Concurrency Limit

const CONCURRENCY_LIMIT = 3 function asyncTaskRunner(tasks) { const results = new Array(tasks.length).fill(0) const totalTasks = tasks.length let currentTaskIndex = 0 const runner = (resolve) => { if (!Array.isArray(tasks)) { throw new Error("tasks must be Array"); } let taskQueue = [] const runNext = async () => { if (taskQueue.length === 0 && currentTaskIndex === totalTasks) { resolve(results) return } if (taskQueue.length >= CONCURRENCY_LIMIT || tasks.length === 0) { return } const taskIndex = currentTaskIndex; currentTaskIndex += 1 const task = tasks.shift() console.log('scheduling task ', taskIndex) taskQueue.push(task) task() .then(result => { console.log('finished task: ', taskIndex) results[taskIndex] = result taskQueue = taskQueue.filter((t => t !== task)) runNext() }) } for (let i = 0 ; i < CONCURRENCY_LIMIT; i++) { if (tasks.length === 0) { break } const taskIndex = currentTaskIndex; currentTaskIndex += 1 const task = tasks.shift() console.log('scheduling task ', taskIndex) taskQueue.push(task) task() .then(result => { console.log('finished task: ', taskIndex) results[taskIndex] = result taskQueue = taskQueue.filter((t => t !== task)) runNext() }) } }; return new Promise((res) => runner(res)); } const t1 = () => new Promise(res => setTimeout(() => res('t1'), 3000)) const t2 = () => new Promise(res => setTimeout(() => res('t2'), 200)) const t3 = () => new Promise(res => setTimeout(() => res('t3'), 1500)) const t4 = () => new Promise(res => setTimeout(() => res('t4'), 5000)) const t5 = () => new Promise(res => setTimeout(() => res('t5'), 4000)) const t6 = () => new Promise(res => setTimeout(() => res('t6'), 1000)) const tasks = [t1, t2, t3, t4, t5, t6] asyncTaskRunner(tasks) .then(console.log)