Flatten an array in javascript

flatten.js

Array.prototype.flatten = function () {
  var flat_arr = [];
  var dirty = false;
  
  for (var i = 0; i < this.length; i++) {
  	if (typeOf(this[i]) == "array") {
    	flat_arr = flat_arr.concat(this[i]);
    	dirty = true;
    } else {
    	flat_arr.push(this[i]);
    }
  }
  
  if (dirty) {
  	return flat_arr.flatten();
  } else {
  	return flat_arr;
  }
}