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JavaScript-100-objective-based-questions

This document contains 100 commonly asked JavaScript interview questions with their answers. These questions cover various aspects of JavaScript including data types, operators, functions, OOP, asynchronous programming, and more.

Table of Contents

  1. Basic JavaScript Questions (Questions 1-25)
  2. Variables and Scope (Questions 26-35)
  3. Functions and Closures (Questions 36-50)
  4. Error Handling (Questions 51-57)
  5. Object-Oriented Programming (Questions 58-62)
  6. Web APIs & Asynchronous JavaScript (Questions 63-67)
  7. Arrays and Data Manipulation (Questions 68-80)
  8. Event Loop and Performance (Questions 81-88)
  9. DOM Manipulation and Best Practices (Questions 89-100)

Basic JavaScript Questions

1. Can we connect JavaScript Directly with Actual Database. can you give reason of it ?

a) Yes;
b) No;
c) Sometime;
d) Some Database
View Answer
    Answer: b) No

2. Which of the following is NOT a JavaScript data type?

a) String
b) Boolean
c) Float
d) Undefined
View Answer
    Answer: c) Float

3. Which symbol is used for single-line comments in JavaScript?

a) //
b) /*
c) #
d) <!--
View Answer
    Answer: a) //

4. What will typeof null return ?

a) "null"
b) "object"
c) "undefined"
d) "string"
View Answer
    Answer: b) "object"

5. How to make immutable object in JavaScript

a) final var ={name:'Anil'}
b) const user={name:'Anil'}
c) var  user={name:'Anil'}; Object.freeze(user);
c) There is no way to make immutable object
View Answer
    Answer: c) var user={name:'Anil'}; Object.freeze(obj);

6. Operators & Expressions What will 2 + “2” evaluate to ?

a) 4
b) "22"
c) NaN
d) Error
View Answer
    Answer: b) "22"

7.Which operator is used for strict equality in JavaScript ?

a) ==
b) !==
c) =
d) !=
View Answer
    Answer: b) !==

8. What does !!“false” evaluate to ?

a) true
b) false
c) undefined
d) Error
View Answer
    Answer: a) true

9. What is the result of 5 == “5” ?

a) true
b) false
View Answer
    Answer: a) true

10. What is the result of type of “5 ” === ” 5” ?

a) true
b) false
View Answer
    Answer: b) false

11. Which loop is guaranteed to execute at least once ?

a) for loop
b) while loop
c) do-while loop
d) None of the above
View Answer
    Answer: c) do-while loop

12. Output of this for loop loop

for (;;) {
  console.log("Loop");
}
a) Infinit Loop
b) Loop will not execute
c) Error
d) Only Run once
View Answer
    Answer: a) Infinit Loop

13. What will console.log(typeof NaN); print ?

a) "number"
b) "NaN"
c) "undefined"
d) "object"
View Answer
    Answer: a) "number"

14. Output of below statment

let x = null;
let y = null;
console.log(x + y);
a) null
b) object
c) 0
d) undefined
View Answer
    Answer: c) 0

15. What will console.log(typeof function(){}); return ?

a) "function"
b) "object"
c) "undefined"
d) "null"
View Answer
    Answer: a) "function"

_ 16. What will console.log(typeof function(){}(); return? _

a) "function"
b) "object"
c) "undefined"
d) "null"
View Answer
    Answer: c) "undefined"

17. What is the default return value of a function in JavaScript if no return statement is used ?

a) null
b) undefined
c) false
d) 0
View Answer
    Answer: b) undefined

18. Which type of function executes immediately after its definition ?

a) Anonymous function
b) Named function
c) IIFE (Immediately Invoked Function Expression)
d) Arrow function
View Answer
    Answer: c) IIFE

19 Output of below statement

console.log(x);
let x = 5;
a) 5
b) undefined
c) ReferenceError
d) NaN
View Answer
    Answer: c) ReferenceError

