Flatten Multidimensional Array with Javascript

Problem

You have a nested array structure and you want to convert it into a single-dimensional array.

const arr = [[1, 2], [3, 4], [5, 6, [7, 8, [9, 10]]]]

Solution

The approach uses Array.prototype.reduce() combined with recursion. The method checks whether each element is an array using Array.isArray(). For arrays, it recursively flattens nested structures; for non-arrays, it appends values directly.

const flatten = (arr) =>
  arr.reduce(
    (acc, val) => (Array.isArray(val) ? acc.concat(flatten(val)) : acc.concat(val)),
    []
  )

Usage

flatten(arr) // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

References

• Array.prototype.reduce() - MDN
• Array.prototype.concat() - MDN