A few vanilla JavaScript utility functions

vanillaJSutils.js

// 1. Add and remove classes jQuery-style

/**
 * Determines if a given thing is in a given array.
 * @param  {any} x An object or primitive whose presence in a is being queried
 * @param  {Array} a Array being queried
 * @return {number}    -1 if x could not be found in a; otherwise index where it was first found, so 0 would be minimum value
 */
function isThingInArray(x, a) {
	if ( Array.isArray(a) ) {
		for (var i = a.length - 1; i >= 0; i--) {
			if ( x === a[i] ) { return a.indexOf(x); }
		};
	}
	// either a is not an array or x is not in a
	return -1;
}

/**
 * Adds class to DOM element if it's not already a class of that element.
 * @param {String} s  Class name to add
 * @param {DOM Node} el Element that will take new class
 */
function addClassToEl(s, el) {
	var elClassList = el.className.split(" ");
	if ( isThingInArray( s, elClassList ) < 0 ) {
		elClassList.push(s);
		el.className = elClassList.join(" ");
	}
}

/**
 * Removes class from DOM element, even if it's present more than once.
 * @param  {String} s  Class name to remove
 * @param  {DOM Node} el Element that will lose class name
 */
function removeClassFromEl(s, el) {
	var elClassList = el.className.split(" "),
		i;
	i = isThingInArray( s, elClassList );

	if ( i >= 0 ) {
		//use while in case s is present multiple times
		while ( i >= 0) {
			// remove ith item in classList
			elClassList.splice(i, 1);
			// update counter within loop
			i = isThingInArray( s, elClassList );
		}
		el.className = elClassList.join(" ");
	}

}

// 2. Wrap code in "ready()"
document.addEventListener('readystatechange', () => {
	if (document.readyState === 'complete') {
		// put code here
	}
});

// 3. Is this a touch device?

/**
 * Return true if device is a touch device
 */
function isTouchDevice() {
	return ('ontouchstart' in document.doucumentElement);
}