javascriptintermediate

JavaScript Array Methods

map, filter, reduce and other array higher-order methods

8 questions

By EZ4Code TeamLast updated: 2026-08-01

1. What does the following code output? const arr = [1, 2, 3] console.log(arr.map(x => x * 2))

const arr = [1, 2, 3]
console.log(arr.map(x => x * 2))

[2, 4, 6]

[1, 2, 3]

Error

Explanation: map calls the callback on each element and returns a new array without modifying the original, resulting in [2, 4, 6].

2. What does the filter method do?

Returns a new array of elements that satisfy the condition

Sorts the array

Accumulates all elements

Modifies the original array

Explanation: filter selects elements based on truthy values returned by the callback, returning a new array without modifying the original.

3. What does the following code output? const arr = [1, 2, 3, 4] console.log(arr.reduce((acc, x) => acc + x, 0))

const arr = [1, 2, 3, 4]
console.log(arr.reduce((acc, x) => acc + x, 0))

[1,2,3,4]

Error

Explanation: reduce accumulates from the initial value 0: 1+2+3+4=10.

4. Which of the following methods modifies the original array?

push

map

filter

slice

Explanation: push adds elements to the end of the original array and returns the new length; map/filter/slice all return new arrays without changing the original.

5. What is the difference between find and filter?

find returns the first element that satisfies the condition, filter returns an array of all satisfying elements

They are completely identical

filter returns a single element

find returns an array

Explanation: find returns the first element satisfying the callback (or undefined if none); filter returns a new array of all satisfying elements.

6. What is the difference between some and every?

some returns true if any is truthy; every returns true only if all are truthy

They are identical

some requires all to be truthy

every requires any to be truthy

Explanation: some returns true as long as one element satisfies the condition; every requires all elements to satisfy the condition to return true.

7. What does the following code output? const arr = [1, 2, 3] console.log(arr.includes(2))

const arr = [1, 2, 3]
console.log(arr.includes(2))

true

false

undefined

Explanation: includes checks whether the array contains the given value, returning a boolean; 2 is in the array, so it returns true.

8. What is the flatMap method equivalent to?

map followed by flat (depth 1)

flat followed by map

Only flat

Only map

Explanation: flatMap calls the callback on each element (returning an array) and flattens the result by one level, equivalent to map followed by flat(1).