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);

1. Some Value
2. Undefined
3. Type Error
4. 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";

1. Some Value
2. undefined
3. Type Error
4. 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";
})();

1. ‘122345’
2. undefined
3. Type Error
4. 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";
  })();
})();

1. ‘122345’
2. undefined
3. ‘1234abe’
4. 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");
  };
})();

1. undefined
2. function
3. ‘Hi I am inside displayFunc’
4. 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);

1. undefined
2. ‘123bcd’
3. ‘abc123’
4. 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);

1. undefined
2. ‘123bcd’
3. ‘abc123’
4. 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();

1. undefined
2. function
3. string
4. 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();

1. undefined
2. Type Error
3. ‘Car’
4. 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);
  }
})();

1. undefined undefined
2. Type Error
3. function function
4. 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));
})();

1. Type Error
2. undefined
3. [“name”, “salary”, “country”, “phoneNo”]
4. [“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));
})();

1. Type Error
2. undefined
3. [“name”, “salary”, “country”, “phoneNo”]
4. [“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);
})();

1. false true
2. false false
3. true false
4. 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);
})();

1. false true
2. false false
3. true false
4. 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);
})();

1. false true
2. false false
3. true false
4. 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);
})();

1. false true
2. false false
3. true false
4. 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());
})();

1. false true
2. false false
3. true false
4. 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());
})();

1. true true true false
2. true false true true
3. true true true true
4. 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);
})();

1. foo bar
2. bar bar
3. foo foo
4. 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);
})();

1. foo bar
2. bar bar
3. foo foo
4. 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);
})();

1. foo bar
2. undefined undefined
3. foo foo
4. 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);
})();

1. undefined undefined
2. [undefined × 100] 100
3. [“100”] 1
4. 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);
})();

1. [] [] [Array[5]] 1
2. [] [undefined × 100] Array[5] 1
3. [] [] [‘1′,2,’3’,4,5.6] 5
4. [] [] [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);
})();

1. 11
2. 5
3. 6
4. 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);
})();

1. 4
2. 5
3. 6
4. 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);
})();

1. [ ‘dog’, ‘rat’, ‘goat’, ‘cow’, ‘horse’, ‘cat’ ]
2. [ ‘cow’, ‘horse’, ‘cat’, ‘dog’, ‘rat’, ‘goat’ ]
3. Type Error
4. 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 -1 4
2. 1 0 -1 4
3. 1 -1 -1 -1
4. 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
2. 1 6 -1
3. 1 1 -1
4. 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);
})();

1. [ 2, 4, 8, 12, 16 ] [ 0, 3, 0, 0, 9, 0, 12]
2. [ 2, 4, 8, 12, 16 ] [ 3, 9, 12]
3. [ 2, 4, 8, 12, 16 ] true
4. [ 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);
})();

1. [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ] [ 2, ’12’, true ]
2. [false, true] [ 2 ] [’12’] [ ]
3. [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true] [2,0,false,””, ’12’, true]
4. [ 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);
})();

1. [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
2. [ ‘bar’, ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’,’ritz ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
3. [ ‘john’, ‘ritz’ ] [ ‘bar’, ‘john’ ] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ] [] [ ‘foo’, ‘bar’, ‘john’, ‘ritz’ ]
4. [ ‘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);
})();

1. [ ‘bar’, ‘john’ ] [] [ ‘foo’ ]
2. [ ‘bar’, ‘john’ ] [] [ ‘bar’, ‘john’ ]
3. [ ‘bar’, ‘john’ ] [ ‘bar’, ‘john’ ] [ ‘bar’, ‘john’ ]
4. [ ‘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);
})();

1. [2, 8, 15, 16, 23, 42]
2. [42, 23, 26, 15, 8, 2]
3. [ 15, 16, 2, 23, 42, 8 ]
4. [ 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());

1. funcA Window {…} innerFunc1 Window {…} innerFunA11 Window {…}
2. undefined
3. Type Error
4. 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);

1. ReferenceError: this.message is not defined
2. undefined
3. Type Error
4. 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());

1. Hello
2. undefined
3. Type Error
4. 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());

1. Type Error
2. Hello
3. undefined
4. 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());

1. Type Error
2. ‘Hello’
3. undefined
4. 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());

1. Type Error
2. ‘Hi John’
3. undefined
4. 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());

1. Type Error
2. ‘Hi John’
3. undefined
4. 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());

1. Type Error
2. ‘Hi John’
3. undefined
4. 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"));

1. 2 2 2
2. 0 2 4
3. undefined
4. 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"));

1. 2 2 2
2. 0 2 4
3. undefined
4. 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();

1. John Person
2. John John
3. John undefined
4. 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);

1. 12345678 Window
2. 12345678 John
3. 12345678 undefined
4. 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);

1. Reference Error
2. aq123
3. bq1uy
4. 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);

1. bq1uy 1BJKSJ bq1uy undefined
2. bq1uy 1BJKSJ bq1uy
3. bq1uy 1BJKSJ kj182
4. 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);
})();

1. foo123 aq123
2. foo123 foo123
3. aq123 aq123
4. 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);
})();

1. Hello World
2. undefined
3. World
4. [ ‘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)());
})();

1. Total amount left in account: 5600 Total amount left in account: 5300
2. undefined undefined
3. Total amount left in account: 3600 Total amount left in account: 3300
4. 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));
})();

1. 5600 5300 5100
2. 3600 3300 3100
3. 5600 3300 5100
4. 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));
})();

1. 5600 5300 5100
2. 3600 3300 3100
3. 5600 3300 5100
4. 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",
})());

1. Hello John
2. Reference Error
3. Window
4. 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);
});

1. John
2. undefined
3. Reference Error
4. 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);
})();

1. [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]
2. undefined undefined undefined
3. [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2]
4. 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);
})();

1. Hi John
2. undefined
3. Reference Error
4. 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);

1. 5
2. undefined
3. 2
4. (2,4,5)

Answer: 1) 5

Q. What would be the output of following code?

function getNumber() {
  return;
}

var numb = getNumber();
console.log(numb);

1. null
2. undefined
3. “”
4. 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));

1. 6, 10
2. undefined undefined
3. Reference Error
4. 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));

1. 6, 10
2. undefined undefined
3. Reference Error
4. 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));

1. 720
2. undefined
3. Reference Error
4. 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 and undefined
• B: Lydia and ReferenceError
• C: ReferenceError and 21
• D: undefined and ReferenceError

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 and 0 1 2
• B: 0 1 2 and 3 3 3
• C: 3 3 3 and 0 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 and 62.83185307179586
• B: 20 and NaN
• C: 20 and 63
• D: NaN and 63

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 and false
• B: false and NaN
• C: false and false

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"} and undefined
• B: Person {firstName: "Lydia", lastName: "Hallie"} and Person {firstName: "Sarah", lastName: "Smith"}
• C: Person {firstName: "Lydia", lastName: "Hallie"} and {}
• D:Person {firstName: "Lydia", lastName: "Hallie"} and ReferenceError

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 ++:

1. Returns the value (this returns 0)
2. Increments the value (number is now 1)

The prefix unary operator ++:

1. Increments the value (number is now 2)
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.