ARRAY FLATTENING METHODS

ECMAScript 2019 added two methods to the Array prototype, flat() and flatMap(), which make array flattening operations much easier. Without these methods, flattening arrays is a nasty business which involved either an iterative or recursive solution.

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NOTE:   flat() and flatMap() are strictly limited to flattening nested arrays. Nested iterable objects such as Map and Set will not be flattened.

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Array.prototype.flatten()

The following is an example of how a simple recursive implementation might look without using these new methods:

function flatten(sourceArray, flattenedArray = []) {
 for (const element of sourceArray) {
  if (Array.isArray(element)) {
   flatten(element, flattenedArray);
  } else {
   flattenedArray.push(element);
  }
 }

 return flattenedArray;
}

const arr = [[0], 1, 2, [3, [4, 5]], 6];

console.log(flatten(arr))
// [0, 1, 2, 3, 4, 5, 6]

In many ways, this example resembles a tree data structure; each element in an...