ReactJS Interview Prep - JavaScript Fundamentals (Part 3)

Question 1

What is the difference between `Array.prototype.find()` and `Array.prototype.filter()`?

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`find()` returns the first element that satisfies the condition, whereas `filter()` returns a new array with all elements that satisfy the condition.

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`filter()` returns the first element, whereas `find()` returns an array.

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`find()` returns a boolean, whereas `filter()` returns an array.

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There is no difference; they are aliases for the same operation.

Question 2

What does the `new` keyword actually do behind the scenes in JavaScript?

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It creates an entirely new thread in the browser's JavaScript engine.

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It creates a new empty object, links its prototype to the constructor function's prototype, binds `this` to the new object, and returns the object.

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It triggers garbage collection to ensure enough memory is available for the constructor.

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It converts a standard function into an asynchronous generator.

Question 3

What is the purpose of the `Set` object's `has()` method?

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To append a new element to the end of the Set.

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To check if an element is present in the Set, returning a boolean. It performs this check in O(1) time complexity.

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To iterate through all elements in the Set.

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To delete an element from the Set if it exists.

Question 4

What happens if you use `await` outside of an `async` function?

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The code executes normally, converting the surrounding function to `async` automatically.

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The JavaScript engine throws a SyntaxError.

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The `await` keyword is ignored and the Promise remains pending.

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The browser halts all execution and displays an alert box.

Question 5

How can you prevent a user from accidentally modifying the length of an array?

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By setting the array's `length` property to a negative number.

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By using `Object.freeze()` or `Object.seal()` on the array.

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By wrapping the array in a `WeakSet`.

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It is impossible; arrays in JavaScript are perpetually mutable.

Question 6

What happens if you omit the `catch()` block at the end of a Promise chain and an error occurs in one of the `then()` blocks?

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The error is silently swallowed and the execution continues normally.

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The browser throws an 'Unhandled Promise Rejection' warning or error.

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The Promise automatically resolves with a value of `undefined`.

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The JavaScript engine instantly crashes the entire tab.

Question 7

Which of the following is true about the `finally()` method in a Promise chain?

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It only executes if the Promise is resolved successfully.

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It receives the resolved value or rejected reason as an argument.

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It executes regardless of whether the Promise was resolved or rejected.

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It is used to explicitly clear memory leaks associated with the Promise.

Question 8

How does `Promise.allSettled()` differ from `Promise.all()`?

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`Promise.allSettled()` rejects immediately if any of the promises reject, whereas `Promise.all()` waits for all to finish.

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`Promise.allSettled()` waits for all promises to resolve or reject, returning an array of objects describing the outcome of each.

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`Promise.allSettled()` only returns the values of the promises that resolved successfully.

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`Promise.allSettled()` executes the promises in sequential order, whereas `Promise.all()` executes them concurrently.

Question 9

What is the result of using the `delete` operator on an array element (e.g., `delete arr[1]`)?

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The element is removed and the array's length is reduced by 1.

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The element is replaced with `undefined`, but the array's length remains unchanged.

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An error is thrown because arrays in JavaScript are immutable by default.

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The element is removed, and all subsequent elements shift down by one index.

Question 10

What is 'Lexical Scoping' in JavaScript?

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A feature that allows variables to be accessed strictly by their alphabetical order.

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A mechanism where a function's scope is determined by its physical location within the source code during the parsing phase.

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A compilation step that removes unused variables to minimize bundle size.

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The process of dynamically binding the `this` keyword at runtime.

Question 11

Why do arrow functions not have their own `arguments` object?

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Because arrow functions do not accept any arguments.

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Because they inherit the `arguments` object from their enclosing lexical scope.

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Because the `arguments` object was deprecated in ES6.

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Because arrow functions are strictly synchronous and cannot handle multiple arguments.

Question 12

What is the primary difference between the `slice()` and `splice()` methods in arrays?

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`slice()` modifies the original array, whereas `splice()` returns a new, copied array.

