The JavaScript array.flat() method

Q: How do I fully flatten a deeply nested array?

Pass Infinity as the depth: arr.flat(Infinity).

Q: What is the difference between flat() and flatMap()?

flat() only flattens; flatMap() maps each element with a callback and then flattens the result one level — equivalent to map().flat().

Method JavaScript All modern browsers Updated June 23, 2026

Quick answer

The flat() method flattens nested arrays into a single array. [1, [2, 3], [4]].flat() gives [1, 2, 3, 4]. By default it flattens one level deep; pass a number for more, or Infinity to flatten completely. It returns a new array and leaves the original alone.

Overview

flat() smooths out nested arrays. An array containing other arrays — [1, [2, 3], [4, [5]]] — becomes flatter when you call flat() on it. By default it goes just one level deep, so that example flattens to [1, 2, 3, 4, [5]] — the doubly-nested [5] stays nested.

To go deeper, pass a depth: flat(2) flattens two levels. When you don't know how deep the nesting goes, flat(Infinity) flattens it completely, however many levels there are. It returns a new array, so the original is untouched, and it also drops empty slots from sparse arrays along the way.

A close relative is flatMap(), which maps each element and then flattens the result one level — handy when a map() callback returns arrays you want merged. If you find yourself writing arr.map(...).flat(), flatMap() does it in one step.

Syntax

array.flat()          // one level (default)
array.flat(depth)     // n levels
array.flat(Infinity)  // fully flatten

[1, [2, [3]]].flat()        // [1, 2, [3]]
[1, [2, [3]]].flat(Infinity) // [1, 2, 3]

Parameters

The array.flat() method accepts the following parameters.

Parameter	Description
`depth`	Optional. How many levels deep to flatten. Defaults to `1`. Use `Infinity` to flatten all levels.

Example

Live example

<pre id="out" style="font:15px ui-monospace,monospace"></pre>
<script>
  const nested = [1, [2, 3], [4, [5, 6]]];

  document.getElementById('out').textContent =
    'flat():    ' + JSON.stringify(nested.flat()) + '\n' +
    'flat(Inf): ' + JSON.stringify(nested.flat(Infinity));
</script>

Best practices

Pass Infinity when the nesting depth is unknown and you want it fully flat.
Use flatMap() instead of map().flat() when a map callback returns arrays.
Remember the default depth is just 1 level.
It returns a new array — the original nested structure is unchanged.

Frequently asked questions

What does flat() do in JavaScript?

It flattens nested arrays into a single array, one level deep by default.

How do I fully flatten a deeply nested array?

Pass Infinity as the depth: arr.flat(Infinity).

What is the difference between flat() and flatMap()?

flat() only flattens; flatMap() maps each element with a callback and then flattens the result one level — equivalent to map().flat().

Does flat() change the original array?

No. It returns a new, flatter array and leaves the original as it was.

array.findLastIndex() array.flatMap()

References