This function creates another function that can be used to loop through a set of steps.

createSequence.js

/**
 * This function creates another function that can be used to loop through a set of steps. 
 * This could be useful, for example, in creating animations or a set of instructions that need to be followed in order.
 *
 * @param {Array} arr - The array of steps to be looped through
 * @param {number} i - The index to start at
 * @param {function} callback - The function to be called on each step
 * @param {string} key - The key to use for the object
 * @returns {function} - A function that can be used to loop through the steps
 */
function createSequence(arr, i, callback, key) {
	i = i || 0;
	const steps = [];
	arr.forEach(step => {
		if (Array.isArray(step)) {
			for (let j = 0; j < step[0]; j++) {
				steps.push(step[1]);
			}
		} else {
			steps.push(step);
		}
	});

	return function(n) {
		n = n == undefined ? 0 : n;
		if (typeof n === "string") i = n = parseInt(n);

		if (i >= steps.length) i = i % steps.length;
		if (i < 0) i = steps.length + (i % steps.length);

		let step = steps[i];
		i += n;

		if (typeof step === "function") step = step();

		switch (typeof callback) {
			case "function":
				callback(step);
				break;

			case "object":
				if (key) callback[key] = step;
				break;
		}
		return step;
	};
}

// Simple usage

// create a sequence of months
const months = createSequence(["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]);

// calling 
console.log(months()) //gives you the current month
console.log(months(1)) //gives you the current month and advances one

// create a sequence of letters and set the value of the object's "letter" property to the current letter
const myObject = {};
const letterSequence = createSequence(["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"], 0, myObject, "letter");

// calling 
console.log(letterSequence(), myObject) //gives you the current letter, also sets it as myObject.letter
console.log(letterSequence(1), myObject) //gives you the current letter and advances one, also sets it as myObject.letter


// create a sequence of numbers and set call a callback on it
const numberSequence = createSequence([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 0, function(step) {
	console.log("Do something with step like multiply by 5 = ", step * 5);
});

// calling 
console.log(numberSequence()) //gives you the current number, calls callback with it
console.log(numberSequence(1)) //gives you the current number and advances one, calls callback with it

// demo of a sequence with a number on the times a item should be inserted
const repeatDemoSequence = createSequence([[2,'Hello'], [4,'World']]);

// calling repeatDemoSequence(1) repeatedly would output
// "Hello", "Hello", "World", "World", "World", "World"