chan.js

class Chan {
    constructor() {
        this.waiting = 0;
        this.reads = [];
        this.available = 0;
        this.buffer = [];
        this.closed = false;
    }

    [Symbol.asyncIterator]() {
        return this 
        // {
        //     next: () => this.next()
        // }
    }

    put(value) {
        if(this.closed) {
            console.log("Can't write to a closed channel");
            return;
        }

        if(this.waiting > 0) {
            const r = this.reads.shift();
            this.waiting--;
            r({value, done: false});
        } else {
            this.available++;
            this.buffer.push(value);
        }
    }

    next() {
        if(this.available > 0) {
            this.available--;
            const value = this.buffer.shift();
            return new Promise(r => r({ value, done: false}));
        } else {
            this.waiting++;
            var that = this;
            const p = new Promise(function(r) {
                that.reads.push(r);
            });
            return p;
        }
    }
   
  }
  
const c = new Chan();

(async () => {
    console.log("1", await c.next())
    console.log("2", await c.next())
    console.log("3", await c.next())
    console.log("4", await c.next())
    console.log("5", await c.next())
    console.log("end");
})();

(async () => {
    c.put(1);
    c.put(2);
    c.put(3);
})();