flatMap/monad in JavaScript

gistfile1.txt

Array.prototype.flatMap = function(lambda) { 
  return [].concat.apply([], this.map(lambda)); 
};

const r = [1, 2, 3].flatMap(x => {
	var arr = new Array(x);
	for (var i=0; i<arr.length; ++i) {
		arr[i] = x;
	}
	return arr;
});

console.log(r);
//output: [ 1, 2, 2, 3, 3, 3 ]