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))

Answer 1

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.}

Answer 2

EDIT 2021

Node > 14.3

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

foo.js

export 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