SOURCE

class PromiseLimit {
    _tasks = []
    _count = 0
    constructor (limit) {
        this.limit = limit
    }

    add (promise) {
        return new Promise((resolve, reject) => {
            let task = this.createTask(promise, resolve, reject)
            if (this._count >= this.limit) {
                this._tasks.push(task)
            } else {
                task()
            }
        })
    }
    createTask (promise, resolve, reject) {
        return () => {
            promise().then(
                v => resolve(v),
                r => reject(r)
            ).finally(() => {
                this._count--
                if (this._tasks.length) {
                    let task = this._tasks.shift()
                    task()
                }
            })
            this._count++
        }
    }
}