Promise doesn't work as expected

Question

I wanna create serial actions with promise and async/await. Code below. Instead of output: 1 2, I sometimes get 2 1. Does somebody know, how fix my gotcha? It is possible do with only promises? with generators? Also it can be, that onResolve() functions works asynchronously, so I need make them synchronous too.

Sequence:

request.get(payload, onResolve, onReject) // first get

.get(payload, onResolve, onReject) // second get

Run first get() do something with payload(_doSomething(payload)), after launch onResolve() or onReject depends on _doSomething() result, after onResolve() or onReject() launch second get() and same operations as in first get().

class Queue {
  constructor() {
    this.data = [];
  }

  add(record) {
    this.data.unshift(record);
  }

  remove() {
    return this.data.pop();
  }

  isEmpty() {
    return this.data.length === 0;
  }
}

class Request {
  constructor() {
    this._queue = new Queue();
    this._isPending = false;
  }

  _doSomething(payload) {
    setTimeout(() => {
      console.log(payload);
    }, Math.random() * 1000);
  }

  async _runCycle() {
    if (this._queue.isEmpty() || this._isPending) return;

    this._isPending = true;
    
    const { urlOne, onResolve, onReject } = this._queue.remove();
    await Promise.resolve(this._doSomething(urlOne))
      .then(() => onResolve())
      .catch(() => onReject());

    this._isPending = false;
    this._runCycle();
  }

  get(urlOne, onResolve, onReject) {
    
    this._queue.add({ urlOne, onResolve, onReject });
    this._runCycle();

    return this;
  }
}

const request = new Request();

request
  .get('1', ()=>1, ()=>0)
  .get('2', ()=>2, ()=>0);

@appleapple, yes, I need run first get() _doSomething() => onResolve() or onReject(), only then run second get(). — Rami Chasygov, Apr 10 '18 at 09:55
OK. well, your `Promise` has no effect in this example. can you change it a bit so I can understand your question more clear? — apple apple, Apr 10 '18 at 09:56
`Promise.resolve(this._doSomething(urlOne))` is a fancy way to write `this._doSomething(urlOne); Promise.resolve(undefined);` — apple apple, Apr 10 '18 at 10:16
Try using waterfall instead https://stackoverflow.com/questions/25705067/using-async-waterfall-in-node-js — Mohit Mutha, Apr 10 '18 at 10:17

apple apple · Accepted Answer · 2018-04-10T10:37:54.410

1

you can use single Promise chain, but if your current code don't use Promise, please reconsider if it's necessary.

class Request {
  constructor() {
    this.resolver = Promise.resolve()
  }

  foo(x,time) {
    return new Promise((res,rej)=>
      setTimeout(() => {
        console.log(x);
        if(x==1)res();
        else rej();
      }, time));
  }

  get(x, time, res, rej) {
    let req = ()=>this.foo(x,time)
    this.resolver = this.resolver.then(req,req).then(res,rej)
    return this;
  }
}

const request = new Request();

request
  .get('1', 1000, ()=>console.log(11), ()=>console.log(10))
  .get('2', 200, ()=>console.log(21), ()=>console.log(20))

edited Apr 10 '18 at 10:37

answered Apr 10 '18 at 10:26

apple apple

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Only one problem left, if onResolve() also async, e.g. `setTimeout(()=>console.log(11), 1000)` and `setTimeout(()=>console.log(21), 200)`, then resolvers doesn't work in order – Rami Chasygov Apr 10 '18 at 11:00
any idea how I can fix it or I need create another question in stackoverflow? – Rami Chasygov Apr 10 '18 at 11:01
1

`setTimeout` cannot be used without translate to `Promise` (like what I do in `foo`). once it's a function which returns `Promise` there should be all right. – apple apple Apr 10 '18 at 11:32
@RamzanChasygov do you really have `setTimeout` in your code or you have a async function (i.e. function returns `Promise`)? – apple apple Apr 10 '18 at 12:30
async emulation – Rami Chasygov Apr 10 '18 at 12:41
1

not sure what it is, anyway, [here is updated code](https://jsfiddle.net/6m8p2s3q/13/) – apple apple Apr 10 '18 at 12:43
@RamzanChasygov you should not use `then(f).catch(g)`. if `f` throw exception or do whatever reject the `Promise`, `g` would be invoke. – apple apple Apr 10 '18 at 12:58

Promise doesn't work as expected

1 Answers1