20. How do you create an object in JavaScript ?

a) let obj = {};
b) let obj = new Object();
c) Both a and b
d) None of the above
View Answer
    Answer: c) Both a and b

21 How do you access a property in an object ?

a) obj[property]
b) obj.property
c) Both a and b
d) None of the above
View Answer
    Answer: c) Both a and b

22. Which method is used to add a new element at the end of an array ?

a) push()
b) pop()
c) shift()
d) unshift()
View Answer
    Answer: a) push()

23 What will console.log([1,2,3].length); return ?

a) 2
b) 3
c) 4
d) undefined
View Answer
    Answer: b) 3

24 How do you remove first 2 element of an array?

a) pop()
b) shift()
c) unshift()
d) splice()
View Answer
    Answer: d) splice()

25 Which keyword allows block-scoped variable declarations ?

a) var
b) let
c) const
d) Both b and c
View Answer
    Answer: d) Both b and c

26 Which of the following is true about const variables?

a) Their values cannot be changed
b) They cannot be reassigned
c) They are always immutable
d) All of the above
View Answer
    Answer: b) They cannot be reassigned

27. What is the output of console.log(typeof([])); ?

a) "object"
b) "array"
c) "undefined"
d) "null"
View Answer
    Answer: a) "object"

28 What is a template literal in JavaScript ?

a) A type of array
b) A string enclosed in backticks (` `)
c) A special function
d) A new ES6 data type
View Answer
    Answer: b) A string enclosed in backticks (` `)

29. What will console.log(…”Hello”); output ?

a) "H e l l o"
b) ["H", "e", "l", "l", "o"]
c) Syntax Error
d) undefined
View Answer
    Answer: a) "H e l l o"

** 30. How do you define an arrow function ?**

a) const add = (a, b) => a + b;
b) const add = function(a, b) { return a + b; };
c) Both a and b
d) None of the above
View Answer
    Answer: a) const add = (a, b) => a + b;

31 What does the spread operator … do in JavaScript ?

a) Combines arrays
b) Expands iterable elements
C) All of the above
View Answer
    Answer: C) All of the above

** 32. What will console.log([…new Set([1, 2, 2, 3])]); return ?**

a) [1, 2, 3]
b) [1, 2, 2, 3]
c) Set {1, 2, 3}
d) {1, 2, 3}
View Answer
    Answer: a) [1, 2, 3]

33. Which statement about arrow functions is true ?

a) They do not bind this
b) They can be used as constructors
c) They have a prototype property
d) They support arguments keyword
View Answer
    Answer: a) They do not bind this

**34 Output of follow code? **

function tryFruits(...fruits) {
  console.log(...fruits);
}


tryFruits("apple", "banana", "grapes");
a) ['apple', 'banana', 'grapes']
b) {'apple', 'banana', 'grapes'}
c) 'apple 'banana grapes'
d) 'apple'
View Answer
    Answer: a) ['apple', 'banana', 'grapes']

35. What is the purpose of JavaScript Promises ?

a) Handle synchronous code
b) Handle asynchronous operations
c) Block execution until resolved
d) Replace all callbacks
View Answer
    Answer: b) Handle asynchronous operations

36. Which state is NOT valid for a Promise ?

a) Pending
b) Fulfilled
c) Rejected
d) Running
View Answer
    Answer: d) Running

37. Use of Await keyword ?

a) wait for an asynchronous operation to finish before continuing the execution
b) make promise
c) atop execution
d) all of above
View Answer
    Answer: a) wait for an asynchronous operation to finish before continuing the execution

38. Which method selects an element by ID?

a) document.getElementofId()
b) document.getElementById()
c) document.selectElementById()
d) document.selectById()
View Answer
    Answer: b) document.getElementById()

39 Which event is triggered when an input field loses focus ?

a) click
b) blur
c) focus
d) change
View Answer
    Answer: b) blur

40. Which method adds an event listener to an element ?

a) element.addEventListener()
b) element.attachEvent()
c) element.onEvent()
d) element.setEventListener()
View Answer
    Answer: a) element.addEventListener()

