Simple sort toggle to be used with angular's sortBy. Allows multilevel sorting.

Sort.js

var defaultSortBy = 'lastName';
var sortBy = defaultSortBy.slice();
function toggleSortBy(sort) {
	// if we are using default, switch to array.
	if (typeof sortBy === 'string')
		sortBy = [];

	// if the value is in decending order
	if (sortBy.indexOf('-' + sort) !== -1) {
		// remove it
		sortBy.splice(sortBy.indexOf('-' + sort), 1);

		// if our array is empty, switch to default
		if (sortBy.length === 0)
			sortBy = defaultSortBy.slice();

	// if the value is in accending order
	} else if (sortBy.indexOf(sort) !== -1) {
		// change it to decending order
		sortBy[sortBy.indexOf(sort)] = '-' + sort;

	// if the value is not present
	} else {

		// if we aren't multilevel sorting
		if (!sortMultiLevel)
			sortBy.length = 0; // clear the array

		// add it
		sortBy.push(sort);
	}
}

function isSortedBy(sort, reverse) {
	// if we are default, don't show any arrows
	if (typeof sortBy === 'string')
		return false;

	// return if sort (or if reversed, -sort) is in the array
	return sortBy.indexOf((reverse ? '-' : '') + sort) !== -1;
}