Question 1
What will be the output of the following code?
console.log(typeof null);
A) “null”
B) “undefined”
C) “object”
D) “number”
Answer: C) “object”
Explanation: In JavaScript, typeof null returns “object”. This is a well-known bug in JavaScript that exists for legacy reasons.
Question 2
What is the output of the following code?
console.log(0 == ‘0’);
A) true
B) false
C) Error
D) Undefined
Answer: A) true
Explanation: The == operator performs type coercion, so 0 and ‘0’ are considered equal.
Question 3
Which method can be used to convert a JSON text into a JavaScript object? A) JSON.stringify()
B) JSON.parse()
C) JSON.toString()
D) JSON.object()
Answer: B) JSON.parse()
Explanation: JSON.parse() converts a JSON string into a JavaScript object.
Question 4
What will the following code output?
console.log(1 + ‘2’ + ‘2’);
A) “32”
B) “122”
C) 5
D) “14”
Answer: B) “122”
Explanation: JavaScript concatenates strings from left to right, so 1 + ‘2’ becomes ’12’ and ’12’ + ‘2’ becomes ‘122’.
Question 5
What will be the output of the following code?
console.log(‘2’ – 1);
A) “21”
B) “1”
C) 1
D) NaN
Answer: C) 1
Explanation: The – operator forces JavaScript to coerce ‘2’ into a number, so the result is 2 – 1 which is 1.
Question 6
Which of the following is not a primitive data type in JavaScript? A) String
B) Number
C) Boolean
D) Object
Answer: D) Object
Explanation: Object is not a primitive data type. The primitive data types are String, Number, Boolean, Null, Undefined, and Symbol.
Question 7
What is the result of the following code?
var foo = function() {
return bar();
var bar = function() {
return 3;
};
function bar() {
return 8;
}
}
console.log(foo());
A) 3
B) 8
C) Error
D) Undefined
Answer: B) 8
Explanation: Function declarations are hoisted above variable declarations and assignments. The function bar defined by function bar() { return 8; } is hoisted.
Question 8
What will be the output of the following code?
let x = {};
let y = x;
console.log(x === y);
y.z = 1;
console.log(x.z);
A) true, undefined
B) false, 1
C) true, 1
D) false, undefined
Answer: C) true, 1
Explanation: Objects are compared by reference, so x and y are referencing the same object. Therefore, x === y is true and adding a property to y reflects in x.
Question 9
What is the result of the following code?
console.log([] + []);
A) “”
B) []
C) “[]”
D) Undefined
Answer: A) “”
Explanation: When you use the + operator with arrays, JavaScript converts them to strings. An empty array becomes an empty string, so [] + [] results in “”.
Question 10
Which of the following methods is used to add elements to the end of an array? A) push()
B) pop()
C) shift()
D) unshift()
Answer: A) push()
Explanation: push() adds one or more elements to the end of an array and returns the new length of the array.
Question 11
What is the output of the following code?
console.log(typeof NaN);
A) “number”
B) “NaN”
C) “undefined”
D) “object”
Answer: A) “number”
Explanation: NaN is a special value representing “Not-a-Number” in JavaScript, but its type is still “number”.
Question 12
What will be the output of the following code?
console.log(‘5’ – ‘2’);
A) “52”
B) 3
C) “3”
D) NaN
Answer: B) 3
Explanation: The – operator converts both strings to numbers and then performs the subtraction.
Question 13
Which of the following statements will throw an error? A) let x = 10;
B) var y = 20;
C) const z = 30;
D) x = 40;
Answer: D) x = 40;
Explanation: The statement x = 40; will throw an error if x was declared with const, but as shown it doesn’t declare a variable in this context. However, this is a poorly worded question; all shown variable declarations are correct.
Question 14
What will the following code output?
console.log(1 + true);
A) “1true”
B) 2
C) 1
D) NaN
Answer: B) 2
Explanation: true is converted to 1, so 1 + true becomes 1 + 1, which is 2.
Question 15
What is the purpose of the bind() method in JavaScript? A) It creates a new function that, when called, has its this keyword set to the provided value.
B) It calls a function with a given this value and arguments provided individually.
C) It calls a function with a given this value and arguments provided as an array.
D) It adds properties to an object.
Answer: A) It creates a new function that, when called, has its this keyword set to the provided value.
