Flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].

flatten.js

String.prototype.replaceAll = function(search, replacement) {
    var target = this;
    return target.split(search).join(replacement);
};

Array.prototype.flatten = function() {
	return JSON.stringify(this).replaceAll('[','').replaceAll(']', '').split(',').map(item => parseInt(item));
};

var a = [[1,2,[3]],4];

console.log(a.flatten())