JavaScript

What is the purpose of the WeakMap in JavaScript? a) To store key-value pairs where the keys are objects and the values can be garbage collected if no other references exist b) To store primitive values and prevent memory leaks c) To create a map with weak references to keys d) To store values that […]

What is the output of the following code: console.log([…”Hello”]);? a) ["H", "e", "l", "l", "o"] b) "Hello" c) [["H", "e", "l", "l", "o"]] d) undefined Answer: a) ["H", "e", "l", "l", "o"] Explanation: The spread operator (…) spreads the elements of an iterable (like a string) into individual elements. When applied to the string "Hello",

How do you create a generator function in JavaScript? a) By using the function* syntax b) By using function generator() c) By using the new Generator() keyword d) By using the Promise() constructor Answer: a) By using the function* syntax Explanation: A generator function in JavaScript is defined using the function* syntax. Generator functions return

What is the Set object used for in JavaScript? a) To store unique values of any type b) To create a collection of key-value pairs c) To perform mathematical operations on arrays d) To sort the elements in an array Answer: a) To store unique values of any type Explanation: The Set object in JavaScript

What is the difference between == and === in JavaScript? a) == performs type coercion, while === checks both type and value b) === performs type coercion, while == does not c) Both perform type coercion d) There is no difference Answer: a) == performs type coercion, while === checks both type and value Explanation:

What is tail call optimization in JavaScript? a) A method where the last function call is optimized to prevent stack overflow b) A method that increases the call stack size c) A technique to sort elements faster d) A function to prevent recursion Answer: a) A method where the last function call is optimized to

What is the purpose of Reflect in JavaScript? a) To perform object operations like proxies without interception b) To create a copy of an object c) To modify the prototype of an object d) To delete properties of an object Answer: a) To perform object operations like proxies without interception Explanation: The Reflect object provides

How can you iterate over the properties of an object in JavaScript? a) for…in loop b) Object.keys() c) Object.entries() d) All of the above Answer: d) All of the above Explanation: In JavaScript, you can iterate over the properties of an object using the for…in loop, Object.keys() to get an array of keys, or Object.entries()

What is the difference between Object.freeze() and Object.seal() in JavaScript? a) Object.freeze() makes an object immutable, while Object.seal() prevents new properties from being added b) Object.freeze() allows new properties, while Object.seal() does not c) Object.seal() removes properties, while Object.freeze() adds properties d) They are identical Answer: a) Object.freeze() makes an object immutable, while Object.seal() prevents

What is the purpose of the Proxy object in JavaScript? a) To intercept and redefine fundamental operations on objects b) To clone an object c) To create a deep copy of an object d) To delete properties from an object Answer: a) To intercept and redefine fundamental operations on objects Explanation: The Proxy object allows

What is the output of the following code: console.log([1, 2, 3] instanceof Array);? a) true b) false c) undefined d) null Answer: a) true Explanation: The instanceof operator checks if an object is an instance of a specific class. In this case, [1, 2, 3] is an array, so the result of [1, 2, 3]

How does the await keyword work in JavaScript? a) It pauses the execution of an async function until the promise is resolved or rejected b) It returns a promise c) It forces a function to execute synchronously d) It runs a function immediately Answer: a) It pauses the execution of an async function until the

What is the purpose of the async keyword in JavaScript? a) To define a function that always returns a promise b) To execute synchronous code c) To delay the execution of a function d) To wait for an event to happen Answer: a) To define a function that always returns a promise Explanation: The async

What is the purpose of Promise.all() in JavaScript? a) To execute multiple promises in parallel and wait until all are resolved b) To execute a promise multiple times c) To create a new promise that resolves to the first settled promise d) To cancel all promises Answer: a) To execute multiple promises in parallel and

How can you create a private variable in JavaScript? a) Using closures b) Using var outside a function c) Using this keyword d) Using let outside a function Answer: a) Using closures Explanation: Private variables in JavaScript can be created using closures. A closure is an inner function that has access to the outer function’s

What is an IIFE (Immediately Invoked Function Expression) in JavaScript? a) A function that runs as soon as it is defined b) A function that runs at the end of the script c) A function that is called multiple times d) A function that waits for a user event Answer: a) A function that runs

How do you create a deep copy of an object in JavaScript? a) JSON.parse(JSON.stringify(object)) b) Object.assign() c) object.slice() d) Array.map() Answer: a) JSON.parse(JSON.stringify(object)) Explanation: To create a deep copy of an object in JavaScript, you can use JSON.parse(JSON.stringify(object)). This method works by first converting the object into a JSON string and then parsing it back

What is the output of the following code: console.log(typeof function(){});? a) "object" b) "function" c) "undefined" d) "object function" Answer: b) "function" Explanation: In JavaScript, the typeof operator returns "function" when applied to a function. Functions in JavaScript are a special type of object, but typeof identifies them as "function". Reference: JavaScript MCQ (Multiple-Choice Questions)

What is the difference between call() and apply() methods in JavaScript? a) call() takes individual arguments, while apply() takes an array of arguments b) call() is used for synchronous code, while apply() is used for asynchronous code c) call() executes a function immediately, while apply() creates a promise d) Both are identical Answer: a) call()

What is the purpose of the Object.assign() method in JavaScript? a) To merge multiple objects into one b) To copy properties from one object to another c) To compare two objects d) To delete properties from an object Answer: b) To copy properties from one object to another Explanation: The Object.assign() method is used to

What is the output of console.log(2 ** 3);? a) 6 b) 8 c) 9 d) 4 Answer: b) 8 Explanation: The ** operator in JavaScript is used for exponentiation. The expression 2 ** 3 means “2 raised to the power of 3”, which equals 8. Reference: JavaScript MCQ (Multiple-Choice Questions)

What will the console.log([1, 2, 3] + [4, 5, 6]) statement output? a) "1,2,34,5,6" b) "1,2,3,4,5,6" c) [1,2,3,4,5,6] d) NaN Answer: a) "1,2,34,5,6" Explanation: In JavaScript, when you use the + operator with arrays, it converts the arrays to strings and concatenates them. Therefore, the result is "1,2,34,5,6", a concatenated string of the two arrays.

What does the finally() method do in a promise? a) Executes a block of code after the promise is settled, regardless of the result b) Executes a block of code only if the promise is resolved c) Executes a block of code only if the promise is rejected d) Cancels the promise Answer: a) Executes

What is a promise in JavaScript? a) An object representing the eventual completion or failure of an asynchronous operation b) A function that always resolves c) A callback function d) A synchronous operation Answer: a) An object representing the eventual completion or failure of an asynchronous operation Explanation: A promise in JavaScript is an object

What will the console.log([] == false) statement output? a) true b) false c) undefined d) NaN Answer: a) true Explanation: When comparing an empty array [] with false using the loose equality operator (==), JavaScript converts the array to a boolean and compares it with false, resulting in true. Reference: JavaScript MCQ (Multiple-Choice Questions)