Best way to return an exported module

Question

On my learning journey, I have started to look at modules for javaScript/Node. What is confusing me is how to return information from a module when the time to complete the functions within a module is unknown.

This returns as expected:

controller.js

const myModule = require('./myModule');
var myWord = myModule.myExp();
console.log(myWord); //returns "Hello World"

myModule.js

module.exports = {
    myExp: function(){
        return mytimeOut();
    }
}

function mytimeOut(){
 var myWord = "Hello World";
 return myWord;
}

What I cannot seem to grasp, is the best way to return myModule when there is an indefinite time to bring back the required result.

How do I get the controller to show "Hello World" and not "Undefined" in the example below. I have read about callbacks, promises, async/await - but without any over-complications, I cannot seem to find a simple solution for this.

controller.js

const myModule = require('./myModule');
var myWord = myModule.myExp();
console.log(myWord); //returns undefined

myModule.js

module.exports = {
    myExp: function(){
        return mytimeOut();
    }
}

function mytimeOut(){
    setTimeout(() => {
        myWord = "Hello World";
        return myWord;

    }, 5000);
}

Possible duplicate of [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — nicholaswmin, Jan 07 '19 at 13:06
You use it the same way you use any asynchronous function. Simply export the function and call it using any async-programming technique, e.g: error-first callbacks, Promises, `async` functions. — nicholaswmin, Jan 07 '19 at 13:07
Possible duplicate of [Node.js: unable to return value from other function inside same file](https://stackoverflow.com/questions/50296339/node-js-unable-to-return-value-from-other-function-inside-same-file) — Dan O, Jan 07 '19 at 13:12
Thank you. This may be a duplicate. I am reading through your suggested duplicates now. — Terence E. Uttley, Jan 07 '19 at 13:24

score 1 · Answer 1 · answered Jan 07 '19 at 13:29

Here's a working copy using async/await

const exports = {
  myExp: function() {
    return mytimeOut();
  }
}

function mytimeOut() {
  return new Promise((resolve) => {
    setTimeout(() => {
      const myWord = "Hello World";
      resolve(myWord);
    }, 5000);
  });
}

(async () => {
  const output = await exports.myExp();
  document.getElementById('output').textContent = output;
})();

<div id="output"></div>

Gamote · Answer 2 · 2019-01-07T14:04:52.413

You can achieve that using async/await.

File: myModule.js

function myTimeout() {
  // Create a promise
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("Hello World"); // resove the promise after those 5 seconds
    }, 5000);
  })
}

module.exports = {
  myExp: function() {
    return myTimeout();
  }
};

File: controller.js

const myModule = require('./myModule');

async function findMyWord() {
  const myWord = await myModule.myExp();
  console.log('My word is:', myWord);
}

findMyWord();

I wrapped everything in an async function because otherwise you will receive the Syntax Error: await is only valid in async error.

David Hellsing · Accepted Answer · 2019-01-07T13:34:24.057

0

Using async/await:

  // myModule.js

  function mytimeOut() { 
    return new Promise(resolve => 
      setTimeout(() => resolve('Hello World'), 5000)
    )
  }

  // controller.js

  async function init() {
    var myWord = await myModule.myExp()
    console.log(myWord)
  }
  init()

edited Jan 07 '19 at 13:34

answered Jan 07 '19 at 13:13

David Hellsing

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2

`mytimeOut` doesn't need to be `async` – James Jan 07 '19 at 13:15
1

Syntax Error: await is only valid in async function – Terence E. Uttley Jan 07 '19 at 13:23
@TerenceE.Uttleyyes you might need to wrap it in an `async` depending on how you call it in controller.js – David Hellsing Jan 07 '19 at 13:33

score 0 · Answer 4 · answered Jan 09 '19 at 14:00

You need to use Promise, because setTimeout does not return a promise that could be awaited.

File: myModule.js

'use strict';

module.exports = {
  myExp: function() {
    return mytimeout();
  }
};

function mytimeout() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('Hello World');
    }, 1000);
  }); 
}

File: controller.js (.then())

'use strict';

const myModule = require('./myModule.js');
myModule.myExp().then(console.log);

Best way to return an exported module

4 Answers4

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