Semaphore

semaphore.js

class S {
    constructor(n) {
        this.n = n
        this.waiting = []
    }
    acquire() {
        const self = this
        let p = new Promise(function (resolve, reject) {
            self.waiting.push(resolve)
        })
        this._check()
        return p
    }
    release() {
        this.n++
        this._check()
    }
    _check() {
        while (this.n > 0 && this.waiting.length != 0) {
            this.n--
            this.waiting.pop()(this.n)
        }
    }
}

function run(s, i) {
    console.log(`Working ${i}`);
    setTimeout(() => s.release(), 3000)
}
(async function() {
    let s = new S(3)
    let i = 0
    while (true) {
        await s.acquire()
        run(s, i++)
    }
})()