ReactJS Interview Prep - JavaScript Fundamentals (Part 5)

Question 1

Which built-in JavaScript object allows you to read the contents of files selected by a user (via an `<input type="file">`) asynchronously?

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Correct Answer

Answer

FileSystem API

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StreamReader

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FileReader

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BlobManager

Question 2

What does the `navigator.onLine` property represent?

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A boolean indicating whether the server is currently accepting WebSocket connections.

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A boolean indicating whether the browser is currently connected to a network (has internet connectivity).

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A string detailing the current connection speed (e.g., '4g', '3g').

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A method that pings the server to verify API health.

Question 3

How do you check if a variable `arr` is an Array in modern JavaScript?

Accepted Answer

Correct Answer

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`typeof arr === 'array'`

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`arr instanceof Array`

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`Array.isArray(arr)`

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Both `typeof arr === 'array'` and `Array.isArray(arr)` work perfectly.

Question 4

What does the `String.prototype.padStart()` method do?

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It removes whitespace from the start of a string.

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It pads the current string with another string (multiple times, if needed) until the resulting string reaches the given length.

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It automatically indents HTML string elements to match a specified tab size.

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It aligns the string to the right inside a DOM text node.

Question 5

What does 'Destructuring Assignment' allow you to do in JavaScript?

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Permanently delete variables from memory to prevent memory leaks.

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Unpack values from arrays, or properties from objects, into distinct, individual variables.

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Merge multiple complex objects into a single flattened object.

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Convert an object into a serialized JSON string.

Question 6

What happens if a module imports itself directly or indirectly in ES6?

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It causes an immediate 'Maximum call stack size exceeded' error.

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The browser freezes and crashes.

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ES6 handles cyclic dependencies gracefully; variables from the cycle will initially be `undefined` until they are fully evaluated.

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The module is silently skipped, resulting in missing variables.

Question 7

What is a 'Symbol.iterator' in JavaScript?

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A built-in class used to loop over deeply nested JSON objects asynchronously.

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A well-known Symbol that specifies the default iterator for an object, used by `for...of` loops.

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A Web API used to iterate through DOM elements in the Shadow DOM.

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An exclusive React feature used to map over child components.

Question 8

Which array method creates a new array by executing a provided function on every element, and strictly flattens the result by one level?

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`.map()`

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`.flattenMap()`

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`.flatMap()`

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`.reduce()`

Question 9

What is the primary role of the `AbortController` interface?

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To automatically cancel infinite loops that block the main thread.

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To provide a signal to a DOM request (like `fetch()`) that allows you to cleanly abort it if needed.

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To intercept and block malicious cross-site scripting attacks.

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To stop Webpack from compiling a bundle if it exceeds a certain size.

Question 10

How does `Object.create()` differ from using the `new` keyword with a constructor?

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They are identical; both call the constructor function to initialize properties.

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`Object.create()` builds a new object with the specified prototype object without executing any constructor function.

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`Object.create()` is significantly slower and should only be used for deep cloning.

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`new` creates an object directly in the global scope, while `Object.create()` limits scope to the current block.

Question 11

What is the primary difference between `localStorage` and `sessionStorage`?

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`localStorage` data persists across browser restarts, while `sessionStorage` data is cleared when the page session ends (e.g., closing the tab).

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`localStorage` has a much smaller storage limit (5MB) compared to `sessionStorage` (100MB).

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`sessionStorage` is securely encrypted, while `localStorage` is stored in plain text.

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`sessionStorage` is shared across all tabs and windows, while `localStorage` is isolated per tab.

Question 12

What is 'Hoisting' specifically regarding `let` and `const` variables?

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Correct Answer

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They are not hoisted at all; the engine ignores them until runtime.

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They are hoisted and initialized to `undefined`, exactly like `var`.

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They are hoisted to the top of their block scope, but remain uninitialized in the Temporal Dead Zone.

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They are hoisted strictly to the global scope to prevent naming collisions.

Question 13

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

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It removes elements from the right side of the array.

