JavaScript Array Methods 2

Question 1

What does `Array.prototype.forEach()` return?

Accepted Answer

undefined

Answer

`forEach()` always returns `undefined`. It is used purely for side effects (like logging or mutating external state), not for transforming data. Use `map()` if you need a new array.

Question 2

What does `Array.from()` do?

Accepted Answer

Creates a new array from an array-like or iterable object

Answer

`Array.from()` creates a new shallow-copied Array from an array-like (has `length` and indexed elements) or iterable object (like `Set`, `Map`, `NodeList`, `String`).

Question 3

What is the result of `[1,2,3].indexOf(4)`?

Accepted Answer

-1

Answer

`indexOf()` returns the first index at which a given element is found. If the element is not found, it returns `-1`. This is the conventional JavaScript way to indicate 'not found'.

Question 4

What does `Array.prototype.every()` do?

Accepted Answer

Returns true only if the callback returns truthy for ALL elements

Answer

`every()` tests whether all elements in the array pass the test implemented by the provided function. It returns `true` only if the callback returns truthy for every element, otherwise returns `false`.

Question 5

What is the difference between `Array.prototype.push()` and `Array.prototype.concat()`?

Accepted Answer

`push()` modifies the original array; `concat()` returns a new array

Answer

`push()` adds elements to the end of the original array (mutates it) and returns the new length. `concat()` returns a new array by merging arrays/values without modifying the originals.

JavaScript Practice Test

JavaScript Practice Test

JavaScript Array Methods 2