41. What does event.preventDefault() do ?

a) Stops the default action of an event
b) Stops event propagation
c) Prevents event from being attached
d) None of the above
View Answer
    Answer: a) Stops the default action of an event

43. What is localStorage used for?

a) Storing session data
b) Storing data persistently in the browser
c) Making API requests
d) Caching images
View Answer
    Answer: b) Storing data persistently in the browser

44 Which method converts a JavaScript object into a JSON string?

a) JSON.stringify()
b) JSON.parse()
c) toJSON()
d) parseJSON()
View Answer
    Answer: a) JSON.stringify()

45 What will console.log(parseInt(“10px”)) return ?

a) 10
b) NaN
c) "10px"
d) Error
View Answer
    Answer: a) 10

46. Which method executes a function repeatedly with a time interval ?

a) setInterval()
b) setTimeout()
c) repeat()
d) setLoop()
View Answer
    Answer: a) setInterval()

47. How do you check if a variable is an array ?

a) typeof x === "array"
b) x.isArray()
c) Array.isArray(x)
d) x instanceof Object
View Answer
    Answer: c) Array.isArray(x)

** 48. What is a closure in JavaScript?**

a) A function inside another function that has access to its parent’s scope
b) A block of code that runs automatically
c) A way to define private variables
d) Both a and c
View Answer
    Answer: d) Both a and c

49. Which of the following is true about closures ?

a) Closures have access to their own scope
b) Closures have access to their parent function's scope
c) Closures have access to global scope
d) All of the above
View Answer
    Answer: d) All of the above

50.What will this code output?

function outer() {
  let count = 0;
  return function inner() {
    count++;
    console.log(count);
  };
}
const counter = outer();
counter();
counter();
a) 1 2
b) 0 1
c) 1 1
d) Error
View Answer
    Answer: a) 1 2

51. Which statement about var and let is true ?

a) Both are function-scoped
b) var is function-scoped, let is block-scoped
c) Both are block-scoped
d) var allows redeclaration, let doesn’t
View Answer
    Answer: b) var is function-scoped, let is block-scoped

52. What will console.log(x); var x = 10; output?

a) 10
b) undefined
c) ReferenceError
d) NaN
View Answer
    Answer: b) undefined

53. Which statement is used for error handling in JavaScript ?

a) try...catch
b) throw
c) finally
d) All of the above
View Answer
    Answer: d) All of the above

54 What happens if an error occurs inside the try block ?

a) The script stops execution
b) The error is caught in the catch block
c) The script crashes
d) The error is ignored
View Answer
    Answer: b) The error is caught in the catch block

55. What will console.log(x); inside a try block with no catch or finally do?

a) Print undefined
b) Print null
c) Throw a ReferenceError
d) Nothing
View Answer
    Answer: c) Throw a ReferenceError

56. Which method is used to generate a custom error ?

a) throw new Error()
b) console.error()
c) generateError()
d) raiseError()
View Answer
    Answer: a) throw new Error()

57. What will finally do in a try-catch-finally block ?

a) Execute only if no error occurs
b) Execute only if an error occurs
c) Always execute
d) None of the above
View Answer
    Answer: c) Always execute

58. OOP (Object-Oriented Programming) in JavaScript Which keyword is used to create a class in JavaScript?

a) class
b) function
c) Class
d) new Class
View Answer
    Answer: a) class

59. What is the purpose of the constructor method in a class ?

a) To create private variables
b) To initialize object properties
c) To call another class
d) None of the above
View Answer
    Answer: b) To initialize object properties

60. Which keyword is used for inheritance in JavaScript ?

a) implements
b) extends
c) inherits
d) prototype
View Answer
    Answer: b) extends

61. Which method in a class is used to call the parent class constructor ?

a) parent()
b) super()
c) this()
d) constructor()
View Answer
    Answer: b) super()

