Recursive Array.flat

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There was much talk about Array.prototype.flat during its early stages, starting with the name alone. Many developers preferred the name flatten but the spec differed from MooTools’ implementation. MooTools would recursively flatten an array but the new, official flat implementation was only one level of flattening.

The current implementation of Array.prototype.flat is:

[1, 2, [3], [[4]]].flat();
// [1,2,3,[4]]

.flat only flattens arrays to one levels, but what if you want a truly flattened array? MDN provides a really great solution to recursively flatten an array:

function flattenDeep(arr1) {
   return arr1.reduce((acc, val) => Array.isArray(val) ? acc.concat(flattenDeep(val)) : acc.concat(val), []);
}

flattenDeep([1, 2, [3], [[4]], [[[[[[6]]]]]]])

// [1,2,3,4,6]

To recursively flatten an array, you must use Array.prototype.reduce with .flat.

I wish a second argument was added to flat to represent if you wanted a recursive flatten, with the default being false to keep the current behavior. I find the method name a bit misleading but I understand why they went to a single level. The method name smush was thrown around, which would’ve been the worst method name since stringify!

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