Flatten One Level

Console

IntermediateArrays

Task

Implement `flatten(arr)` that merges nested arrays **one level deep** into a single array: `[[1,2],[3,[4]]]` → `[1,2,3,[4]]` (inner-nested values stay as arrays). Note the built-in arr.flat() solves exactly this — implement it yourself first, then compare.

Hints

0 / 2

concat can merge everything: [].concat(...arr) or arr.reduce((acc, x) => acc.concat(x), []).
The built-in arr.flat() with no arguments flattens exactly one level — same behavior you're implementing.

Verification tests

flatten([[1,2],[3],[4,5]])→ [1,2,3,4,5]
flatten([["a"],["b"]])→ ["a","b"]
flatten([[1],[2,[3]]])→ [1,2,[3]]
flatten([])→ []

⚠️ Reveal official solutions

Try solving it yourself first — struggling is where the learning happens.

Use when asked to implement from scratch without built-ins.

function flatten(arr) {
  const out = [];
  for (const item of arr) {
    if (Array.isArray(item)) {
      for (const x of item) out.push(x);
    } else {
      out.push(item);
    }
  }
  return out;
}

Recommended — prefer the built-in in real code.

function flatten(arr) {
  return arr.flat();
}