Deep flattens an array.

Use recursion.
Use Array.prototype.concat() with an empty array ([]) and the spread operator (...) to flatten an array.
Recursively flatten each element that is an array.

代码片段

const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));

使用样例

deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]