mutable array flatten. An array flattening implementation that does not require more memory than the original deep array.

flatten.js

/**
* Multi-level array flattening. Takes an array of arbitrarily deep nested arrays, and returns an array of values.
* It mutates the original array instead of creating a new one.
**/
function flatten(arr) {
    for (let i = 0, inc=1; i < arr.length; i += inc) { 
        if (Array.isArray(arr[i])) {
            const flat = flatten(arr[i]);
            arr.splice(i, 1, ...flat);
            inc = flat.length;
        } else { inc = 1; }
    }
    return arr;
}