How can I use 'finally' within a 'Promise' clause in NodeJS?

Question

I want something like this:

let promises = [];
for (let i = 0; i < some_length; i++) {
    let output = '';
    promises.push(some_func(i)
        .then((result) => {
            output = result;
        })
        .catch((error) => {
            output = error.message;
        })
        .finally(() => {
            console.log(output);
        })
    );
}
return Promise.all(promises);

But I get a runtime error .then(...).catch(...).finally is not a function.

How can I resolve this?

`finally` is not yet in the standard, and therefore not supported by Node. You can use [this polyfill](https://stackoverflow.com/a/32362233/1048572) though — Bergi, Oct 26 '17 at 07:07
@Bergi: OK, so in other words, switch to the (newer) `async`/`await` paradigm, which we are planning to do at some point anyway, but were hoping that there would be a similar mechanism in `Pomise` paradigm. Thank you!!! — goodvibration, Oct 26 '17 at 07:24
@goodvibration `async`/`await` is still using the promise paradigm, it just has more syntactic sugar. — Bergi, Oct 26 '17 at 08:35
Btw, for the example in your question I would recommend to simplify to `some_func(i).catch(err => err.message).then(console.log)` (which is probably even better than any `finally`+`await`, as there you always need that `output` variable) — Bergi, Oct 26 '17 at 08:37

Forivin · Answer 1 · 2018-09-11T19:21:43.957

2

Node 10 finally added support for it. Tested with node 10.7.0.

Promise.resolve().finally(() => {
    console.log("It finally works!")
})

It finally works!

(Chrome and Firefox also support it btw.)

edited Sep 11 '18 at 19:21

answered Jul 19 '18 at 11:58

Forivin

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score 0 · Answer 2 · answered May 16 '18 at 11:43

Actually, I Think your some-func function isn't return Promise, with returning JavaScript Promisees the then, catch and finally has meaning, so I think you must declare some_func function as a new instance of Promise object, see below code:

let promises = [];
for (let i = 0; i < some_length; i++) {
    let output = '';
    let some_func = (i) => {
       return new Promise(function(resolve, reject) {
            setTimeout(resolve(`success: ${i}`), 100, 'foo');
       });
    }
    promises.push(some_func(i)
        .then((result) => {
            output = result;
        })
        .catch((error) => {
            output = error.message;
        })
        .finally(() => {
            console.log(output);
        })
    );
}
return Promise.all(promises);

Maybe this code have some other errors, I don't know because I do not test it, but undoubtedly your error is for just like I said in above sentences.

How can I use 'finally' within a 'Promise' clause in NodeJS?

2 Answers2