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It functions exactly like `reduce()`, but iterates over the array from right-to-left.

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It aligns the array elements to the right side of the memory stack.

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It strictly converts the final accumulated value into an array.

Question 14

What does a 'MutationObserver' do in modern JavaScript?

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It observes changes to the application's global Redux state.

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It monitors network requests and detects slow API responses.

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It provides a way to asynchronously observe changes made to the DOM tree.

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It tracks garbage collection metrics for memory leak detection.

Question 15

Which of the following describes a JavaScript 'Thunk'?

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A heavily optimized string comparison algorithm.

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A function that acts as a wrapper to delay the evaluation of an expression or the execution of a function.

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An error thrown when a Web Worker runs out of allocated memory.

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A native implementation of a linked list data structure.

Question 16

What is the purpose of the `Set.prototype.clear()` method?

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To remove a specific value from the Set.

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To remove all elements from a Set object, leaving it empty.

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To reset the internal pointer of a Set iterator back to the first element.

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To clear the memory reference of the Set, triggering immediate garbage collection.

Question 17

When comparing objects using `===`, what does JavaScript check?

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It deeply compares every property and value within the objects.

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It checks if both objects share the exact same prototype chain.

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It checks if both variables reference the exact same object in memory.

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It checks if the stringified JSON output of both objects is identical.

Question 18

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

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The starting index position of the searched substring.

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An array containing all matched occurrences.

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A boolean indicating whether one string may be found within another string.

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The number of times the substring appears in the string.

Question 19

What happens if you omit the `new` keyword when calling a Constructor function (e.g., `Person()`)?

Accepted Answer

Correct Answer

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The JavaScript engine automatically inserts the `new` keyword for you.

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It throws a TypeError halting the application.

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The function executes normally, but its `this` context points to the global object, potentially polluting it.

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It creates a new object successfully, but without attaching the prototype chain.

Question 20

What is the function of the `URLSearchParams` interface?

Accepted Answer

Correct Answer

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To encode complex JSON objects into base64 format for safe transmission.

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To define utility methods to work with the query string of a URL.

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To securely generate random hashes for routing identifiers.

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To prevent Cross-Site Request Forgery (CSRF) by sanitizing URL parameters.

Question 21

What is a 'Polyfill' specifically designed for in modern web development?

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Bundling CSS and JS into a single highly optimized file.

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Providing fallback code to allow older browsers to use new features that they do not natively support.

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Translating React JSX syntax directly into native DOM manipulation code.

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Automatically generating documentation based on JSDoc comments.

Question 22

How does `Object.entries()` handle non-enumerable properties?

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It includes them in the returned array but flags them as read-only.

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It throws an error if it encounters a non-enumerable property.

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It completely ignores them; they are not included in the returned array.

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It groups them at the end of the array.

Question 23

What is a 'Tag Function' in the context of Template Literals?

Accepted Answer

Correct Answer

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A function used to automatically inject HTML tags around strings.

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A function that parses a template literal, allowing you to manipulate the strings and embedded expressions before the final string is returned.

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A built-in method that sanitizes SQL queries to prevent injection attacks.

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A utility that converts template literals back into standard strings for older browsers.

Question 24

What is the purpose of the `Performance` API (e.g., `performance.now()`)?

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Correct Answer

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To upgrade the browser's hardware rendering acceleration.

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To automatically detect memory leaks in the V8 engine.

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To provide high-resolution timestamps for highly accurate performance measurement and benchmarking.

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To optimize the loading of large image files in the DOM.

Question 25

What does the `DocumentFragment` interface provide?

Accepted Answer

Correct Answer

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A minimal document object that has no parent, used to build offscreen DOM structures efficiently before appending them to the active DOM.

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A built-in class used to parse XML fragments retrieved from legacy backend APIs.

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A graphical fragment representing the visible portion of the viewport.

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A secure sandbox for executing potentially unsafe third-party scripts.

Academy

ReactJS Interview Prep - JavaScript Fundamentals (Part 5)