JavaScript

Flatten Nested Array

by @admin

12h ago

Apr 28, 2026

Public

Recursively flattens an array of arbitrary nesting depth into a single flat array. Accepts an optional depth limit — pass Infinity to flatten completely. Uses the native Array.flat when the browser supports it, otherwise falls back to a recursive reduce.

JavaScript

const flatDeep = (arr, depth = Infinity) => arr.flat(depth);

// Manual fallback for environments without Array.flat:
function flatDeepManual(arr, depth = Infinity) {
  return depth > 0
    ? arr.reduce(
        (acc, val) =>
          acc.concat(Array.isArray(val) ? flatDeepManual(val, depth - 1) : val),
        []
      )
    : arr.slice();
}

// Usage
const nested = [1, [2, [3, [4, [5]]]]];
console.log(flatDeep(nested));        // [1, 2, 3, 4, 5]
console.log(flatDeep(nested, 2));     // [1, 2, 3, [4, [5]]]

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