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`slice()` returns a shallow copy of a portion of an array and does not modify the original, whereas `splice()` modifies the array in place.

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`slice()` can only remove elements, whereas `splice()` can only add elements.

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There is no difference; `slice()` is just an alias for `splice()`.

Question 13

In a `try...catch...finally` block, if a `return` statement is executed inside the `try` block, what happens to the `finally` block?

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The `finally` block is skipped entirely.

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The `finally` block executes before the function actually returns.

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An error is thrown because you cannot use `return` inside a `try` block.

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The `finally` block executes after the function returns, asynchronously.

Question 14

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

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It removes all duplicate elements from an array.

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It removes all falsy values (null, undefined, 0, false) from an array.

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It creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

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It flattens an array of objects into a single object.

Question 15

What is a 'Higher-Order Function' in JavaScript?

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A function that is placed at the top of a file so it gets hoisted first.

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A function that takes one or more functions as arguments, or returns a function as its result.

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A function that is executed exclusively by the root Window object.

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A native C++ function implemented directly in the V8 engine.

Question 16

What happens when you declare a variable without using `var`, `let`, or `const` (in non-strict mode)?

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The script throws a SyntaxError immediately.

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The variable is successfully created as a block-scoped variable.

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The variable is implicitly created as a property of the global object (e.g., `window`), polluting the global scope.

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The variable is accessible only within the function it was defined in.

Question 17

Which statement accurately describes the global object in a web browser environment?

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It is the `document` object.

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It is the `window` object.

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It is the `global` object.

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It is the `process` object.

Question 18

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

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It returns true if ALL elements in the array pass the provided test function.

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It returns true if AT LEAST ONE element in the array passes the provided test function.

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It returns the first element that passes the provided test function.

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It executes a callback on every element and returns a modified array.

Question 19

What is the result of `"5" + 3` compared to `"5" - 3` in JavaScript?

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Both result in `8`.

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Both result in `"53"`.

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`"5" + 3` results in `"53"` (string concatenation), while `"5" - 3` results in `2` (numeric subtraction).

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Both throw a TypeError due to type mismatch.

Question 20

What is 'Shadowing' in variable scope?

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Creating a duplicate variable in the same scope to increase memory allocation.

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When a variable declared in an inner scope has the same name as a variable in an outer scope, preventing access to the outer variable.

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The process of copying an object without copying its prototype.

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When a `let` declaration overrides a global `const` declaration.

Question 21

What is the primary advantage of utilizing ES6 template literals over standard string concatenation?

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They execute code 50% faster in the browser.

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They automatically sanitize HTML inputs, preventing XSS attacks.

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They provide syntactic sugar for multi-line strings and easy expression interpolation without using the `+` operator.

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They enforce strict typing on all variables injected into them.

Question 22

How does the `Promise.any()` method behave?

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It resolves when all promises have successfully resolved.

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It rejects immediately if the first promise to finish rejects.

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It resolves as soon as ANY ONE of the promises fulfills, and only rejects if ALL promises reject.

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It returns a random promise from the provided iterable array.

Question 23

What is 'Duck Typing' in the context of JavaScript?

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A strict type-checking mechanism built into the ES6 specification.

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Determining an object's suitability by checking for the presence of certain methods and properties, rather than its actual inheritance or type.

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A compilation error that occurs when strings are mixed with integers.

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A method of encrypting variables to prevent cross-site scripting.

Question 24

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

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To enforce strict type definitions on object properties.

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To deeply clone complex nested objects.

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To copy the values of all enumerable own properties from one or more source objects to a target object.

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To lock an object so its prototype cannot be modified.

Question 25

How does `JSON.stringify()` handle functions and `undefined` values inside an object?

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It converts them to string representations.

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It throws a TypeError and stops execution.

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It ignores them, completely omitting those keys from the resulting JSON string.

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It converts them to `null`.

Academy

ReactJS Interview Prep - JavaScript Fundamentals (Part 3)