ReactJS Interview Prep - JavaScript Fundamentals (Part 6)

Question 1

What is an 'Iterable' in JavaScript?

Accepted Answer

Correct Answer

Answer

Any variable declared using the `let` keyword.

Answer

An object that implements the `[Symbol.iterator]()` method, defining how it should be iterated over.

Answer

An object that contains only primitive data types.

Answer

A strictly numerical array used for high-performance loops.

Question 2

What is 'Tree Shaking' in modern JavaScript tooling?

Accepted Answer

Correct Answer

Answer

The process of reorganizing DOM nodes to improve rendering performance.

Answer

A dead-code elimination process used by bundlers (like Webpack or Rollup) to remove unused exports from the final bundle.

Answer

An algorithm used by the garbage collector to clear deep object prototypes.

Answer

A security procedure that removes potentially malicious scripts before executing.

Question 3

How do you construct a deeply nested copy of an object (Deep Clone) purely using native JavaScript APIs?

Accepted Answer

Correct Answer

Answer

`Object.assign({}, originalObject)`

Answer

`{ ...originalObject }`

Answer

`structuredClone(originalObject)`

Answer

`Object.create(originalObject)`

Question 4

What is the result of applying `Object.is(NaN, NaN)` compared to `NaN === NaN`?

Accepted Answer

Correct Answer

Answer

Both evaluate to `false`.

Answer

Both evaluate to `true`.

Answer

`Object.is(NaN, NaN)` is `true`, while `NaN === NaN` is `false`.

Answer

`Object.is(NaN, NaN)` is `false`, while `NaN === NaN` is `true`.

Question 5

What is the purpose of the `BigInt` data type?

Accepted Answer

Correct Answer

Answer

To increase the precision of floating-point numbers during complex mathematical operations.

Answer

To safely represent and manipulate integers larger than `Number.MAX_SAFE_INTEGER`.

Answer

To provide a strongly-typed integer format that prevents automatic type coercion.

Answer

To efficiently store large binary data blobs directly in memory.

Question 6

What is 'Metaprogramming' in JavaScript?

Accepted Answer

Correct Answer

Answer

Writing scripts that automatically compile into HTML and CSS.

Answer

The ability of a program to read, generate, analyze, or transform other programs, and even modify itself while running (e.g., using Proxies or Reflect).

Answer

Structuring an application using only Pure Functions.

Answer

The practice of writing code directly inside the browser's developer console.

Question 7

What does the `Array.prototype.at()` method do?

Accepted Answer

Correct Answer

Answer

It returns a boolean indicating whether an element exists at a given index.

Answer

It inserts an element into the array at a specific index.

Answer

It returns the item at a given index, supporting negative integers to count back from the end of the array.

Answer

It permanently deletes an element at the specified index.

Question 8

What is a 'Tail Call Optimization' (TCO)?

Accepted Answer

Correct Answer

Answer

A feature that automatically removes the last element of an array to save memory.

Answer

An engine optimization that allows recursive functions to execute without growing the call stack, preventing stack overflow errors.

Answer

A minification process that removes trailing whitespaces from the final build file.

Answer

An automated testing procedure that verifies the final return statement of a module.

Question 9

Which of the following is true about JavaScript's `Date` object?

Accepted Answer

Correct Answer

Answer

It natively supports strict timezone manipulation without external libraries.

Answer

It tracks time based on the number of milliseconds elapsed since January 1, 1970 UTC.

Answer

Months in the `Date` object are 1-indexed (1 = January, 12 = December).

Answer

It uses a 64-bit integer format exclusively, making it immune to the Year 2038 problem.

Question 10

What is the behavior of the `Object.fromEntries()` method?

Accepted Answer

Correct Answer

Answer

It converts an object into a Map.

Answer

It transforms an array of key-value pairs (like `[['key', 'value']]`) back into a fully formed object.

Answer

It deeply freezes an object, preventing iteration.

Answer

It retrieves the prototype properties of a given object.

Question 11

What does a 'Generator' function return when initially called?

Accepted Answer

Correct Answer

Answer

The first yielded value.

Answer

The final returned value.

Answer

An Iterator object.

Answer

A Promise.

Question 12

How do you detect if a script is running on the client (browser) or server (Node.js)?

Accepted Answer

Correct Answer

Answer

By checking if `typeof window !== 'undefined'`.

Answer

By checking if `navigator.onLine === true`.

Answer

By checking if `process.env.CLIENT === true`.

Answer

By evaluating `document.isReady`.

Question 13

What does the `queueMicrotask()` function do?

