ReactJS Interview Prep - JavaScript Fundamentals (Part 4)

Question 1

Which of the following is true about 'Strict Mode' (`"use strict";`)?

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It allows you to use newer syntax that older browsers wouldn't normally support.

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It eliminates some JavaScript silent errors by changing them to throw errors (e.g., assigning to undeclared variables).

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It forces all functions to be declared as arrow functions.

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It speeds up execution by skipping the garbage collection phase.

Question 2

What does the `Math.floor()` function do?

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It rounds a number up to the next largest integer.

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It returns the absolute positive value of a number.

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It rounds a number down to the largest integer less than or equal to a given number.

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It truncates the decimal portion entirely, identical to `Math.trunc()` in all scenarios.

Question 3

How do you detect if a property exists directly on an object, rather than on its prototype?

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Using the `in` operator (e.g., `'prop' in obj`).

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Using `Object.hasOwn(obj, 'prop')`.

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Using `typeof obj.prop !== 'undefined'`.

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Using `Reflect.getPrototypeOf(obj)`.

Question 4

What is the primary difference between `Object.keys()` and `Object.getOwnPropertyNames()`?

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`Object.keys()` returns all properties including inherited ones, while `Object.getOwnPropertyNames()` only returns own properties.

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`Object.keys()` returns only enumerable own properties, while `Object.getOwnPropertyNames()` returns all own properties (both enumerable and non-enumerable).

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They are identical and always return the same array of string keys.

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`Object.keys()` only returns string keys, while `Object.getOwnPropertyNames()` returns both string and Symbol keys.

Question 5

Which built-in object is used to execute background tasks on a separate thread, avoiding blocking the main UI thread?

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Promise

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AsyncAwait

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Web Worker

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Service Registry

Question 6

How do you correctly throw a custom error in JavaScript?

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`throw new Error('Something went wrong');`

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`return new Error('Something went wrong');`

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`throw Exception('Something went wrong');`

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`console.error('Something went wrong');`

Question 7

What is the purpose of the `fetch()` API's `Response.json()` method?

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It instantly converts a JavaScript object into a JSON string to be sent to the server.

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It returns a Promise that resolves with the result of parsing the response body text as JSON.

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It synchronously parses the raw response bytes into a JavaScript object.

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It forces the browser to set the 'Content-Type' header to 'application/json'.

Question 8

When making a cross-origin request with `fetch()`, what is a CORS 'Preflight' request?

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A heavy download process that caches images before rendering the page.

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An automatic retry mechanism triggered when a server goes offline.

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An initial `OPTIONS` request sent by the browser to check if the server permits the actual request.

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A mechanism that encrypts the request payload using an SSL certificate.

Question 9

What does the `Object.entries()` method return?

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An array containing all the values of an object.

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An array containing all the keys of an object.

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An array of a given object's own enumerable string-keyed property [key, value] pairs.

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A deeply cloned copy of the original object.

Question 10

What does the `Optional Chaining` operator (`?.`) do?

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It provides a default fallback value if a variable evaluates to null or undefined.

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It permits reading the value of a property located deep within a chain of connected objects without having to check that each reference in the chain is valid.

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It forces the JavaScript engine to type-check an object property at runtime.

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It checks if an object is an instance of a specific class.

Question 11

What is the primary function of the Nullish Coalescing Operator (`??`)?

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It acts identical to the logical OR (`||`) operator, but executes faster.

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It returns its right-hand operand when its left-hand operand is null or undefined (and only null or undefined).

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It returns the right-hand operand if the left-hand operand is any falsy value (like 0, false, or '').

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It securely deletes null values from complex nested objects.

Question 12

Which statement correctly describes 'Garbage Collection' in V8 (Chrome/Node.js)?

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It requires developers to manually invoke `window.gc()` to prevent memory overflow.

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It automatically tracks memory usage based on the 'Reference Counting' algorithm only.

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It primarily uses a 'Mark and Sweep' algorithm to determine which objects are no longer reachable from the root and reclaims their memory.

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It deletes all variables as soon as the active browser tab is moved to the background.

Question 13

What does the `window.requestAnimationFrame()` method do?

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It triggers a CSS animation precisely after a specified millisecond delay.

