Highly performant ES6 String extras polyfills for modern browsers.

string-extras.js

if (!('startsWith' in String.prototype))
	Object.defineProperty(String.prototype, 'startsWith', {
		value: function (searchString, position) {
			if (Object.prototype.toString.call(searchString) === "[object RegExp]")
				throw new TypeError("The search string cannot be a regular expression.");
			if (position === undefined)
				position = 0;
			else
				position = Math.min(Math.max(position, 0), this.length);
			return this.lastIndexOf(searchString, position) === position;
		},

		configurable: true, writable: true,
	});

if (!('endsWith' in String.prototype))
	Object.defineProperty(String.prototype, 'endsWith', {
		value: function (searchString, endPosition) {
			if (Object.prototype.toString.call(searchString) === "[object RegExp]")
				throw new TypeError("The search string cannot be a regular expression.");
			if (endPosition === undefined)
				endPosition = this.length;
			else
				endPosition = Math.min(Math.max(endPosition, 0), this.length);
			var start = endPosition - searchString.length;
			if (start < 0) return false;
			return this.indexOf(searchString, start) === start;
		},

		configurable: true, writable: true,
	});

if (!('contains' in String.prototype))
	Object.defineProperty(String.prototype, 'contains', {
		value: function (searchString, position) {
			if (Object.prototype.toString.call(searchString) === "[object RegExp]")
				throw new TypeError("The search string cannot be a regular expression.");
			if (position === undefined)
				position = 0;
			else
				position = Math.min(Math.max(position, 0), this.length);
			return this.indexOf(searchString, position) !== -1;
		},

		configurable: true, writable: true,
	});

if (!('repeat' in String.prototype))
	Object.defineProperty(String.prototype, 'repeat', {
		value: function (count) {
			if (count < 0 || count === Infinity)
				throw new RangeError("The number of times the string is to be repeated must be in [0, Infinity).");
			if (count === 0)
				return '';
			else if (count === 1)
				return this.valueOf();

			var result = '', pattern = this;
			for (;;) {
				if (count & 1) result += pattern;
				if (count >>= 1) pattern += pattern;
				else return result;
			}
		},

		configurable: true, writable: true,
	});

Copied from http://www.snip2code.com/Snippet/69036/Highly-performant-ES6-String-extras-poly