Easily insert a string within another string.

String.prototype.insert.js

String.prototype.insert = function(strToInsert, num_rgx_str, opt_after) {
    return ({}).toString.call(num_rgx_str) == '[object Number]'
        ? this.slice(0, num_rgx_str + strToInsert + this.slice(num_rgx_str)
        : this.replace(num_rgx_str, function(match) {
            return opt_after ? match + strToInsert : (strToInsert + match);
        });
};