ReactJS Interview Prep - JavaScript Fundamentals

Question 1

Which of the following statements accurately describes the difference between `undefined` and `null` in JavaScript?

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`null` is a function, whereas `undefined` is an object.

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`undefined` means a variable has been declared but not assigned a value, while `null` represents an intentional absence of any object value.

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`undefined` is an object, and `null` is a primitive type.

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They are strictly identical and can be used interchangeably.

Question 2

What is a key advantage of using a `Map` over a standard `Object` in JavaScript?

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A `Map` allows keys of any data type and maintains the insertion order of its elements.

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An `Object` maintains insertion order reliably across all data types, while a `Map` does not.

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A `Map` performs significantly slower than an `Object` for frequent additions and removals.

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An `Object` allows keys of any data type, whereas a `Map` only allows string keys.

Question 3

Which of the following is a feature provided by template literals in modern JavaScript?

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They are enclosed by single quotes and prevent string injection attacks.

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They enforce single-line strings and throw errors on line breaks.

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They are enclosed by backticks (`) and allow multi-line strings and expression interpolation.

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They cannot contain variables, only static text.

Question 4

What is the primary function of the `bind()` method in JavaScript?

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It immediately invokes the target function with a specified `this` context.

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It creates and returns a new function with its `this` keyword permanently set to the provided value.

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It is a specialized method only available on arrays to bind elements to the DOM.

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It prevents a function from accepting any future arguments when invoked.

Question 5

When performing object destructuring, what is the default behavior regarding variable names?

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The new variable names must exactly match the property keys of the object being destructured.

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It only functions correctly on arrays, not objects.

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You are not allowed to assign default values to the destructured variables.

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It mutates the original object by removing the destructured properties.

Question 6

What does the expression `typeof null` evaluate to in JavaScript?

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`'null'`

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`'undefined'`

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`'object'`

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`'number'`

Question 7

How is the value of the `this` keyword determined within a regular (non-arrow) function?

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It is permanently bound to the global object (or `undefined` in strict mode).

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It is bound lexically based on where the function is written in the code.

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Its value is determined dynamically by how and where the function is called at runtime.

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It always references the function's own internal execution context.

Question 8

Which statement accurately describes the timing behavior of the `setTimeout` function?

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It guarantees that the callback will execute exactly at the specified millisecond delay.

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It severely blocks the main thread, halting rendering during the delay period.

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It specifies a minimum delay; the callback is placed in the macro-task queue and waits for the call stack to clear.

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It is part of the core ECMAScript standard and behaves identically in all programming languages.

Question 9

What is the core purpose of the array `reduce()` method?

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To filter out unwanted elements and return an array of a reduced length.

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To execute a reducer callback on each element, accumulating a single final output value.

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To strictly manipulate arrays of numbers by applying basic arithmetic operations.

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To loop through an array without accepting or utilizing an initial value.

Question 10

Which of the following is a characteristic of ES6 Modules in JavaScript?

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They rely heavily on the `require()` function and `module.exports` object.

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They operate entirely synchronously and block the parsing of the HTML document.

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They utilize `import` and `export` syntax and are executed in strict mode by default.

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They automatically leak all declared variables directly into the global window scope.

Question 11

What condition causes a promise returned by `Promise.race()` to settle?

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It settles only after every promise in the provided iterable has successfully resolved.

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It settles as soon as the first promise in the iterable settles (either resolves or rejects).

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It settles only if the first promise to finish is successfully fulfilled, ignoring rejections.

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It settles by returning an array containing the results of the fastest executing promises.

Question 12

Which of the following best describes the behavior of a `Set` in JavaScript?

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It is a collection of unique values of any type, where duplicates are automatically ignored.

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It is an ordered collection that allows duplicate elements.

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It is a collection of key-value pairs similar to an Object.

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It is a specialized array that only accepts primitive data types.

Question 13

What is a significant difference between a traditional `for` loop and the `forEach` method when iterating over an array?

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A `for` loop cannot be used with `async/await` syntax.

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A `for` loop can be terminated early using the `break` statement, whereas `forEach` cannot.

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`forEach` executes significantly faster than a traditional `for` loop in all modern engines.

