ECMAScript 2015 call Promises sequentially

Question

I am trying to rewrite a synchronous junction to work with promises, and am a bit stuck with it. I have a function that call different routines A, B, and C, depending on arguments and results:

const worker = (v, r, ok, er)=>{
  if(v > 10) {
    ok(r)
  } else {
    er(r)
  }
};

const A = v=>{let r = null; worker(v, 'a', _r=>r=_r, ()=>{});return r};
const B = v=>{let r = null; worker(v, 'b', _r=>r=_r, ()=>{});return r};
const C = v=>{let r = null; worker(v, 'c', _r=>r=_r, ()=>{});return r};

const mainSync = (obj)=>{
  let result = null;
  if(obj.a) {
    result = A(obj.a);
  }
  if (!result && obj.b) {
    result = B(obj.b);
  }
  if (!result && obj.c) {
    result = C(obj.c);
  }
  return result;
}

which works fine https://repl.it/JcjE/0 with synchronous A, B, and C:

mainSync({a:4}) === null;
mainSync({a:4, b:14}) === 'b';
mainSync({a:14, b:14}) === 'a';
mainSync({b:4, c:14}) === 'c';
// etc

Now A, B, and C become Promises:

const worker = (v, r, ok, er)=>{
  if(v > 10) {
    ok(r)
  } else {
    er(r)
  }
};

const A = v=>new Promise((ok, er)=>worker(v, 'a', ok, er));
const B = v=>new Promise((ok, er)=>worker(v, 'b', ok, er));
const C = v=>new Promise((ok, er)=>worker(v, 'c', ok, er));

and I am not quite sure how to handle it:

const mainAsync = (obj)=>{
    // what todo here?
}

I am happy with mainAsync to return Promise itself, like

mainAsync({a:4}).then(r=>r === null);
mainAsync({a:4, b:14}).then(r=>r === 'b');
mainAsync({a:14, b:14}).then(r=>r === 'a');
mainAsync({b:4, c:14}).then(r=>r === 'c');

The problem is that call to B depends on result of A, and call to C depends on results of both A and B, and no async/await available yet.

I have tried my naive approach https://repl.it/Jcjw/0 but it is terrible and doesn't quite work on real life scale.

PS: I am looking for vanilla javascript if possible, and am aware about similar questions like

• Sequential call of promises,
• How to execute promises sequentially, passing the parameters from an array?,

etc, but couldn't figure out how to apply them to my case.

Answer 1

Depending on which type of browsers you are trying to target, you can use the async/await features that are in chrome for sure.

function promiseA(){return Promise.resolve(20)}
function promiseB(arg){return Promise.resolve(arg * 2)}
function promiseC(arg){return Promise.resolve(arg + 10)}

(async function(){
  let a = await promiseA();
  console.log(a)
  let b = await promiseB(a);
  console.log(b)
  let c = await promiseC(b);
  console.log(c)
})();

Answer 2

To call the promises in sequence, you can call the next one in the .then callback. Your conditions (if (!result && ...)) translate pretty easily:

function mainAsync(obj) {
  return (obj.a ? A(obj.a) : Promise.resolve())
    .then(result => !result && obj.b ? B(obj.b) : result)
    .then(result => !result && obj.c ? C(obj.c) : result);
}

---

If you need to do this for many properties, then you can avoid repeating yourself too much by using a lookup table and a loop (Array#reduce in this case):

const funcs = {
  a: A,
  b: B,
  c: C,
};
const props = ['a', 'b', 'c'];

function mainAsync(obj) {
  return props.reduce(
    (promise, prop) => promise.then(
      result => !result && obj[prop] ? funcs[prop](obj[prop]) : result
    ).catch(() => Promise.resolve(null)),
    Promise.resolve(null)
  );
}

Answer 3

mainAsync = mainSync;

or maybe more easy:

function mainAsync({a,b,c}){
  if(c) return C(c);
  if(b) return B(b);
  if(a) return A(a);
}

If you want all promises to be fullfilled before returning:

function mainAsync({a,b,c}){
 var promises=[];
  if(a) promises.push(A(a));
  if(b) promises.push(B(b));
  if(c) promises.push(C(c));
  return Promise.all(promises).then(val=>val.pop())
}

mainAsync({a:1,b:2,c:3});

Answer 4

If using ES7 transpilers (for async/await) is not an option, you can implement an async control flow using ES2015 generators. Here's a naive example implementation:

function *main() {
  let a = yield Promise.resolve(5);
  let b = yield Promise.resolve(a + 10);
  let c = yield Promise.resolve(b + 15);

  // place the rest of your code here
  console.log(c); // prints 30
}

function execAsync(generator, previousValue) {
  const nextValue = generator.next(previousValue);
  if (!nextValue.done) {
    let promise = nextValue.value;
    // FIXME: promise rejection is not handled
    promise.then(function (value) {
      execAsync(generator, value);
    });
  }
}

// ...    

// start the execution
execAsync(main());