CODEWITHSUNDEEP
javascriptnode.js
j.doe
j.doe

Reputation: 4859

await is only valid in async function

I wrote this code in lib/helper.js:

var myfunction = async function(x,y) {
    ....
    return [variableA, variableB]
}
exports.myfunction = myfunction;

Then I tried to use it in another file:

var helper = require('./helper.js');   
var start = function(a,b){
    ....
    const result = await helper.myfunction('test','test');
}
exports.start = start;

I got an error:

await is only valid in async function

What is the issue?

Upvotes: 374

Views: 846718

Answers (15)

Orelsanpls
Orelsanpls

Reputation: 23495

The error is not refering to myfunction but to start.

async function start() {
   ....

   const result = await helper.myfunction('test', 'test');
}


// My function
const myfunction = async function(x, y) {
  return [
    x,
    y,
  ];
}

// Start function
const start = async function(a, b) {
  const result = await myfunction('test', 'test');
  
  console.log(result);
}

// Call start
start();

I use the opportunity of this question to advise you about an known anti pattern using await which is : return await.

WRONG

async function myfunction() {
  console.log('Inside of myfunction');
}

// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for as well
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later

// useless async here
async function start() {
  // useless await here
  return await myfunction();
}

// Call start
(async() => {
  console.log('before start');

  await start();
  
  console.log('after start');
})();

CORRECT

async function myfunction() {
  console.log('Inside of myfunction');
}

// Here we wait for the myfunction to finish
// and then returns a promise that'll be waited for as well
// It's useless to wait the myfunction to finish before to return
// we can simply returns a promise that will be resolved later

// Also point that we don't use async keyword on the function because
// we can simply returns the promise returned by myfunction
function start() {
  return myfunction();
}

// Call start
(async() => {
  console.log('before start');

  await start();
  
  console.log('after start');
})();

Also, know that there is a special case where return await is correct and important : (using try/catch)

Are there performance concerns with `return await`?

Upvotes: 427

hx_hxl
hx_hxl

Reputation: 103

I encounter this problem when I'm using TypeScript. finally I figured out that I should set target: "ES2017" in tsconfig.json so that the transpiled javascript code keep the async function literals instead of __await ... code under ES6 behavior.

Upvotes: 0

Satyam Pathak
Satyam Pathak

Reputation: 6912

To use await, its executing context needs to be async in nature

As it said, you need to define the nature of your executing context where you are willing to await a task before anything.

Just put async before the fn declaration in which your async task will execute.

var start = async function(a, b) { 
  // Your async task will execute with await
  await foo()
  console.log('I will execute after foo get either resolved/rejected')
}

Explanation:

 var helper = require('./helper.js');   
 var start = async function(a,b){
     ....
     const result = await helper.myfunction('test','test');
 }
 exports.start = start;

Wondering what's going under the hood

await consumes promise/future / task-returning methods/functions and async marks a method/function as capable of using await.

Also if you are familiar with promises, await is actually doing the same process of promise/resolve. Creating a chain of promise and executes your next task in resolve callback.

For more info you can refer to MDN DOCS.

Upvotes: 42

aGuegu
aGuegu

Reputation: 2299

In later nodejs (>=14), top await is allowed with { "type": "module" } specified in package.json or with file extension .mjs.

https://www.stefanjudis.com/today-i-learned/top-level-await-is-available-in-node-js-modules/

Upvotes: 13

Fattie
Fattie

Reputation: 12582

A common problem in Express:

The warning can refer to the function, or where you call it.

Express items tend to look like this:

app.post('/foo', ensureLoggedIn("/join"), (req, res) => {
    const facts = await db.lookup(something)
    res.redirect('/')
})

Notice the => arrow function syntax for the function.

The problem is NOT actually in the db.lookup call, but right here in the Express item.

Needs to be:

app.post('/foo', ensureLoggedIn("/join"), async function (req, res) {
    const facts = await db.lookup(something)
    res.redirect('/')
})

Basically, nix the => and add async function .

Upvotes: 1

YMH
YMH

Reputation: 3093

Found the code below in this nice article: HTTP requests in Node using Axios

const axios = require('axios')

const getBreeds = async () => {
  try {
    return await axios.get('https://dog.ceo/api/breeds/list/all')
  } catch (error) {
    console.error(error)
  }
}

const countBreeds = async () => {
  const breeds = await getBreeds()

  if (breeds.data.message) {
    console.log(`Got ${Object.entries(breeds.data.message).length} breeds`)
  }
}

countBreeds()

Or using Promise:

const axios = require('axios')

const getBreeds = () => {
  try {
    return axios.get('https://dog.ceo/api/breeds/list/all')
  } catch (error) {
    console.error(error)
  }
}

const countBreeds = async () => {
  const breeds = getBreeds()
    .then(response => {
      if (response.data.message) {
        console.log(
          `Got ${Object.entries(response.data.message).length} breeds`
        )
      }
    })
    .catch(error => {
      console.log(error)
    })
}

countBreeds()

Upvotes: 2

Lahiru Pinto
Lahiru Pinto

Reputation: 1681

If you have called async function inside foreach update it to for loop

Upvotes: 2

Ameba
Ameba

Reputation: 1

This in one file works..

