Here you can find the source of startsWith(string)
// String.prototype.startsWith String.prototype.startsWith = function (string) { var index = arguments.length < 2 ? 0 : arguments[1]; return this.slice(index).indexOf(string) === 0; };
String.prototype.startsWith = function(str) { if (str == this.substring(0, str.length)) return true; return false;
String.prototype.startsWith = function(str) { return this.substr(0, str.length) == str; String.prototype.endsWith = function(str) { return this.substr(this.length - str.length) == str;
String.prototype.startsWith = function(str) { return this.indexOf(str) === 0; };
String.prototype.startsWith = function(str, ignoreCase) { return (ignoreCase ? this.toUpperCase() : this) .indexOf(ignoreCase ? str.toUpperCase() : str) == 0; };
String.prototype.startsWith = function (string) { var index = arguments.length < 2 ? 0 : arguments[1]; return this.slice(index).indexOf(string) === 0; };
String.prototype.startsWith = function(string){ if (typeof(string) !== "string") { throw new Error("Input argument was not a string"); } else if (this.substring(0, string.length) === string) { return true; } else { return false; }; ...
String.prototype.startsWith = function (string){ return this.indexOf(string) === 0; };
String.prototype.startsWith = function(substr) { for (var i = 0; i < substr.length; i++) { if (this[i] !== substr[i]) { return false; return true;
String.prototype.startsWith = function(substr) { return (this.indexOf(substr) == 0); };