JavaScript Array Methods

Question 1

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

Accepted Answer

A new array with each element transformed by the callback

Answer

`map()` creates and returns a new array populated with the results of calling the provided callback function on every element in the original array. The original array is not modified.

Question 2

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

Accepted Answer

Applies a function against an accumulator and each element to reduce the array to a single value

Answer

`reduce()` executes a reducer function on each element of the array, accumulating the result into a single output value. It takes a callback and an optional initial value.

Question 3

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

Accepted Answer

`find()` returns the first matching element; `filter()` returns an array of all matches

Answer

`find()` returns the first element that satisfies the testing function, or `undefined` if none found. `filter()` returns a new array containing ALL elements that satisfy the test.

Question 4

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

Accepted Answer

Creates a new array with sub-arrays concatenated into it up to a specified depth

Answer

`flat()` creates a new array with all sub-array elements concatenated into it recursively up to the specified depth (default: 1). `flat(Infinity)` flattens all nesting levels.

Question 5

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

Accepted Answer

true if at least one element passes the test, otherwise false

Answer

`some()` tests whether at least one element in the array passes the test implemented by the provided function. It returns `true` immediately when a matching element is found, `false` if none match.

JavaScript Practice Test

JavaScript Practice Test

JavaScript Array Methods