Array.prototype.concat()
with an empty array ([]
) and the spread operator (...
) to flatten an array.const deepFlatten = arr =>
[].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));
deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]
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