flatMap

The flatMap() method applies a transform function to each element and then automatically flattens the result by one level. It therefore combines map() and flat(1) in a single step.

Interactive example

Syntax

Classic signature

function flatMap<
	GenericInput extends readonly unknown[],
	GenericOutput extends unknown
>(
	input:  GenericInput,
	theFunction: (
		element: GenericInput[number],
		params: ArrayFlatMapParams<GenericInput>
	) => GenericOutput
): FlatArray<GenericOutput, 1>[]

Curried signature

function flatMap<
	GenericInput extends readonly unknown[],
	GenericOutput extends unknown
>(
	theFunction: (
		element: GenericInput[number],
		params: ArrayFlatMapParams<GenericInput>
	) => GenericOutput
): (input: GenericInput) => FlatArray<GenericOutput, 1>[]

Helper parameters

interface ArrayFlatMapParams<
	GenericInputArray extends readonly unknown[]
> {
	index: number;
	self: GenericInputArray;
}

Parameters

• input: The source array.
• theFunction: Function applied to each element. It returns an array (or a value) that will be flattened by one level.
• params.index: Position of the current element.
• params.self: The whole array (useful for comparing the length or accessing neighbors during the transformation).

Return value

A new array with the transformed values flattened by one level. Input tuples keep precise typing thanks to FlatArray.

See also

• map - Applies a function to each element
• flat - Flattens a nested array

Sources

• MDN Web Docs - Array.prototype.flatMap()