function flatten(arr) {
  return arr.reduce(
    (acc, item) => acc.concat(Array.isArray(item) ? flatten(item) : item),
    [],
  );
}
flatten([1, [2, [3, [4]]]]); // [1, 2, 3, 4]

arr.flat(Infinity) exists, but interviews usually want the recursion. Nested empty arrays ([[], [1], []]) must yield [1].

sandbox.js

function flatten(arr) {
  // TODO
}

Output

Hit “Run tests” to check your solution.

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Next question

JavaScriptMedium10/15

Write flatten(arr) that flattens a nested array at any depth

By FrontendPro Editorial Team Updated 8/7/2026

Explore the JavaScript interview guide

Examples

Write flatten so that the following cases hold.

Inputflatten([1,[2,[3,[4]]]])
Output[1,2,3,4]
Inputflatten([[],[1],[]])
Output[1]
Inputflatten([1,2,3])
Output[1,2,3]

1 more hidden test cases - 4 in total will be graded.

Answer

function flatten(arr) {
  return arr.reduce(
    (acc, item) => acc.concat(Array.isArray(item) ? flatten(item) : item),
    [],
  );
}
flatten([1, [2, [3, [4]]]]); // [1, 2, 3, 4]