Looks like await only is applied to the local function which has to be async..

I also am struggling now with a more complex structure and in between different files. That's why I made this small test code.

edit: i forgot to say that I'm working with node.js.. sry. I don't have a clear question. Just thought it could be helpful with the discussion..

    function helper(callback){



    function doA(){

        var array = ["a ","b ","c "];

        var alphabet = "";

        return new Promise(function (resolve, reject) {

            array.forEach(function(key,index){

            alphabet += key;

                if (index == array.length - 1){

                    resolve(alphabet);

                };

            });

        });

    };



    function doB(){

        var a = "well done!";

        return a;

    };



    async function make() {

        var alphabet = await doA();
        var appreciate = doB();

        callback(alphabet+appreciate);

    };

    make();

};

helper(function(message){

    console.log(message);

});

Upvotes: 0

gianlucaparadise
gianlucaparadise

Reputation: 1763

If you are writing a Chrome Extension and you get this error for your code at root, you can fix it using the following "workaround":

async function run() {
    // Your async code here
    const beers = await fetch("https://api.punkapi.com/v2/beers");
}

run();

Basically you have to wrap your async code in an async function and then call the function without awaiting it.

Upvotes: 15

Rafiq
Rafiq

Reputation: 11445

async/await is the mechanism of handling promise, two ways we can do it

functionWhichReturnsPromise()
            .then(result => {
                console.log(result);
            })
            .cathc(err => {
                console.log(result);

            });

or we can use await to wait for the promise to full-filed it first, which means either it is rejected or resolved.

Now if we want to use await (waiting for a promise to fulfil) inside a function, it's mandatory that the container function must be an async function because we are waiting for a promise to fulfiled asynchronously || make sense right?.

async function getRecipesAw(){
            const IDs = await getIds; // returns promise
            const recipe = await getRecipe(IDs[2]); // returns promise
            return recipe; // returning a promise
        }

        getRecipesAw().then(result=>{
            console.log(result);
        }).catch(error=>{
            console.log(error);
        });

Upvotes: 4

mn_test347
mn_test347

Reputation: 452

"await is only valid in async function"

But why? 'await' explicitly turns an async call into a synchronous call, and therefore the caller cannot be async (or asyncable) - at least, not because of the call being made at 'await'.

Upvotes: -3

Emil
Emil

Reputation: 204

I had the same problem and the following block of code was giving the same error message:

repositories.forEach( repo => {
        const commits = await getCommits(repo);
        displayCommit(commits);
});

The problem is that the method getCommits() was async but I was passing it the argument repo which was also produced by a Promise. So, I had to add the word async to it like this: async(repo) and it started working:

repositories.forEach( async(repo) => {
        const commits = await getCommits(repo);
        displayCommit(commits);
});

Upvotes: 17

Rodney P. Barbati
Rodney P. Barbati

Reputation: 2090

Yes, await / async was a great concept, but the implementation is completely broken.

For whatever reason, the await keyword has been implemented such that it can only be used within an async method. This is in fact a bug, though you will not see it referred to as such anywhere but right here. The fix for this bug would be to implement the await keyword such that it can only be used TO CALL an async function, regardless of whether the calling function is itself synchronous or asynchronous.

Due to this bug, if you use await to call a real asynchronous function somewhere in your code, then ALL of your functions must be marked as async and ALL of your function calls must use await.

This essentially means that you must add the overhead of promises to all of the functions in your entire application, most of which are not and never will be asynchronous.

If you actually think about it, using await in a function should require the function containing the await keyword TO NOT BE ASYNC - this is because the await keyword is going to pause processing in the function where the await keyword is found. If processing in that function is paused, then it is definitely NOT asynchronous.

So, to the developers of javascript and ECMAScript - please fix the await/async implementation as follows...

  • await can only be used to CALL async functions.
  • await can appear in any kind of function, synchronous or asynchronous.
  • Change the error message from "await is only valid in async function" to "await can only be used to call async functions".

Upvotes: -3

user835611
user835611

Reputation: 2366

The current implementation of async / await only supports the await keyword inside of async functions Change your start function signature so you can use await inside start. 

 var start = async function(a, b) {

 }

For those interested, the proposal for top-level await is currently in Stage 2: https://github.com/tc39/proposal-top-level-await

Upvotes: 3

John Langford
John Langford

Reputation: 1435

When I got this error, it turned out I had a call to the map function inside my "async" function, so this error message was actually referring to the map function not being marked as "async". I got around this issue by taking the "await" call out of the map function and coming up with some other way of getting the expected behavior.

var myfunction = async function(x,y) {
    ....
    someArray.map(someVariable => { // <- This was the function giving the error
        return await someFunction(someVariable);
    });
}

Upvotes: 17

Related Questions