Cooperative multitasking for javascript

multitasking.js

function multiTasking(schedule = setTimeout) {
	const taskList = new Set();
	let isRunning = false;
	let isScheduled = false;

	const next = () => {
		isScheduled = false;
		taskList.forEach(task => {
			if (!isRunning) {
				return;
			}
			if (task.next().done) {
				remove(task);
			}
		});
		if (isRunning && taskList.size) {
			run();
		} else {
			stop();
		}
	};

	const run = () => {
		if (!isScheduled) {
			isScheduled = true;
			console.log('scheduling');
			schedule(next);
		}
	}

	const start = () => {
		if (!isRunning) {
			isRunning = true;
			console.log('starting');
			run();
		}
	};

	const stop = () => {
		console.log('stopped');
		isRunning = false;
	};

	const add = (...tasks) => {
		tasks.forEach(task => {
			taskList.add(task);
			start();
		});
		return tasks.length > 1 ? tasks : tasks[0];
	};

	const remove = (...tasks) => {
		tasks.forEach(task => {
			taskList.delete(task);
		});
	};

	const clear = () => {
		taskList.clear();
	};

	return {
		start,
		stop,
		add,
		remove,
		clear
	};
}

Each task needs to be a generator function that yields control back to the task manager (cooperative multitasking):

function* task1() {
  while(true) {
    // do some work, then
    yield;
  }
}

You can pass a custom scheduler function, e.g. one that runs always one second after the last run:

const timeoutScheduler = (timeout) => (next) => setTimeout(next, timeout);
const tm = multiTasking(timeoutScheduler(1000));