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`forEach` allows you to break out of the loop using the `return` statement.

Question 14

How does the `await` keyword affect the execution of an `async` function?

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It forces the async function to return a string representing the Promise state.

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It pauses the execution of the `async` function until the Promise settles (resolves or rejects).

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It can be used in any regular, non-async JavaScript function.

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It blocks the main browser thread until the asynchronous operation completes.

Question 15

What is the primary role of the Event Loop in JavaScript's concurrency model?

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It spawns separate threads to handle heavy computational tasks concurrently.

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It ensures that synchronous code is executed in a random order for security.

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It constantly monitors the call stack and task queue, pushing asynchronous callbacks to the stack when it is empty.

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It is a DOM API specifically designed to handle user interface click events.

Question 16

Which of the following is the most accurate definition of a Closure in JavaScript?

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A method used to explicitly close database connections and browser tabs.

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A strict object-oriented concept that enforces data privacy through classes.

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A bug in JavaScript that leads to severe memory leaks if not handled.

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A function that retains access to variables in its lexical scope, even after the outer function has finished executing.

Question 17

How does JavaScript handle 'hoisting' during the execution phase?

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It physically moves variable initializations to the top of the file.

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It conceptually moves variable and function declarations to the top of their respective scopes before code execution.

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It is an exclusive feature of the `let` and `const` keywords that prevents temporal dead zones.

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It is a strict-mode only feature that optimizes memory allocation.

Question 18

What does a Promise represent in JavaScript?

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A synchronous operation that blocks execution until a value is returned.

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A callback function that cannot be chained or passed as an argument.

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An event listener that can be triggered multiple times over its lifecycle.

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An object representing the eventual completion (or failure) of an asynchronous operation and its resulting value.

Question 19

Which of the following is a key characteristic of Arrow functions compared to regular functions?

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They must always return an object explicitly.

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They cannot accept arguments or default parameters.

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They do not have their own bindings to the `this` keyword; they inherit it from the parent scope.

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They have their own dedicated `this` binding that overrides the global object.

Question 20

What is the primary scoping difference between variables declared with `var` and those declared with `let`?

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`var` is block-scoped, while `let` is function-scoped.

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`let` allows you to redeclare the same variable multiple times in the same scope.

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`let` is block-scoped, while `var` is function-scoped or globally scoped.

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Both keywords create block-scoped variables, but `var` allows reassignment.

Question 21

Why is it generally recommended to use the strict equality operator (`===`) instead of the loose equality operator (`==`)?

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`==` checks both value and type, whereas `===` only checks the value.

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`==` performs type coercion before comparison, whereas `===` checks both value and type without coercion.

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Both perform type coercion, but `===` executes faster.

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Neither performs type coercion, but `===` supports comparing objects.

Question 22

What is the primary purpose of the spread operator (`...`) in JavaScript?

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To combine multiple separate elements into a single array structure.

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To expand an iterable (like an array or object) into its individual elements.

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To define an indefinite number of arguments within a function declaration.

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To deeply clone nested objects and modify the original array in place.

Question 23

How does the `Promise.all()` method handle a provided iterable of promises?

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It resolves immediately when the first promise in the iterable resolves successfully.

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It only rejects if every single promise in the iterable is rejected.

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It resolves when all promises in the iterable resolve, but rejects immediately if any single promise rejects.

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It executes the promises sequentially, waiting for one to finish before starting the next.

Question 24

What is the key difference between the Array methods `map()` and `forEach()`?

Accepted Answer

Correct Answer

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`map()` returns a new array containing the results of the callback, while `forEach()` returns `undefined`.

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`forEach()` returns a new modified array, while `map()` mutates the original array and returns `undefined`.

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Both return a new array, but `map()` can only be used with asynchronous callbacks.

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Neither can be chained with other array methods like `filter()` or `reduce()`.

Question 25

Which statement accurately describes the 'rest' parameter syntax (`...`) in a function definition?

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It must be the first parameter declared in the function's parameter list.

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It collects an indefinite number of remaining arguments into a standard JavaScript array.

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It expands an existing array into individual, comma-separated arguments.

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It cannot be utilized within arrow functions or anonymous functions.

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ReactJS Interview Prep - JavaScript Fundamentals