es6-string-prototype-startswith.js: String.prototype.startsWith の Polyfill (ES6 規定)

es6-string-prototype-startswith.js

/**
 * es6-string-prototype-startswith.js
 * String.prototype.startsWith (ECMA-262 6th Edition / ECMAScript 2015)
 *
 *
 * @version 1.0.0
 * @author think49
 * @url https://gist.github.com/think49/908b8d5f08c9945beea7
 * @license http://www.opensource.org/licenses/mit-license.php (The MIT License)
 * @see <a href="http://www.ecma-international.org/ecma-262/6.0/#sec-string.prototype.startswith">21.1.3.18 String.prototype.startsWith – ECMA-262 6th Edition</a>
 */
'use strict';

if (typeof String.prototype.startsWith !== 'function') {
  (function (Object, String, RegExp, max, min) {
    Object.defineProperty(this, 'startsWith',
      {
        writable: true,
        enumerable: false,
        configurable: true,
        value: function startsWith (searchString /* [, position] */) {
          var thisArg, position, length;

          if (this === null || typeof this === 'undefined') {
            throw new TypeError('String.prototype.startsWith called on null or undefined');
          }

          if (searchString instanceof RegExp) {
            throw new TypeError('First argument to String.prototype.startsWith must not be a regular expression');
          }

          thisArg = String(this);
          searchString = String(searchString);
          length = searchString.length;
          position = arguments.length < 2 ? 0 : min(max(~~arguments[1], 0), length);

          if (position + length > thisArg.length) {
            return false;
          }

          return thisArg.lastIndexOf(searchString, position) === position;
        }
      });
  }.call(String.prototype, Object, String, RegExp, Math.max, Math.min));
}