startswith.js

/*! https://mths.be/startswith v0.2.0 by @mathias */
if (!String.prototype.startsWith) {
	(function() {
		'use strict'; // needed to support `apply`/`call` with `undefined`/`null`
		var defineProperty = (function() {
			// IE 8 only supports `Object.defineProperty` on DOM elements
			try {
				var object = {};
				var $defineProperty = Object.defineProperty;
				var result = $defineProperty(object, object, object) && $defineProperty;
			} catch(error) {}
			return result;
		}());
		var toString = {}.toString;
		var startsWith = function(search) {
			if (this == null) {
				throw TypeError();
			}
			var string = String(this);
			if (search && toString.call(search) == '[object RegExp]') {
				throw TypeError();
			}
			var stringLength = string.length;
			var searchString = String(search);
			var searchLength = searchString.length;
			var position = arguments.length > 1 ? arguments[1] : undefined;
			// `ToInteger`
			var pos = position ? Number(position) : 0;
			if (pos != pos) { // better `isNaN`
				pos = 0;
			}
			var start = Math.min(Math.max(pos, 0), stringLength);
			// Avoid the `indexOf` call if no match is possible
			if (searchLength + start > stringLength) {
				return false;
			}
			var index = -1;
			while (++index < searchLength) {
				if (string.charCodeAt(start + index) != searchString.charCodeAt(index)) {
					return false;
				}
			}
			return true;
		};
		if (defineProperty) {
			defineProperty(String.prototype, 'startsWith', {
				'value': startsWith,
				'configurable': true,
				'writable': true
			});
		} else {
			String.prototype.startsWith = startsWith;
		}
	}());
}