Run an asynchronous operation on each item in an array, waiting for the previous iteration to complete before doing the next one

slowLoop.js

/**
 * Execute the loopBody function once for each item in the items array, 
 * waiting for the done function (which is passed into the loopBody function)
 * to be called before proceeding to the next item in the array.
 * @param {Array} items - The array of items to iterate through
 * @param {Function} loopBody - A function to execute on each item in the array.
 *		This function is passed 3 arguments - 
 *			1. The item in the current iteration,
 *			2. The index of the item in the array,
 *			3. A function to be called when the iteration may continue.
 * @returns {Promise} - A promise that is resolved when all the items in the 
 *		in the array have been iterated through.
 */
function slowLoop(items, loopBody) {
	return new Promise(f => {
		let done = arguments[2] || f;
		let idx = arguments[3] || 0;
		let cb = items[idx + 1] ? () => slowLoop(items, loopBody, done, idx + 1) : done;
		loopBody(items[idx], idx, cb);
	});
}