1. What is the output of the following code?
console.log(typeof null);
A) “object”
B) “null”
C) “undefined”
D) “boolean”
Answer: A) “object”
2. How can you create an immutable object in JavaScript?
A) Object.freeze(obj)
B) Object.seal(obj)
C) Object.preventExtensions(obj)
D) Object.immutable(obj)
Answer: A) Object.freeze(obj)
3. What does the reduce method do?
A) Iterates over an array and returns a new array
B) Iterates over an array and returns a single value
C) Iterates over an object and returns an array
D) Iterates over an array and returns a boolean
Answer: B) Iterates over an array and returns a single value
4. Which of the following is not a valid way to create a new array?
A) new Array()
B) Array.of()
C) Array.create()
D) []
Answer: C) Array.create()
5. What will the following code output?
console.log([1, 2, 3] + [4, 5, 6]);
A) “1,2,34,5,6”
B) “1,2,3,4,5,6”
C) [1, 2, 3, 4, 5, 6]
D) “1,2,34,5,6”
Answer: A) “1,2,34,5,6”
6. Which method is used to merge two or more arrays?
A) Array.concat()
B) Array.merge()
C) Array.join()
D) Array.add()
Answer: A) Array.concat()
7. What will the following code output?
let a = 5;
let b = a++;
console.log(a, b);
A) 5, 5
B) 6, 5
C) 5, 6
D) 6, 6
Answer: B) 6, 5
8. Which of the following is a JavaScript data type?
A) Integer
B) Float
C) Number
D) Double
Answer: C) Number
9. What does the splice method do?
A) Adds elements to the end of an array
B) Removes elements from an array and optionally inserts new elements in their place
C) Sorts the elements of an array
D) Removes the first element from an array
Answer: B) Removes elements from an array and optionally inserts new elements in their place
10. What will the following code output?
console.log(“Hello” instanceof String);
A) true
B) false
C) TypeError
D) undefined
Answer: B) false
11. Which of the following will NOT create an empty object?
A) {}
B) new Object()
C) Object.create(null)
D) Object.empty()
Answer: D) Object.empty()
12. How can you determine if a variable is an array?
A) typeof variable === ‘array’
B) variable instanceof Array
C) Array.isArray(variable)
D) B and C
Answer: D) B and C
13. What is the output of the following code?
console.log(0.1 + 0.2 === 0.3);
A) true
B) false
C) TypeError
D) undefined
Answer: B) false
14. Which statement is used to handle exceptions in JavaScript?
A) try…catch
B) try…except
C) catch…try
D) try…finally
Answer: A) try…catch
15. What does the map method return?
A) A single value
B) A new array with the results of calling a provided function on every element
C) The first element that matches a condition
D) A boolean value
Answer: B) A new array with the results of calling a provided function on every element
16. How can you create a new promise?
A) new Promise((resolve, reject) => {})
B) Promise.create((resolve, reject) => {})
C) new Promise.create((resolve, reject) => {})
D) Promise((resolve, reject) => {})
Answer: A) new Promise((resolve, reject) => {})
17. What is the purpose of the super keyword in JavaScript?
A) To refer to the parent class constructor
B) To call a superclass method
C) To create a new object
D) To define a new class
Answer: A) To refer to the parent class constructor
18. Which method is used to return the keys of an object?
A) Object.values()
B) Object.keys()
C) Object.entries()
D) Object.getKeys()
Answer: B) Object.keys()
19. What does the finally block do?
A) It executes if no error occurs in the try block
B) It executes only if an error occurs in the try block
C) It executes regardless of whether an error occurs or not
D) It catches errors
Answer: C) It executes regardless of whether an error occurs or not
20. What is the output of the following code?
console.log(typeof NaN);
A) “number”
B) “NaN”
C) “undefined”
D) “object”
Answer: A) “number”
21. Which statement is true about the for…in loop?
A) It iterates over all enumerable properties of an array
B) It iterates over all enumerable properties of an object
C) It iterates over the values of an array
D) It iterates over the values of an object
Answer: B) It iterates over all enumerable properties of an object
22. What is the output of the following code?
let x = 10;
if (x) {
console.log(“x is true”);
}
A) “x is true”
B) Nothing
C) “x is false”
D) TypeError
Answer: A) “x is true”
23. Which of the following is not a primitive data type in JavaScript?
A) String
B) Number
C) Boolean
D) Object
Answer: D) Object
24. What will the following code output?
console.log(2 ** 3);
A) 6
B) 8
C) 9
D) 12
Answer: B) 8
25. Which method removes the last element from an array?
A) shift()
B) unshift()
C) pop()
D) push()
Answer: C) pop()
26. What is the output of the following code?
console.log(‘2’ + 2);
A) 4
B) “22”
C) “4”
D) TypeError
Answer: B) “22”
27. How can you make a shallow copy of an array?
A) Array.copy()
B) Array.slice()
C) Array.clone()
D) Array.duplicate()
Answer: B) Array.slice()
28. What will the following code output?
let obj = { a: 1, b: 2 };
console.log(‘a’ in obj);
A) true
B) false
C) “a”
D) TypeError
Answer: A) true
29. What is the result of the following expression?
null == undefined;
A) true
B) false
C) TypeError
D) undefined
Answer: A) true
30. Which of the following methods returns a promise that resolves after all of the promises in the iterable have resolved?