Accepted Answer

Correct Answer

Answer

It pauses execution of the current function for one millisecond.

Answer

It places a function in the microtask queue to be executed immediately after the currently executing script and before control is returned to the event loop.

Answer

It forces heavy computations into a background Web Worker.

Answer

It manages the state queue in React components.

Question 14

What is the primary characteristic of an 'Immutable' data structure?

Accepted Answer

Correct Answer

Answer

It is encrypted upon creation, hiding its properties.

Answer

Its state cannot be modified after it is created. Any changes result in the creation of a completely new data structure.

Answer

It can only store primitive strings and numbers.

Answer

It is permanently pinned to the global scope to prevent garbage collection.

Question 15

What does the `String.prototype.matchAll()` method return?

Accepted Answer

Correct Answer

Answer

A boolean indicating if the string matches the regex.

Answer

A single string representing the first match.

Answer

An iterator containing all results matching a string against a regular expression, including capturing groups.

Answer

An integer indicating the total number of matches.

Question 16

What is the difference between `Math.max()` and `Math.max.apply()`?

Accepted Answer

Correct Answer

Answer

`Math.max()` can accept an array directly, whereas `.apply()` requires individual arguments.

Answer

`Math.max()` takes a comma-separated list of arguments. Using `.apply(null, arr)` allows you to pass an array of numbers to it.

Answer

There is no difference; both require arrays.

Answer

`.apply()` is an asynchronous version of `Math.max()`.

Question 17

How does JavaScript manage block-scoped variables in memory?

Accepted Answer

Correct Answer

Answer

They are appended to the global object.

Answer

They are managed via a 'Lexical Environment', which creates a new declarative environment record for every block.

Answer

They are converted into global variables during the minification process.

Answer

They are strictly stored on the hard drive using IndexedDB.

Question 18

What is the purpose of the `btoa()` and `atob()` window methods?

Accepted Answer

Correct Answer

Answer

They convert strings to ArrayBuffers and vice-versa.

Answer

They compress and decompress large text files.

Answer

They encode and decode strings in Base64 format.

Answer

They securely hash passwords using the SHA-256 algorithm.

Question 19

What does it mean when an object property is 'configurable'?

Accepted Answer

Correct Answer

Answer

It can be iterated over using a `for...in` loop.

Answer

Its value can be changed using standard assignment.

Answer

The property can be deleted from the object, and its attributes (like enumerability or writability) can be modified.

Answer

It can be serialized into a JSON string.

Question 20

Which feature enables you to export multiple values from a module, and import them using their exact names?

Accepted Answer

Correct Answer

Answer

Default Exports

Answer

Named Exports

Answer

Anonymous Exports

Answer

Destructured Exports

Question 21

What is the function of the `crypto.getRandomValues()` method?

Accepted Answer

Correct Answer

Answer

To randomly sort an array of objects.

Answer

To generate cryptographically strong random values.

Answer

To create a fake mock backend API for testing.

Answer

To evaluate unpredictable math logic equations.

Question 22

What does `void 0` evaluate to in JavaScript?

Accepted Answer

Correct Answer

Answer

`null`

Answer

`0`

Answer

`undefined`

Answer

It throws a SyntaxError.

Question 23

What happens if you define a function inside a `for` loop and assign it to an event listener without using a closure or block-scoped variable?

Accepted Answer

Correct Answer

Answer

The function executes perfectly, isolating the loop variable for each listener.

Answer

The function fails to compile due to a syntax error.

Answer

Every event listener will reference the final value of the loop variable after the loop has completed.

Answer

The event listener automatically disconnects once the loop finishes.

Question 24

Which of the following is true regarding 'Web Components' (Custom Elements)?

Accepted Answer

Correct Answer

Answer

They are a specialized feature only available in React.

Answer

They require a robust build system like Webpack to function.

Answer

They allow developers to create reusable, encapsulated HTML tags natively supported by modern browsers.

Answer

They are exclusively used to style elements using CSS modules.

Question 25

Congratulations, you have mastered JavaScript Fundamentals! What is the primary benefit of mastering vanilla JavaScript before learning React?

Accepted Answer

Correct Answer

Answer

React removes the need for JavaScript entirely.

Answer

It ensures you can memorize React hooks faster.

Answer

React is 'Just JavaScript'. A strong grasp of closures, `this`, array methods, and async logic drastically reduces the learning curve and debugging time in React.

Answer

Vanilla JavaScript guarantees your React code will never have memory leaks.

ReactJS Interview Prep - JavaScript Fundamentals (Part 6)

Academy

ReactJS Interview Prep - JavaScript Fundamentals (Part 6)