Promise.resolve() pass callback parameters

Question

How to return values from promise in this case:

 async foo() {
   let [error, data] = await this.promiseFoo();
 ....
 }

 promiseFoo() {
   return Promise.resolve(this.asyncFoo()).then((error, data) => 
   [error, data]);
 }

Getting error and data undefined inside then(). How can I return error and data from asyncFoo? When callback it looks like: this.asyncFoo((error, data) => {...});

But I what to avoid using callback this time.

`asyncFoo` takes the callback as an **argument**. You might be interested in `promisify` found in libraries like Bluebird. Alternatively, create a wrapper for asyncFoo which creates a new Promise. — cbr, Oct 27 '17 at 17:11
A promise never resolves with *both* an error and a result. Why would you want to return them as a pair? Why not use the normal `try { let data = await this.asyncFoo(); … } catch(error) { … }`? — Bergi, Oct 28 '17 at 08:19
it's necessary for business logic to return both error and data — MolecularMan, Oct 28 '17 at 09:58

score 1 · Accepted Answer · answered Oct 28 '17 at 08:20

1

I think you're looking for

promiseFoo() {
    return this.asyncFoo().then(data => [null, data], error => [error, null]);
}

answered Oct 28 '17 at 08:20

Bergi

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well your answer was right if he wanted to do try catch but he want to get the error as a resolve and not as catch – Amit Wagner Oct 28 '17 at 17:53
2

@AmitWagner The onrejected handler fulfills the returned promise as well? The code does pretty much the same as your answer but is safer, simpler, shorter and more efficient. – Bergi Oct 28 '17 at 20:36
i checked what you said and its works great. to bad i didn't know that before. could have saved some code writing good tip(: – Amit Wagner Oct 28 '17 at 22:01
@AmitWagner See the [difference between `.then(…, …)` and `.then(…).catch(…)`](https://stackoverflow.com/q/24662289/1048572) for details – Bergi Oct 28 '17 at 22:35

Amit Wagner · Answer 2 · 2017-10-28T06:06:20.063

0

try change your promiseFoo to

promiseFoo() {
   return new Promise ((resolve,reject) =>{
     asyncFoo()
     .then( data =>{
        resolve([null, data])
     }  )
     .catch(err =>{
        resolve([err,null])
     })
   })
 }

edited Oct 28 '17 at 06:06

answered Oct 27 '17 at 17:57

Amit Wagner

3,134
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Avoid the [`Promise` constructor antipattern](https://stackoverflow.com/q/23803743/1048572?What-is-the-promise-construction-antipattern-and-how-to-avoid-it)! – Bergi Oct 28 '17 at 08:18

Promise.resolve() pass callback parameters

2 Answers2