Playing around with implementing ES5 Array extras

array-extras.js

/**
 * Attach function to Array.prototype, with the given implementation.
 */
function augmentArrayPrototypeWith(func, impl) {
	if (typeof Array.prototype[func] !== 'function') {
		Array.prototype[func] = function() {
			return impl.apply(this, arguments);
		};
	}
}

augmentArrayPrototypeWith('indexOf', function(obj/*, fromIndex*/) {
	var len = checkArray.apply(this);
	
	var k = normalizeFromIndex.call(this, 0, arguments);
	for (; k < len; k++) {
		if (this[k] === obj) {
			return k;
		}
	}
	return -1;
});

augmentArrayPrototypeWith('lastIndexOf', function(obj/*, fromIndex*/) {
	var len = checkArray.apply(this);
	
	var k = normalizeFromIndex.call(this, len, arguments);
	for(; k >= 0; k--) {
		if (this[k] === obj) {
			return k;
		}
	}
	return -1;
});

function checkArray() {
	if (this && this.constructor !== Array) {
		throw new TypeError();
	}
	return (this.length !== 0 ? this.length : -1);
}

function normalizeFromIndex(other, args) {
	var fromIndex = other;
	if (args.length > 1) {
		fromIndex = Number(args[1]);
		if (fromIndex !== fromIndex) { //NaN check
			fromIndex = other;
		} else if (fromIndex !== 0 && fromIndex !== (1 / 0) && fromIndex !== -(1 / 0)) {
			fromIndex = (fromIndex > 0 || -1) * Math.floor(Math.abs(fromIndex));
		}
	}
	return fromIndex >= 0 ? fromIndex : other;
}