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# flatMap

> For the full documentation index, see: https://docs.mulesoft.com/llms.txt

## flatMap<T, R>(@StreamCapable items: Array<T>, mapper: (item: T, index: Number) -> Array<R>): Array<R>

Iterates over each item in an array and flattens the results.

Instead of returning an array of arrays (as `map` does when you iterate over the values within an
input like `[ [1,2], [3,4] ]`), `flatMap` returns a flattened array that looks like this:
`[1, 2, 3, 4]`. `flatMap` is similar to `flatten`, but `flatten` only acts on the values of the
arrays, while `flatMap` can act on values and indices of items in the array.

### Parameters

<table><colgroup><col> <col></colgroup><thead><tr><th>Name</th><th>Description</th></tr></thead><tbody><tr><td><p><code>items</code></p></td><td><p>The array to map.</p></td></tr><tr><td><p><code>mapper</code></p></td><td><p>Expression or selector for an <code>item</code> and/or <code>index</code> in the array to flatten.</p></td></tr></tbody></table>

### Example

This example returns an array containing each value in order. Though it names the optional `index`
parameter in its anonymous function `(value, index) → value`, it does not use `index` as a selector
for the output, so it is possible to write the anonymous function using `(value) → value`. You can
also use an anonymous parameter for the value to write the example like this:
`[ [3,5], [0.9,5.5] ] flatMap $`. Note that this example produces the same result as
`flatten([ [3,5], [0.9,5.5] ])`, which uses `flatten`.

#### Source

```DataWeave
%dw 2.0
output application/json
---
[ [3,5], [0.9,5.5] ] flatMap (value, index) -> value
```

#### Output

```JSON
[ 3, 5, 0.9, 5.5]
```

## flatMap<T, R>(@StreamCapable value: Null, mapper: (item: Nothing, index: Nothing) -> Any): Null

Helper function that enables `flatMap` to work with a `null` value.