A) Promise.all()
B) Promise.race()
C) Promise.resolve()
D) Promise.any()
Answer: A) Promise.all()
31. What will the following code output?
console.log(typeof function() {});
A) “object”
B) “function”
C) “undefined”
D) “string”
Answer: B) “function”
32. What does the Object.assign() method do?
A) Copies the values of all enumerable own properties from one or more source objects to a target object
B) Creates a new object with the specified prototype object and properties
C) Seals an object, preventing new properties from being added and marking all existing properties as non-configurable
D) Freezes an object, making it immutable
Answer: A) Copies the values of all enumerable own properties from one or more source objects to a target object
33. What is the purpose of the Symbol type?
A) To create unique identifiers for object properties
B) To create new primitive data types
C) To create large integers
D) To create non-enumerable properties
Answer: A) To create unique identifiers for object properties
34. What will the following code output?
let [a, b] = [1, 2];
[a, b] = [b, a];
console.log(a, b);
A) 1, 2
B) 2, 1
C) undefined, undefined
D) 1, undefined
Answer: B) 2, 1
35. What does the Array.prototype.flat() method do?
A) Flattens an array of arrays into a single array
B) Maps and flattens an array
C) Filters and flattens an array
D) Reduces and flattens an array
Answer: A) Flattens an array of arrays into a single array
36. What is the result of the following code?
console.log(1 == ‘1’);
console.log(1 === ‘1’);
A) true, true
B) false, false
C) true, false
D) false, true
Answer: C) true, false
37. How can you make an object immutable?
A) Object.preventExtensions(obj)
B) Object.freeze(obj)
C) Object.seal(obj)
D) Object.lock(obj)
Answer: B) Object.freeze(obj)
38. What is the output of the following code?
let arr = [1, 2, 3];
console.log(arr.includes(2));
A) true
B) false
C) TypeError
D) undefined
Answer: A) true
39. What is the output of the following code?
console.log(2 + ‘2’ – 2);
A) 22
B) 2
C) 20
D) “22”
Answer: C) 20
40. What is the purpose of the Proxy object?
A) To intercept and redefine fundamental operations for an object
B) To create new objects with dynamic behavior
C) To clone objects
D) To create immutable objects
Answer: A) To intercept and redefine fundamental operations for an object
41. What will the following code output?
console.log([…’hello’]);
A) [“h”, “e”, “l”, “l”, “o”]
B) [“hello”]
C) [“h”, “e”, “l”, “l”]
D) [“h”, “e”, “l”, “o”]
Answer: A) [“h”, “e”, “l”, “l”, “o”]
42. Which method creates a new, shallow-copied array instance from an array-like or iterable object?
A) Array.of()
B) Array.from()
C) Array.create()
D) Array.new()
Answer: B) Array.from()
43. What will the following code output?
console.log(typeof NaN === ‘number’);
A) true
B) false
C) undefined
D) TypeError
Answer: A) true
44. What does the Object.create() method do?
A) Creates a new object with the specified prototype object and properties
B) Creates a new empty object
C) Copies all properties from one object to another
D) Freezes an object
Answer: A) Creates a new object with the specified prototype object and properties
45. What is the output of the following code?
console.log(‘5’ – 3);
A) “2”
B) 2
C) “53”
D) TypeError
Answer: B) 2
46. Which method adds one or more elements to the end of an array?
A) push()
B) pop()
C) shift()
D) unshift()
Answer: A) push()
47. What is the purpose of the async keyword in JavaScript?
A) To define an asynchronous function that returns a promise
B) To pause the execution of a function
C) To handle synchronous code
D) To create a callback function
Answer: A) To define an asynchronous function that returns a promise
48. What will the following code output?
console.log([1, 2, 3].map(x => x * 2));
A) [1, 2, 3]
B) [2, 4, 6]
C) [3, 6, 9]
D) TypeError
Answer: B) [2, 4, 6]
49. What is the purpose of the finally block in a try…catch statement?
A) To execute code after the try block, regardless of the result
B) To handle errors that are not caught
C) To throw new errors
D) To define asynchronous code
Answer: A) To execute code after the try block, regardless of the result
50. What does the reduce method return when applied to an empty array with no initial value?
A) undefined
B) null
C) 0
D) TypeError
Answer: D) TypeError