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It forces the browser to render 60 frames per second regardless of hardware limits.

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It tells the browser you wish to perform an animation and requests that the browser call a specified function to update an animation before the next repaint.

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It parses and executes GIF and SVG animations automatically.

Question 14

How can you convert a NodeList (returned by `document.querySelectorAll()`) into a standard JavaScript Array?

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You cannot; a NodeList is strictly tied to the DOM.

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By using `Array.from()` or the spread operator (`[...nodeList]`).

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By calling the `nodeList.toArray()` method.

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By passing the NodeList through `JSON.parse(JSON.stringify(nodeList))`.

Question 15

What is the difference between `e.target` and `e.currentTarget` in a DOM event handler?

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`e.target` refers to the element that triggered the event, while `e.currentTarget` refers to the element the event listener is attached to.

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`e.target` is used for click events, while `e.currentTarget` is used for keyboard events.

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`e.currentTarget` refers to the element that triggered the event, while `e.target` refers to the root `document`.

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There is no difference; they both always reference the exact same element.

Question 16

Which of the following describes the difference between `script defer` and `script async` in HTML?

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`async` scripts block HTML parsing, whereas `defer` scripts do not.

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`defer` scripts execute sequentially in the order they appear after parsing completes, whereas `async` scripts execute immediately once downloaded, regardless of order.

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`async` is for external scripts, while `defer` is for inline scripts.

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Both block parsing, but `defer` waits for images to load.

Question 17

What is the 'Same-Origin Policy' (SOP)?

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A mechanism that forces all APIs to be built using the exact same programming language.

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A security measure ensuring that all data packets originate from the same IP address.

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A browser security concept that restricts how a document or script loaded from one origin can interact with a resource from another origin.

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A deployment rule requiring the frontend and backend to share the same physical server.

Question 18

In regular expressions, what does the `g` flag indicate (e.g., `/pattern/g`)?

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Global search: meaning it finds all matches rather than stopping after the first match.

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Greedy search: it attempts to match the longest possible string.

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Group matching: it strictly captures characters in parenthesis.

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Generic mode: it ignores case sensitivity.

Question 19

What is the purpose of the `Intl` object in JavaScript?

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To securely manage internal application routing and history state.

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To provide language-sensitive string comparison, number formatting, and date and time formatting.

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To intercept and parse incoming HTTP integer requests.

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To handle complex multi-threading in WebAssembly.

Question 20

What is the 'Spread' syntax primarily used for when dealing with Arrays?

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Grouping multiple array elements into a single nested array.

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Calculating the statistical variance (spread) of numeric arrays.

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Expanding an iterable array into its individual elements.

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Removing whitespace from all string elements in an array.

Question 21

What happens when you declare a function using the `function` keyword (Function Declaration)?

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The function is executed immediately upon compilation.

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The function declaration is hoisted to the top of its enclosing scope, allowing you to call it before it is defined in the code.

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The function is permanently locked to the global scope, regardless of where it is defined.

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The function cannot accept an indefinite number of arguments via the rest parameter.

Question 22

What is a 'Polyfill'?

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A Webpack plugin used to reduce the size of JavaScript bundles.

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A piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.

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A strict design pattern used to fill memory leaks in single-page applications.

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A native HTML element used to draw complex 2D and 3D graphics.

Question 23

Which of the following defines a 'Method' in JavaScript?

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A strictly synchronous API request.

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A function that is defined as a property of an object.

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Any function that utilizes the `return` keyword.

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A built-in class that manages state updates.

Question 24

What does the `isNaN()` function do?

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It checks if a variable strictly equals the global `NaN` object.

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It determines whether a value is Not-A-Number or cannot be coerced into a valid number.

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It verifies if a string contains alphabetical characters.

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It converts a string representation of a number into a strict integer.

Question 25

What is the core distinction between the DOM (Document Object Model) and the BOM (Browser Object Model)?

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The DOM manages CSS styles, while the BOM manages HTML tags.

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The DOM represents the page content (HTML/XML documents), while the BOM interacts with the browser itself (window, history, navigator).

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The DOM is used by React, while the BOM is strictly used by Angular.

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There is no distinction; they are two names for the same core API.

Academy

ReactJS Interview Prep - JavaScript Fundamentals (Part 4)