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Array.map()

Pushes support back to at least IE6.

/**
 * Array.map() polyfill
 */
// From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map
// Production steps of ECMA-262, Edition 5, 15.4.4.19
// Reference: http://es5.github.io/#x15.4.4.19
if (!Array.prototype.map) {

	Array.prototype.map = function(callback/*, thisArg*/) {

		var T, A, k;

		if (this == null) {
			throw new TypeError('this is null or not defined');
		}

		// 1. Let O be the result of calling ToObject passing the |this|
		//    value as the argument.
		var O = Object(this);

		// 2. Let lenValue be the result of calling the Get internal
		//    method of O with the argument "length".
		// 3. Let len be ToUint32(lenValue).
		var len = O.length >>> 0;

		// 4. If IsCallable(callback) is false, throw a TypeError exception.
		// See: http://es5.github.com/#x9.11
		if (typeof callback !== 'function') {
			throw new TypeError(callback + ' is not a function');
		}

		// 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
		if (arguments.length > 1) {
			T = arguments[1];
		}

		// 6. Let A be a new array created as if by the expression new Array(len)
		//    where Array is the standard built-in constructor with that name and
		//    len is the value of len.
		A = new Array(len);

		// 7. Let k be 0
		k = 0;

		// 8. Repeat, while k < len
		while (k < len) {

			var kValue, mappedValue;

			// a. Let Pk be ToString(k).
			//   This is implicit for LHS operands of the in operator
			// b. Let kPresent be the result of calling the HasProperty internal
			//    method of O with argument Pk.
			//   This step can be combined with c
			// c. If kPresent is true, then
			if (k in O) {

				// i. Let kValue be the result of calling the Get internal
				//    method of O with argument Pk.
				kValue = O[k];

				// ii. Let mappedValue be the result of calling the Call internal
				//     method of callback with T as the this value and argument
				//     list containing kValue, k, and O.
				mappedValue = callback.call(T, kValue, k, O);

				// iii. Call the DefineOwnProperty internal method of A with arguments
				// Pk, Property Descriptor
				// { Value: mappedValue,
				//   Writable: true,
				//   Enumerable: true,
				//   Configurable: true },
				// and false.

				// In browsers that support Object.defineProperty, use the following:
				// Object.defineProperty(A, k, {
				//   value: mappedValue,
				//   writable: true,
				//   enumerable: true,
				//   configurable: true
				// });

				// For best browser support, use the following:
				A[k] = mappedValue;
			}
			// d. Increase k by 1.
			k++;
		}

		// 9. return A
		return A;
	};
}