How to Flatten Javascript Arrays?

Given an array in Javascript, Flatten its elements so it becomes one-dimension. For example, flatten([1, [[2], 3, 4], 5]) becomes [1, 2, 3, 4, 5].

In ES6, you can use the array.Prototype.flatten method which flattens the elements of an array. The parameter specifies the depth the flattening process goes to – default is two. You can pass in Infinity to flattens the array no matter how deep the elements are.

const a = [1, 2, [3, 4, 5, [6, 7]], 8];
undefined
a.flat(Infinity)
(8) [1, 2, 3, 4, 5, 6, 7, 8]
a.flat()
(7) [1, 2, 3, 4, 5, Array(2), 8]

Before ES6, you can write a flatten method like this:

const flatten = (arr) => { 
  let result = [];
  arr.forEach(function (v) {
    if (Array.isArray(v)) {
      result = result.concat(flatten(v));
    } else {
      result.push(v);
    }
  });
  return result;
} 

The idea is to iterate the array using forEach and if the element itself is an array (Array.isArray) – we recursively flatten and concat to the result array. Otherwise, the element is simply pushed to result array.

NodeJs / Javascript

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