How to Flatten a Nested Array in JavaScript

The built-in Array.prototype.flat() method flattens nested arrays into a single-level array up to a given depth.

[1, [2, 3], [4, [5, 6]]].flat(Infinity)
// [1, 2, 3, 4, 5, 6]

Steps

1. Call .flat(depth) on the array, where depth is how many levels of nesting to flatten
2. Pass Infinity as the depth to flatten arbitrarily deep nesting in one call

How it works

flat() returns a new array; it does not mutate the original. Without an argument, it defaults to a depth of 1, flattening only the first level of nested arrays.

Things to watch for

• flat() is supported in all modern browsers and Node 11+; for older environments use a recursive reduce() implementation instead
• flatMap() combines map() and a flat(1) in one pass and is often faster when you're transforming and flattening at the same time

FAQ

How do I flatten and map in one step?

Use array.flatMap(fn) — it runs fn over each element and flattens the result by one level, avoiding a separate .map().flat(1) call.