Bullet-proof function to transform an array into a map using a callback function to determine the key

mapAssociative.js

// Similar to Array.prototype.map, this function takes an array-like object and turns it into an associative map
// Instead of the callback function being used to transform the value in the returned array, it is called to get
// the key instead. Unlike map, this function will use the second param in place of `this`, if `this` isn't bound
function mapAssociative(callback, thisArg) {
	var arr, len;

	/*jshint validthis:true */
	arr = this || thisArg;
	/*jshint validthis:false */
	if (arr === window) {
		arr = thisArg;
	}

	if (arr == null || !(length in arr)) {
		throw new TypeError(' this is not an array or array-like');
	}

	if (typeof callback !== 'function') {
		throw new TypeError(callback + ' is not a function');
	}

	arr = Object(arr);      // transform to object
	len = arr.length >>> 0; // transform to uint32

	var obj = {};
	for (var i = 0; i < len; i++) {
		if (i in arr) {
			obj[callback.call(thisArg, arr[i], i, arr)] = arr[i];
		}
	}
	return obj;
}