JavaScript implementation Ruby's str.insert method

String.insert.js

/**
 * 'Hello world'.insert(6, 'happy ') == 'Hello happy world'
 * 'Hello world'.insert(-1, '!!!') == 'Hello world!!!'
 * @param at {Number} index
 * @param text {String}
 * @see http://ruby-doc.org/core/classes/String.html#M000773
 */
String.prototype.insert = function (at, text) {
    if (at < 0) {
        at += this.length + 1;
    }
    return this.substr(0, at) + text + this.substr(at);
}