Pure JavaScript (JS) Array Flatten method (like Ruby Array#flatten)

js_array_flatten.js

Array.prototype.flatten = function(limit) {
  var level = 0;
  if(limit === undefined) limit = Number.POSITIVE_INFINITY;
  if(arguments.length > 1) level = arguments[1];

  return this.reduce(function(a, b) {
    if((b instanceof Array) && level < limit)
      a = a.concat(b.flatten(limit, level + 1));
    else
      a.push(b);
    return a;
  }, []);
};

console.log([1, 2, 3].flatten()); // => [1, 2, 3]
console.log([[1], 2, 3].flatten()); // => [1, 2, 3]
console.log([[1], 2, 3].flatten(1)); // => [1, 2, 3]
console.log([[[1]], 2, 3].flatten(1)); // => [[1], 2, 3]
console.log([[[[[1]],2]],3].flatten(1)); // => [[[[1]], 2], 3]
console.log([[[[[1]],2]],3].flatten(2)); // => [[[1]], 2, 3]
console.log([[[[[1]],2]],3].flatten(3)); // => [[1], 2, 3]