62 Which statement about JavaScript classes is true ?

a) They support multiple inheritance
b) They are syntactic sugar over prototypes
c) They can be redeclared
d) They do not support inheritance
View Answer
    Answer: b) They are syntactic sugar over prototypes

63 Web APIs & Asynchronous JavaScript

Which API is used for making HTTP requests in JavaScript?
a) XMLHttpRequest
b) Fetch API
c) Axios
d) All of the above
View Answer
    Answer: d) All of the above

64. Which method sends a GET request using Fetch API ?

a) fetch(url)
b) fetch(url, { method: 'GET' })
c) Both a and b
d) None of the above
View Answer
    Answer: c) Both a and b

65. What does navigator.geolocation.getCurrentPosition() do ?

a) Gets user’s IP address
b) Gets user’s location
c) Opens a Google Maps page
d) None of the above
View Answer
    Answer: b) Gets user’s location

66. Which storage API stores data persistently ?

a) localStorage
b) sessionStorage
c) cookies
d) All of the above
View Answer
    Answer: a) localStorage

67. How can you set an interval in JavaScript ?

a) setTimeout()
b) setInterval()
c) setRepeat()
d) Interval()
View Answer
    Answer: b) setInterval()

68 Which method removes an element from an array ?

a) splice()
b) slice()
c) remove()
d) delete()
View Answer
    Answer: a) splice()

69. Which JavaScript engine is used in Google Chrome ?

a) SpiderMonkey
b) V8
c) Chakra
d) Nitro
View Answer
    Answer: b) V8

70. Which method converts a string into a number ?

a) parseInt()
b) Number()
c) + (unary plus)
d) All of the above
View Answer
    Answer: d) All of the above

71. Which function generates a random number between 0 and 1 ?

a) Math.random()
b) random()
c) generateRandom()
d) Math.rand()
View Answer
    Answer: a) Math.random()

72. Which of the following is a falsy value in JavaScript ?

a) "false"
b) "0"
c) undefined
d) "undefined"
View Answer
    Answer: c) undefined

73 What will console.log([] == false); return ?

a) true
b) false
c) undefined
d) Error
View Answer
    Answer: a) true

74. Which of the following is NOT a primitive data type in JavaScript ?

a) Number
b) String
c) Object
d) Symbol
View Answer
    Answer: c) Object

75 How do you deep clone an object in JavaScript ?

a) Object.assign({}, obj)
b) JSON.parse(JSON.stringify(obj))
c) obj.clone()
d) obj.copy()
View Answer
    Answer: b) JSON.parse(JSON.stringify(obj))

76. What is the output of console.log(2 + “2” - 1); ?

a) "21"
b) 21
c) "22"
d) 1
View Answer
    Answer: b) 21

77. Which method is used to filter elements from an array ?

a) map()
b) filter()
c) reduce()
d) slice()
View Answer
    Answer: b) filter()

78 Which function combines array elements into a single value ?

a) reduce()
b) map()
c) join()
d) concat()
View Answer
    Answer: a) reduce()

79. What does the following code return ?

console.log([1, 2, 3].map((num) => num * 2));
a) [2, 4, 6]
b) [1, 4, 9]
c) [1, 2, 3]
d) [2, 3, 4]
View Answer
    Answer: a) [2, 4, 6]

80. Which of the following is NOT an immutable operation?*

a) map()
b) filter()
c) splice()
d) concat()
View Answer
    Answer: c) splice()

81. What is the event loop in JavaScript ?

a) A process that handles function calls
b) A mechanism that allows async operations
c) A feature that prevents infinite loops
d) A method to execute code
View Answer
    Answer: b) A mechanism that allows async operations

82. Which of the following executes first in the event loop ?

a) setTimeout()
b) setInterval()
c) Promise.resolve().then()
d) console.log()
View Answer
    Answer: d) console.log()

83. Which queue does setTimeout() use in JavaScript ?

a) Microtask queue
b) Callback queue
c) Event loop queue
d) Execution stack
View Answer
    Answer: b) Callback queue

