ReactJS Interview Prep - JavaScript Fundamentals (Part 2)

Question 1

What is the primary difference between a `WeakMap` and a standard `Map` in JavaScript?

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A `WeakMap` only accepts primitive values as keys, whereas a `Map` accepts objects.

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Keys in a `WeakMap` are weakly held, meaning they do not prevent garbage collection if there are no other references to the key object.

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A `WeakMap` allows iteration over its keys using `for...of`, whereas a `Map` does not.

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A `WeakMap` restricts the number of elements it can hold to optimize memory usage.

Question 2

Which of the following best explains 'Event Delegation' in JavaScript?

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Passing a function as an argument to another function to be executed later.

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Attaching a single event listener to a parent element to manage events triggered by all its descendant elements.

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The automatic transfer of events from the browser window object to the document body.

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Delegating asynchronous event handling to Web Workers to prevent blocking the main thread.

Question 3

What is the purpose of the `Symbol` primitive type introduced in ES6?

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To create guaranteed unique identifiers for object properties to avoid naming collisions.

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To define strictly typed enumerations similar to TypeScript enums.

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To encrypt sensitive data within the client-side browser storage.

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To replace standard strings when defining highly performant DOM query selectors.

Question 4

In JavaScript, what is the 'Prototype Chain'?

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A strict sequence of function calls required to resolve a Promise.

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The hierarchical mechanism by which objects inherit properties and methods from other objects.

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A design pattern used to chain array methods like `.map().filter().reduce()`.

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The internal list of modules imported by Webpack during the bundling process.

Question 5

What does the `Reflect` API provide in modern JavaScript?

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A built-in utility for analyzing component rendering performance in React.

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A way to mirror the DOM structure in a Web Worker for multi-threading.

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A collection of methods that intercept standard JavaScript operations, mirroring the methods of Proxy handlers.

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A mechanism to dynamically load CSS modules into the shadow DOM.

Question 6

How do generator functions (`function*`) differ from standard JavaScript functions?

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They execute infinitely until the browser tab is closed.

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They can pause their execution yielding multiple values over time, and can be resumed later.

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They run entirely on a separate thread, bypassing the main event loop.

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They automatically parse and evaluate incoming strings as JavaScript code.

Question 7

What is 'Currying' in JavaScript?

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The process of optimizing recursive functions to prevent stack overflow errors.

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Transforming a function that takes multiple arguments into a sequence of functions, each taking a single argument.

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Bundling multiple small JavaScript files into one large file to reduce HTTP requests.

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The process of deep cloning objects using JSON serialization.

Question 8

What is the primary difference between `Function.prototype.call()` and `Function.prototype.apply()`?

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`call()` invokes the function synchronously, while `apply()` executes asynchronously.

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`call()` takes arguments as a comma-separated list, whereas `apply()` takes an array of arguments.

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`apply()` permanently binds the `this` context, while `call()` only binds it for the single execution.

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`call()` is used for arrow functions, whereas `apply()` is exclusively for standard functions.

Question 9

What is the output of `0.1 + 0.2 === 0.3` in JavaScript, and why?

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`true`, because JavaScript uses strict mathematical precision.

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`false`, because JavaScript numbers are represented as 64-bit floating point, leading to precision rounding errors.

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`true`, because the V8 engine automatically rounds floating-point numbers.

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`false`, because the strict equality operator requires type matching, and the left side evaluates to a string.

Question 10

Which of the following is a common cause of memory leaks in single-page applications?

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Failing to clean up global variables and event listeners when components unmount.

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Using strict equality (`===`) instead of loose equality (`==`) in deeply nested loops.

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Declaring variables with `let` instead of `var` in global scope.

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Using `Promise.all()` with more than 10 concurrent promises.

Question 11

What is the purpose of the `Object.freeze()` method?

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It deeply clones an object, returning a completely new reference.

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It prevents new properties from being added to an object, but allows existing properties to be modified.

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It prevents modifications to existing properties and prevents new properties from being added, effectively making the object immutable.

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It pauses object execution, preventing its methods from being invoked.

Question 12

How does a 'Debounce' function optimize performance?

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By caching the results of expensive function calls and returning the cached result when identical inputs occur.

