Unable to get the value from Promise.allSettled in NodeJs 12 with Typescript 3.8.3 version

user_13000512 0
Viewed 13693

I am learning NodeJs 12 with Promise.allSettled() function and its usage. I have written the following code. I am able to print the status in the console but unable to print the value as it is giving compilation issue.

        const p1 = Promise.resolve(50);
        const p2 = new Promise((resolve, reject) =>
            setTimeout(reject, 100, 'geek'));
        const prm = [p1, p2];

        Promise.allSettled(prm).
        then((results) => results.forEach((result) =>
            console.log(result.status,result.value)));

I am getting the following compilation issue. enter image description here

I provide below the tsconfig.json.

{
  "compilerOptions": {
    "target": "es2017",
    "lib": ["es6","esnext", "dom"],
    "allowJs": true,
    "module": "commonjs",
    "moduleResolution": "node",
    "declaration": true,
    "outDir": "./lib",
    "strict": true,
    "esModuleInterop": true,
    "typeRoots": [ "./types", "./node_modules/@types"]
  },
  "include": ["src"],
  "exclude": ["**/__tests__/*"]
}
es6-promise typescript node.js
4 Answers

You might want something like this:

  Promise.allSettled(prm).
    then((results) => results.forEach(result => {
       if (result.status === 'fulfilled') {
         console.log(result.status,result.value);
       } else {
         console.log(result.status,result.reason);
       }
    });

value only exists if the status is fulfilled, but it doesn't cover cases where one of the promises had an error.

user_809110

If one of the promises will be rejected, your object will have no value. To warn you about this possibility, TypeScript uses this type:

enter image description here

So, to get value from the result you should use guards

const isFilled = <T extends {}>(v: PromiseSettledResult<T>): v is PromiseFulfilledResult<T> => v.status === 'fulfilled';

Promise.allSettled(prm).then((results) => results.forEach((result) =>
    console.log(result.status, isFilled(result) ? result.value : result.status)));

or to cast it yourself

Promise.allSettled(prm).then((results) => results.forEach((result) =>
    console.log(result.status, (result as PromiseFulfilledResult<any>).value)));
user_55755950

Using typeguards to correctly type PromiseSettledResult like in but without creating a specific isFullfilled function and using filter + map

const promises = [
  Promise.reject("Error"),
  Promise.resolve(42),
  Promise.reject("Another error")
]
const results = await Promise.allSettled(promises)
const successes = results
  .filter((x): x is PromiseFulfilledResult<number> => x.status === "fulfilled")
  .map(x => x.value)
const failures: any[] = results
  .filter((x): x is PromiseRejectedResult => x.status === "rejected")
  .map(x => x.reason)

I believe the typing system is flexible enough to be extended to correctly type

const successes = results
  .filter(x => x.status === "fulfilled")
  .map(x => x.value)
const failures = results
  .filter(x => x.status === "rejected")
  .map(x => x.reason)

but I wasn't able to figure that out myself.

user_15071240

I see two problems here.

first

The second promise you create is of type Promise<unknown> because typescript cannot infer its underlying type.

When you create an array with the two promises, unknown "swallows" everything and you are left with an array of type Promise<unknown>[].

If you type p2 like this:

const p2 = new Promise<number>((resolve, reject) =>
  setTimeout(reject, 100, 'geek'));

you won't have this problem.

second

You probably have to do something like this for typescript to know if the promise was a success or not:

Promise.all(prm).then((results) =>
  results.forEach((result) => {
    if (result.status === "fulfilled") {
      console.log("fulfilled", result.value);
    } else {
      console.log("rejected", result.reason);
    }
  })
);
user_65488020
Related