node / java script: perform a list of tasks with limit parallelism

concurrent.js

async function doConcurrent(parallelity, tasks) {
	const promises = [];
	const chain = async (f1, f2) => {
		await f1();
		await f2();
	};
	const next = () => {
		if (tasks.length) {
			// console.log('start next, task count is ' + tasks.length);
			var task = tasks.shift();
			promises.push(chain(task, next));
		}
	}
	const initial = tasks.slice(0, parallelity);
	tasks = tasks.slice(parallelity);
	initial.forEach(t => promises.push(chain(t, next)));
	while (tasks.length) {
		// console.log('await all, promise count is ' + promises.length);
		await Promise.all(promises);
	}
	await Promise.all(promises);
	// console.log('everything finished');
}


function sleep(ms){
    return new Promise(resolve=>{
        setTimeout(resolve,ms)
    });
}


async function task(name) {
	console.log('start ' + name + '..');
	await sleep(500 + Math.floor(Math.random() * 2000));
	console.log('...finished ' + name);
}


await doConcurrent(10, [() => task('test')])
await doConcurrent(2, [1,2,3,4,5].map(v => () => task(v)));
// line 42 reserved for answer to big question