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By grouping a sudden burst of sequential function calls into a single execution after a specified quiet period.

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By enforcing a strict maximum execution rate, ensuring a function executes at most once every specified time window.

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By distributing complex calculations across multiple Web Worker threads.

Question 13

How does a 'Throttle' function differ from 'Debounce'?

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Throttling executes the function once per time window regardless of how many times the event fires, whereas debouncing waits for a pause in events.

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Throttling is used for synchronous tasks, whereas debouncing is used for asynchronous tasks.

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Throttling caches results in memory, whereas debouncing caches them in local storage.

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Throttling prevents the event from firing entirely, whereas debouncing delays the event.

Question 14

What is the meaning of 'Temporal Dead Zone' (TDZ) in JavaScript?

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The period during page load before the DOM is fully parsed.

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The delay between an asynchronous API request and its response.

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The timeframe where variables declared with `let` and `const` exist but cannot be accessed before their initialization line.

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The timeframe where garbage collection pauses the main thread execution.

Question 15

What does the `yield*` expression do within a generator function?

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It multiplies the yielded value by the subsequent argument.

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It permanently stops the generator and throws an error.

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It delegates yielding to another iterable object or generator function.

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It forces the generator to resolve all promises concurrently.

Question 16

What is the primary characteristic of an 'Immediately Invoked Function Expression' (IIFE)?

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It is a function that executes immediately after it is created.

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It is exclusively used for handling highly concurrent web socket messages.

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It automatically binds the global `window` object to its `this` context.

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It prevents garbage collection on all variables instantiated within it.

Question 17

How can you determine if an object property is an 'own property' rather than inherited from the prototype chain?

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By utilizing the `Reflect.has()` method.

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By checking if the property returns `undefined`.

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By using the `hasOwnProperty()` method or `Object.hasOwn()`.

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By passing the object through `JSON.stringify()`.

Question 18

What is 'Memoization'?

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An optimization technique that caches the results of expensive function calls.

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A strict mode feature that prevents the modification of global variables.

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A design pattern used to structure component state in React.

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A memory management system built directly into the V8 engine.

Question 19

Which of the following is true regarding 'Pure Functions'?

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They frequently modify variables existing outside their local scope.

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They always return identical outputs for identical inputs and produce no observable side effects.

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They are the only functions that can be passed as arguments in JavaScript.

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They require asynchronous syntax (`async/await`) to execute properly.

Question 20

In the context of the DOM, what is 'Event Bubbling'?

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The process where an event starts at the target element and triggers listeners moving upward through its ancestors.

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The process where an event starts at the window object and moves downward to the target element.

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The creation of multiple identical events resulting from a rapid click sequence.

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The memory leak caused when event listeners are not properly removed.

Question 21

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

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To route HTTP requests securely to cross-origin servers.

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To wrap another object and intercept/redefine fundamental operations like property lookup, assignment, and function invocation.

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To strictly evaluate variables and coerce them to specific data types.

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To mock database connections during automated unit testing.

Question 22

How does `Object.seal()` differ from `Object.freeze()`?

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`Object.seal()` allows properties to be added, but not removed. `Object.freeze()` prevents both.

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`Object.seal()` prevents adding or removing properties, but allows modifying existing writable properties. `Object.freeze()` prevents modifications completely.

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They behave identically, but `Object.seal()` executes faster.

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`Object.seal()` deep freezes nested objects, whereas `Object.freeze()` only affects the top-level properties.

Question 23

What is the concept of 'Type Coercion' in JavaScript?

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The manual conversion of a variable from one data type to another using methods like `String()` or `Number()`.

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The automatic, implicit conversion of values from one data type to another by the JavaScript engine.

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The strict enforcement of variable types using TypeScript.

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The process of serializing complex objects into JSON strings.

Question 24

What does the `typeof NaN` expression return?

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`'undefined'`

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`'NaN'`

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`'number'`

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`'error'`

Question 25

What is the role of the `super` keyword in JavaScript classes?

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It provides access to the global object scope, overriding any local bindings.

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It is used to call functions on an object's parent class.

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It immediately returns an instance of the class without calling the constructor.

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It explicitly upgrades standard class methods to be fully asynchronous.

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ReactJS Interview Prep - JavaScript Fundamentals (Part 2)