Flat array from a multidimensional array

JavaScriptJunior~1 min read

Example:

javascript

const arr = [1, [2, 3], [4, [5, 6]]];

We want to get -> [1, 2, 3, 4, 5, 6]

1. The simplest way - `.flat()` (ES2019+)

javascript

const flattened = arr.flat();
console.log(flattened); // [1, 2, 3, 4, [5, 6]]

By default flat() "flattens" only one level of nesting.

2. Specify the flattening depth

javascript

const deeplyFlattened = arr.flat(2);
console.log(deeplyFlattened); // [1, 2, 3, 4, 5, 6]

flat(1) -> one level
flat(2) -> two levels
flat(Infinity) -> completely, regardless of depth

Example:

javascript

const nested = [1, [2, [3, [4]]]];
console.log(nested.flat(Infinity)); // [1, 2, 3, 4]

This is the most convenient and modern way to "flatten" an array.

3. Alternative - recursion (the old way, before flat)

javascript

function flatten(arr) {
  return arr.reduce(
    (acc, val) => acc.concat(Array.isArray(val) ? flatten(val) : val),
    []
  );
}

console.log(flatten([1, [2, [3, 4]], 5]));
// [1, 2, 3, 4, 5]

We check each element: if it's an array -> call flatten recursively.
If not -> add it to the result.

4. Via `toString()` (only for numbers/strings)

javascript

[1, [2, [3, 4]]].toString().split(',');
// ["1", "2", "3", "4"]

Works only if there are no objects or null, because everything is turned into strings.

5. Via `flatMap()` (for combining + flatten)

javascript

const arr = [1, 2, [3, 4]];
const result = arr.flatMap(x => (Array.isArray(x) ? x : [x]));

console.log(result); // [1, 2, 3, 4]

flatMap() simultaneously transforms elements and flattens one level.

Short version:

Method	Works with	Depth	Mutates	Modern
`arr.flat()`	array	1 by default	No	Yes (ES2019+)
`arr.flat(Infinity)`	multidimensional	all levels	No	Yes
Recursion	array	any	No	Yes (universal)
`flatMap()`	array	1 level	No	Yes
`toString().split()`	numbers, strings	all levels	No	limited

Conclusion: To "flatten" an array:

javascript
const flat = arr.flat(Infinity);

this is short, safe, and clear.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.