Explanation: The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
Question 16
Which of the following is not a looping structure in JavaScript? A) for
B) while
C) do-while
D) foreach
Answer: D) foreach
Explanation: foreach is not a native looping structure in JavaScript; it is a method of arrays. The native loops are for, while, and do-while.
Question 17
What will be the output of the following code?
console.log(‘5’ + 5);
A) 10
B) “55”
C) 5
D) NaN
Answer: B) “55”
Explanation: The + operator concatenates the string ‘5’ with the number 5, resulting in the string “55”.
Question 18
What is the output of the following code?
console.log(0.1 + 0.2 === 0.3);
A) true
B) false
C) Undefined
D) Error
Answer: B) false
Explanation: Due to floating-point precision, 0.1 + 0.2 does not exactly equal 0.3. It equals 0.30000000000000004.
Question 19
What does the Array.prototype.map() method do? A) It mutates the original array.
B) It executes a provided function once for each array element.
C) It creates a new array with the results of calling a provided function on every element in the calling array.
D) It sorts the array elements.
Answer: C) It creates a new array with the results of calling a provided function on every element in the calling array.
Explanation: The map() method creates a new array populated with the results of calling a provided function on every element in the calling array without modifying the original array.
Question 20
What is the result of the following code?
console.log([10, 20, 30].reduce((acc, val) => acc + val, 0));
A) “102030”
B) 60
C) “60”
D) [10, 20, 30]
Answer: B) 60
Explanation: The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value. Here, it sums the values, resulting in 60.
Question 21
Which statement is used to stop a loop in JavaScript? A) break
B) stop
C) halt
D) exit
Answer: A) break
Explanation: The break statement is used to terminate the current loop, switch, or label statement.
Question 22
What will be the output of the following code?
console.log(typeof undefined);
A) “null”
B) “undefined”
C) “object”
D) “number”
Answer: B) “undefined”
Explanation: typeof undefined returns “undefined” as undefined is a primitive value in JavaScript.
Question 23
Which method is used to add one or more elements to the beginning of an array? A) push()
B) pop()
C) shift()
D) unshift()
Answer: D) unshift()
Explanation: The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array.
Question 24
What will be the output of the following code?
console.log(Boolean(”));
A) true
B) false
C) “false”
D) undefined
Answer: B) false
Explanation: An empty string ” is a falsy value in JavaScript, so Boolean(”) returns false.
Question 25
What does the typeof operator do? A) It returns the type of a variable.
B) It returns the value of a variable.
C) It checks if a variable is defined.
D) It checks if a variable is an array.
Answer: A) It returns the type of a variable.
Explanation: The typeof operator returns a string indicating the type of the unevaluated operand.
Question 26
What will the following code output?
let a = [1, 2, 3];
let b = a;
b.push(4);
console.log(a);
A) [1, 2, 3]
B) [1, 2, 3, 4]
C) [1, 2, 3, 4, 4]
D) Error
Answer: B) [1, 2, 3, 4]
Explanation: Both a and b reference the same array, so changes to b are reflected in a.
Question 27
What is the output of the following code?
console.log(1 + “2” + 3);
A) “6”
B) “123”
C) 123
D) “15”
Answer: B) “123”
Explanation: JavaScript performs string concatenation from left to right, so 1 + “2” becomes “12” and “12” + 3 becomes “123”.
Question 28
Which method is used to remove the first element from an array? A) push()
B) pop()
C) shift()
D) unshift()
Answer: C) shift()
Explanation: The shift() method removes the first element from an array and returns that removed element.
Question 29
What will be the output of the following code?
console.log(typeof NaN);
A) “NaN”
B) “undefined”
C) “object”
D) “number”
Answer: D) “number”
Explanation: NaN stands for “Not-a-Number” but its type is “number” in JavaScript.
Question 30
Which statement is used to handle exceptions in JavaScript? A) handle
B) catch
C) try
D) except
Answer: C) try
Explanation: The try…catch statement marks a block of statements to try and specifies a response should an exception be thrown.
Question 31
What will be the output of the following code?
console.log(10 === ’10’);
A) true
B) false
C) “true”
D) “false”
Answer: B) false
Explanation: The === operator checks for both value and type equality, so 10 (number) is not equal to ’10’ (string).
Question 32
Which of the following methods is used to join two or more arrays? A) concat()
B) join()
C) merge()
D) link()
Answer: A) concat()
Explanation: The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.
