Access promise instance that returns from async function

Question

Async function automatically returns promise - I wonder if there is a way somehow to get this instance of this promise inside the function For example if I return an actual promise like this:

const getSomePromise = () => {
    const promise = new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve('success');
       }, 1000);
    })

    promise.someProp = 'myProp';
    return promise;
}

const promise = getSomePromise();
console.log(promise.someProp);

I want to achieve the same thing with pure async function:

const sleep = ts => new Promise(resolve => setTimeout(resolve, ts));

const getSomePromise =  async () => {
     const p = await sleep(1000);
     // some how access the instance of the promise from within the async function 
     // for example this['someProp'] = 'myProp';
     // and return the all promise with this prop
     return 'sucess';
}
const promise = getSomePromise();
console.log(promise.someProp);

Can I do that ?

Thanks

No, this is not possible. What are you trying to do, what do you want to use the .prop for? — Bergi
– Bergi, Commented Oct 14, 2018 at 13:01
add some flag to my promise object - lets say cancellable like described here: github.com/fangj/make-cancelable/blob/master/index.js - I want to do the same thing only with pure async function - not use new Promise — Adidi
– Adidi, Commented Oct 14, 2018 at 13:02
Take a look at Fluture => github.com/fluture-js/Fluture. It can wrap promises and it already provides cancellation. — Matías Fidemraizer
– Matías Fidemraizer, Commented Oct 14, 2018 at 13:05
@Adidi A cancellation signalisation should be an input (parameter of your function), not be part of the output. — Bergi
– Bergi, Commented Oct 14, 2018 at 13:12

T.J. Crowder · Accepted Answer · 2018-10-14 13:20:47Z

2

Adding a property to the promise is almost certainly a bad idea (more on that later, under However), but just to talk about how you would continue to do it:

I wonder if there is a way somehow to get this instance of this promise inside the function

No, there isn't. You could create a promise within the function and return it, but that wouldn't be the promise the function returns (it would just affect how the promise the function returns resolves).

If you want to add a property to the promise being returned, you'll have to use a non-async function. You might make the function's entire code non-async:

const sleep = ts => new Promise(resolve => setTimeout(resolve, ts));

const getSomePromise = () => {
    const p = sleep(1000).then(() => 'success');
    p.someProp = 'myProp';
    return p;
}
const promise = getSomePromise();
console.log(promise.someProp);

...or you might use an inner async function so you can use await semantics and such:

const sleep = ts => new Promise(resolve => setTimeout(resolve, ts));

const getSomePromise = () => {
    const p = (async () => {
        await sleep(1000);
        return 'success';
    })();
    p.someProp = 'myProp';
    return p;
}
const promise = getSomePromise();
console.log(promise.someProp);

However: Adding a property to the promise is almost certainly a bad idea. Instead, have the promise resolve to an object with properties both for the resolution and the extra someProp:

const sleep = ts => new Promise(resolve => setTimeout(resolve, ts));

const getSomePromise = async () => {
    const p = await sleep(1000);
    // some how access the instance of the promise from within the async function 
    // for example this['someProp'] = 'myProp';
    // and return the all promise with this prop
    return {
        result: 'success',
        someProp: 'myProp'
    };
}
getSomePromise()
    .then(resolution => {
        console.log(resolution.someProp);
    });

edited Oct 14, 2018 at 13:20

answered Oct 14, 2018 at 12:59

T.J. Crowder

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4 Comments

Adidi Over a year ago

Can you explain why is it almost certainly a bad idea ? If i want to add ability to the instance of the promise like cancellation: github.com/fangj/make-cancelable/blob/master/index.js - why is it bad idea ? it seems very elegant to me other then now return a complex object with keys I need to remember

T.J. Crowder Over a year ago

@Adidi - Because it doesn't survive standard promise operations (like then). If you really want to carry the information on the promise itself rather than on its resolution value, your best bet would be a Promise subclass (which would mean you couldn't use an async function).

Benjamin Gruenbaum Over a year ago

Probably pass a token in for cancellation.

Adidi Over a year ago

@BenjaminGruenbaum - Yes - this is what axios did and this is what I try to do - wrapped their solution so I always get the cancel function cause I think their api cancellation is very verbose and clunky

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