How can I export promise result?

Question

This code works correctly. And I just need to export data variable after all promises successfully resolved.

I cannot put this code to function and export variable. Because in this case, this function will export an empty array.

'use strict'

import urls from './urls'
import getData from './get-data'

getData(urls).then((responses) => {
    const data = []
    const results = responses.map(JSON.parse)

    for (let i = 0, max = results.length; i < max; i++) {
        // some magic and pushing 
    }

    return data
}).catch(error => console.log(error))

You don't, you use the call back `then`. That is when you have access to the data, you do not return it directly. Maybe I am not understanding what you mean by `need to export data variable after....` — Igor, Mar 22 '17 at 17:11
Possible duplicate of [How do I return the response from an asynchronous call?](http://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — Jared Smith, Mar 22 '17 at 17:16
See https://stackoverflow.com/questions/41279796/exporting-node-module-from-promise-result/41279904 — jchi2241, Jul 28 '18 at 18:16
Possible duplicate of [Exporting Node module from promise result](https://stackoverflow.com/questions/41279796/exporting-node-module-from-promise-result) — jchi2241, Jul 28 '18 at 18:16

Bergi · Answer 1 · 2019-02-16T20:53:52.673

21

You could easily assign it to an exported variable, but you should not do that - the assignment happens asynchronously, and the variable might be read before that in the modules where it is imported.

So instead, just export the promise¹!

// data.js
import urls from './urls'
import getData from './get-data'

export default getData(urls).then(responses =>
    responses.map(JSON.parse).map(magic)
);

// main.js
import dataPromise from './data'

dataPromise.then(data => {
    console.log(data);
    …
}, console.error);

_{1: Until the proposed top-level await comes along and you can just wait for the value before exporting it.}

edited Feb 16 '19 at 20:53

answered Mar 22 '17 at 17:23

Bergi

630,263
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3

I am looking forward to top-level await! – basickarl Dec 03 '19 at 15:59
how to pass parameter here? – Balaji Jul 12 '20 at 05:15
@ßãlãjî Pass a parameter from where to where? Do you want to export a promise-returning function instead of a promise? – Bergi Jul 12 '20 at 10:55

Sceat · Answer 2 · 2022-07-13T09:00:39.780

7

EDIT 2021

Node > 14.3

Since node now support top-level-await the solutions is extra basic

foo.js

export default await new Promise()

index.js

import foo from './foo.js' // the import wait for the promise to be resolved

console.log(foo)

Below node 14.3

Nowadays u do

foo.js

const wait = ms => new Promise(resolve => setTimeout(resolve, ms))

async function foo() {
    console.log('called')
    await wait(1000)
    return 'hi'
}

export default foo()

index.js

import foo from './foo'

void (async function() {
    console.log(foo)
    console.log(await foo)
    console.log(await foo)
})()

output

> called
> Promise { <pending> }
> hi
> hi

It's usefull to fetch datas at start

edited Jul 13 '22 at 09:00

answered May 22 '19 at 17:42

Sceat

1,427
14
12

`export await new Promise()` -> Unexpect token export error. Change it to `export default await new Promise()` works fine. – vee Jul 09 '22 at 18:44
oh right, i edited – Sceat Jul 13 '22 at 09:00

score 0 · Answer 3 · answered Oct 30 '18 at 17:50

You can export your promise result by simply resolving the promise in the module from which to want to export, and in then() block use exports.variable_name = promiseResult;

For example: I want to use a database connection in my whole app. but the connect call return me a promise. so I can simply call the promise.then and i then block, export my desired result. Connection.js file

async function connect() {
    connection = await oracledb.getConnection(config)
if (connection) {
    return connection;
} else {
    return null;
  }
}

connect().then((res) => {
    exports.connection = res;
});

then in main.js file, just simply require the connection.js file.

But this is not good practice because if your promise fails or take too much time to resolve, and it is used somewhere before it was resolved, may cause errors.

score 0 · Answer 4 · edited Jun 16 '21 at 04:23

You should use Promise constructor with a custom

mypromise.js

export function get_data(){
  const get_data = new Promise(function(resolve, reject) {
    try {
      //do your stuff 
      return resolve('success'); // change this to whatever data your want
    } catch (err) {
      return reject(err);
    }
  });
  //return promise 
  return get_data;
}

Import

import * as data from './mypromise'

data.get_data()
  .then((result) => console.log(result))
  .catch((error) => console.error(error));

How can I export promise result?

4 Answers4

Node > 14.3

foo.js

index.js

Below node 14.3

foo.js

index.js

output

Linked

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