
.
Q. What is the output?
function getAge(...args) { console.log(typeof args); } getAge(21);
- A:
"number"
- B:
"array"
- C:
"object"
- D:
"NaN"
Answer: C
The rest parameter (...args
.) lets us “collect” all remaining arguments into an array. An array is an object, so typeof args
returns "object"
Q. What is the output?
function getAge() { "use strict"; age = 21; console.log(age); } getAge();
- A:
21
- B:
undefined
- C:
ReferenceError
- D:
TypeError
Answer: C
With "use strict"
, you can make sure that you don’t accidentally declare global variables. We never declared the variable age
, and since we use "use strict"
, it will throw a reference error. If we didn’t use "use strict"
, it would have worked, since the property age
would have gotten added to the global object.
Q. What is value of sum
?
const sum = eval("10*10+5");
- A:
105
- B:
"105"
- C:
TypeError
- D:
"10*10+5"
Answer: A
eval
evaluates codes that’s passed as a string. If it’s an expression, like in this case, it evaluates the expression. The expression is 10 * 10 + 5
. This returns the number 105
.
Q. How long is cool_secret accessible?
sessionStorage.setItem("cool_secret", 123);
- A: Forever, the data doesn’t get lost.
- B: When the user closes the tab.
- C: When the user closes the entire browser, not only the tab.
- D: When the user shuts off their computer.
Answer: B
The data stored in sessionStorage
is removed after closing the tab.
If you used localStorage
, the data would’ve been there forever, unless for example localStorage.clear()
is invoked.
Q. What is the output?
var num = 8; var num = 10; console.log(num);
- A:
8
- B:
10
- C:
SyntaxError
- D:
ReferenceError
Answer: B
With the var
keyword, you can declare multiple variables with the same name. The variable will then hold the latest value. You cannot do this with let
or const
since they’re block-scoped.
Q. What is the output?
const obj = { 1: "a", 2: "b", 3: "c" }; const set = new Set([1, 2, 3, 4, 5]); obj.hasOwnProperty("1"); obj.hasOwnProperty(1); set.has("1"); set.has(1);
- A:
false
true
false
true
- B:
false
true
true
true
- C:
true
true
false
true
- D:
true
true
true
true
Answer: C
All object keys (excluding Symbols) are strings under the hood, even if you don’t type it yourself as a string. This is why obj.hasOwnProperty('1')
also returns true.
It doesn’t work that way for a set. There is no '1'
in our set: set.has('1')
returns false
. It has the numeric type 1
, set.has(1)
returns true
.
Q. What is the output?
const obj = { a: "one", b: "two", a: "three" }; console.log(obj);
- A:
{ a: "one", b: "two" }
- B:
{ b: "two", a: "three" }
- C:
{ a: "three", b: "two" }
- D:
SyntaxError
Answer: C
If you have two keys with the same name, the key will be replaced. It will still be in its first position, but with the last specified value.
Q. The JavaScript global execution context creates two things for you: the global object, and the “this” keyword.
- A: true
- B: false
- C: it depends
Answer: A
The base execution context is the global execution context: it’s What is accessible everywhere in your code.
Q. What is the output?
for (let i = 1; i < 5; i++) { if (i === 3) continue; console.log(i); }
- A:
1
2
- B:
1
2
3
- C:
1
2
4
- D:
1
3
4
Answer: C
The continue
statement skips an iteration if a certain condition returns true
.
Q. What is the output?
String.prototype.giveLydiaPizza = () => { return "Just give Lydia pizza already!"; }; const name = "Lydia"; name.giveLydiaPizza();
- A:
"Just give Lydia pizza already!"
- B:
TypeError: not a function
- C:
SyntaxError
- D:
undefined
Answer: A
String
is a built-in constructor, which we can add properties to. I just added a method to its prototype. Primitive strings are automatically converted into a string object, generated by the string prototype function. So, all strings (string objects) have access to that method!
Q. What is the output?
const a = {}; const b = { key: "b" }; const c = { key: "c" }; a[b] = 123; a[c] = 456; console.log(a[b]);
- A:
123
- B:
456
- C:
undefined
- D:
ReferenceError
Answer: B
Object keys are automatically converted into strings. We are trying to set an object as a key to object a
, with the value of 123
.
However, when we stringify an object, it becomes "[Object object]"
. So what we are saying here, is that a["Object object"] = 123
. Then, we can try to do the same again. c
is another object that we are implicitly stringifying. So then, a["Object object"] = 456
.
Then, we log a[b]
, which is actually a["Object object"]
. We just set that to 456
, so it returns 456
.
Q. What is the output?
const foo = () => console.log("First"); const bar = () => setTimeout(() => console.log("Second")); const baz = () => console.log("Third"); bar(); foo(); baz();
- A:
First
Second
Third
- B:
First
Third
Second
- C:
Second
First
Third
- D:
Second
Third
First
Answer: B
We have a setTimeout
function and invoked it first. Yet, it was logged last.
This is because in browsers, we don’t just have the runtime engine, we also have something called a WebAPI
. The WebAPI
gives us the setTimeout
function to start with, and for example the DOM.
After the callback is pushed to the WebAPI, the setTimeout
function itself (but not the callback!) is popped off the stack.
Now, foo
gets invoked, and "First"
is being logged.
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