Question 33
What is the output of the following code?
console.log([1, 2, 3] instanceof Array);
A) true
B) false
C) “true”
D) “false”
Answer: A) true
Explanation: The instanceof operator tests whether an object has in its prototype chain the prototype property of a constructor. Arrays are instances of the Array constructor.
Question 34
What will be the output of the following code?
console.log(‘Hello’.toUpperCase());
A) HELLO
B) hello
C) “HELLO”
D) “hello”
Answer: C) “HELLO”
Explanation: The toUpperCase() method returns the calling string value converted to uppercase.
Question 35
What is the purpose of the Object.keys() method? A) To return an array of the object’s property names.
B) To return an array of the object’s values.
C) To create a new object with the specified prototype.
D) To merge two or more objects.
Answer: A) To return an array of the object’s property names.
Explanation: The Object.keys() method returns an array of a given object’s own enumerable property names.
Question 36
What will the following code output?
console.log(‘5’ – 3);
A) 2
B) “2”
C) “53”
D) NaN
Answer: A) 2
Explanation: The – operator converts the string ‘5’ to a number, resulting in 5 – 3, which equals 2.
Question 37
Which method can be used to create a new array with all elements that pass a test implemented by the provided function? A) map()
B) filter()
C) reduce()
D) every()
Answer: B) filter()
Explanation: The filter() method creates a new array with all elements that pass the test implemented by the provided function.
Question 38
What is the result of the following code?
let a = 5;
let b = 10;
[a, b] = [b, a];
console.log(a, b);
A) 5, 10
B) 10, 5
C) undefined
D) Error
Answer: B) 10, 5
Explanation: The array destructuring assignment swaps the values of a and b.
Question 39
Which of the following is not a primitive data type in JavaScript? A) String
B) Number
C) Boolean
D) Array
Answer: D) Array
Explanation: Arrays are not primitive data types; they are objects.
Question 40
What will be the output of the following code?
console.log(typeof function() {});
A) “object”
B) “function”
C) “undefined”
D) “boolean”
Answer: B) “function”
Explanation: The typeof operator returns “function” for functions.
Question 41
Which method removes the last element from an array and returns that element? A) pop()
B) push()
C) shift()
D) unshift()
Answer: A) pop()
Explanation: The pop() method removes the last element from an array and returns that element.
Question 42
What is the result of the following code?
console.log(’10’ * 2);
A) 20
B) “102”
C) NaN
D) “20”
Answer: A) 20
Explanation: The * operator converts the string ’10’ to a number and performs the multiplication, resulting in 20.
Question 43
Which operator is used to assign a value to a variable? A) *
B) =
C) ==
D) ===
Answer: B) =
Explanation: The = operator is used to assign a value to a variable.
Question 44
What will be the output of the following code?
console.log([1, 2, 3].length);
A) 3
B) 2
C) 1
D) undefined
Answer: A) 3
Explanation: The length property of an array returns the number of elements in that array.
Question 45
Which method is used to remove whitespace from both ends of a string? A) slice()
B) substr()
C) trim()
D) strip()
Answer: C) trim()
Explanation: The trim() method removes whitespace from both ends of a string.
Question 46
What will be the output of the following code?
console.log([1, 2, 3].map(x => x * x));
A) [1, 4, 9]
B) [1, 2, 3]
C) [1, 8, 27]
D) [2, 3, 4]
Answer: A) [1, 4, 9]
Explanation: The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.
Question 47
Which method is used to create a shallow copy of a portion of an array into a new array object? A) copy()
B) slice()
C) splice()
D) concat()
Answer: B) slice()
Explanation: The slice() method returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included).
Question 48
What is the result of the following code?
let a = 1;
let b = 2;
let c = a++;
console.log(a, b, c);
A) 2, 2, 2
B) 2, 2, 1
C) 1, 2, 2
D) 1, 2, 1
Answer: B) 2, 2, 1
Explanation: The a++ operator increments a but returns the original value of a, which is 1. Therefore, c is 1, and a is incremented to 2.
Question 49
Which of the following is a correct way to create a new object in JavaScript? A) var obj = {};
B) var obj = new Object();
C) var obj = Object.create(null);
D) All of the above
Answer: D) All of the above
Explanation: All the methods shown are correct ways to create a new object in JavaScript.
Question 50
What is the result of the following code?
console.log(’10’ – ‘4’);
A) 6
B) “104”
C) NaN
D) “6”
Answer: A) 6
Explanation: The – operator converts both strings to numbers and then performs the subtraction, resulting in 6.
