Array.prototype.flatMap()

flatMap(callbackFn, thisArg)

flatMap() combines map() and flat() in a single method. It applies a function to each element, then flattens the result by one level. This is more efficient than chaining .map().flat() or .map().flat(1) because it only iterates once.

Syntax

flatMap(callbackFn, thisArg)

Parameters

Parameter	Type	Default	Description
callbackFn	function	—	Function that produces an element for the new array
thisArg	any	undefined	Value to use as this when executing callbackFn

callbackFn Parameters

Parameter	Type	Description
currentValue	any	The current element being processed
index	number	The index of the current element
array	Array	The array flatMap() was called on

Examples

Basic usage

const words = ["hello", "world"];
const chars = words.flatMap(word => word.split(''));
console.log(chars);
// ['h', 'e', 'l', 'l', 'o', 'w', 'o', 'r', 'l', 'd']

Flattening nested arrays

const nested = [[1, 2], [3, 4], [5]];
const flattened = nested.flatMap(arr => arr.map(x => x * 2));
console.log(flattened);
// [2, 4, 6, 8, 10]

Removing empty slots

const arr = [1, , 3].flatMap(x => [x, x]);
console.log(arr);
// [1, 1, 3, 3]

Common Patterns

Flattening and transforming in one pass

Instead of:

const result = arr.map(x => [x, x * 2]).flat();

Use:

const result = arr.flatMap(x => [x, x * 2]);

Conditional flattening

const items = [1, 2, 3, 4, 5];
const result = items.flatMap(x => x % 2 === 0 ? [x] : [Global_Objects::eval]);
console.log(result);
// [2, 4]

See Also

• array::map
• array::flat
• array::filter