84. What will be the output of this code ?

console.log("A");
setTimeout(() => console.log("B"), 0);
console.log("C");
a) A B C
b) A C B
c) B A C
d) C A B
View Answer
    Answer: b) A C B

85. Which of the following is a best practice in JavaScript ?

a) Using == instead of ===
b) Avoiding global variables
c) Using var instead of let
d) Nesting loops as deep as possible
View Answer
    Answer: b) Avoiding global variables

86. What does “debouncing” do in JavaScript?

a) Delays function execution until a pause in events
b) Executes a function immediately
c) Runs a function continuously
d) None of the above
View Answer
    Answer: a) Delays function execution until a pause in events

87. What does “throttling” do?

a) Executes a function only at fixed intervals
b) Prevents a function from running
c) Removes unnecessary function calls
d) Stops event propagation
View Answer
    Answer: a) Executes a function only at fixed intervals

88. Which of the following improves JavaScript performance ?

a) Minifying JavaScript files
b) Using lazy loading
c) Avoiding unnecessary DOM manipulations
d) All of the above
View Answer
    Answer: d) All of the above

89. What is the best way to check if a variable is null or undefined ?

a) if (x == null)
b) if (typeof x === "null")
c) if (x === null || x === undefined)
d) if (x == undefined)
View Answer
    Answer: c) if (x === null || x === undefined)

90. What does document.createElement(‘div’) do ?

a) Creates and appends a div
b) Creates a div but does not append it
c) Selects an existing div
d) Deletes all div elements
View Answer
    Answer: b) Creates a div but does not append it

91. Which API is used to create animations in JavaScript ?

a) WebGL
b) requestAnimationFrame()
c) animateCSS()
d) window.setInterval()
View Answer
    Answer: b) requestAnimationFrame()

92. Which function removes whitespace from both ends of a string ?

a) trim()
b) slice()
c) removeSpace()
d) strip()
View Answer
    Answer: a) trim()

93. Which method removes the last element from an array ?

a) pop()
b) shift()
c) splice()
d) removeLast()
View Answer
    Answer: a) pop()

94. What is the output of the following code ?

console.log(myFunc);
function myFunc() {
  return "Hello";
}
A) undefined
B) ReferenceError: myFunc is not defined
C) [Function: myFunc]
D) TypeError: myFunc is not a function
View Answer
    Answer: c) ƒ myFunc() { return "Hello"; }

95. Which of the following is an example of a higher-order function ?

a) A function that returns another function
b) A function with a return type of void
c) A function that has a loop inside
d) A function that only contains if-else statements
View Answer
    Answer: a) A function that returns another function

96. Which method is used to handle asynchronous functions in JavaScript ?

a) setTimeout()
b) Promise.then()
c) async/await
d) All of the above
View Answer
    Answer: d) All of the above

97. Which of the following is NOT true about closures ?

a) A closure allows a function to retain access to variables from its outer scope.
b) Closures are created every time a function is invoked.
c) Closures help in data encapsulation.
d) Closures cannot access global variables.
View Answer
    Answer: d) Closures cannot access global variables.

98. What will be the output of the following code ?

const obj = {
  value: 42,
  getValue: () => {
    return this.value;
  },
};
console.log(obj.getValue());
a) 42
b) undefined
c) ReferenceError
d) null
View Answer
    Answer: b) undefined

99 What will be the output of the following asynchronous function ?

async function foo() {
  return "Hello";
}
console.log(foo());
a) "Hello"
b) Promise { "Hello" }
c) undefined
d) Error
View Answer
    Answer: b) Promise { "Hello" }

100. What is currying in JavaScript?

a) A technique where a function is transformed into a sequence of unary (one-argument) functions.
b) A method to execute functions asynchronously.
c) A way to cache function results for optimization.
d) A technique to convert a function into a class.
View Answer
    Answer: a) A technique where a function is transformed into a sequence of unary functions.