Here you can find the source of startswith(str)
String.prototype.startswith = function(str) { return this.slice(0, str.length) === str; };
String.prototype.startsWith = String.prototype.startsWith || function startsWith(searchString, position) { position = position || 0; return this.indexOf(searchString, position) === position; };
String.prototype.startsWith = function startsWith(prefix) { return this.indexOf(prefix) === 0; };
String.prototype.startswith = function(entity) { if (!entity) return false; if ('string' !== typeof(entity)) return false; return 0 === this.indexOf(entity); };
String.prototype.startswith = function (needle) { return this.indexOf(needle) === 0; };
String.prototype.startswith = function(s) { return (this.match("^"+s)==s)
String.prototype.startswith = function(str) { return (this.substring(0, str.length) === str) ? true : false;
String.prototype.startWith = function(pre){ return new RegExp("^("+pre+")(.*)").test(this); };
String.prototype.startWith = function (s) { return this.indexOf(s) == 0
String.prototype.startWith = function(s) { if (s === null || s === "" || this.length === 0 || s.length > this.length) return false; if (this.substr(0, s.length) == s) return true; else return false; return true;