Using async requires async function, but my function is async

Question

I'm adapting a library that uses callback to use Promises. It's working when I use then(), but it doesn't work when I use await.

> dbc.solve
[AsyncFunction]
> await dbc.solve(img)
await dbc.solve(img)
^^^^^

SyntaxError: await is only valid in async function

The code for dbc.solve is:

module.exports = DeathByCaptcha = (function() {
  function DeathByCaptcha(username, password, endpoint) {
    ...
  }

  DeathByCaptcha.prototype.solve = async function(img) {
    return new Promise(
      function(resolve, reject) {
        ...
      }
    );
  };
})();

I believe this has something with the fact solve is member of prototype, but I couldn't find any information about it. I found that node didn't always supported async await for class methods, so I upgraded from node 7, now I'm using node 9.4.0.

Denys Séguret · Accepted Answer · 2018-02-08 12:58:59Z

25

You don't read that error message right: the problem isn't the function you're calling but the function you're in.

You may do

(async function(){
    await dbc.solve(img);
    // more code here or the await is useless
})();

Note that this trick should soon enough not be needed anymore in node's REPL: https://github.com/nodejs/node/issues/13209

answered Feb 8, 2018 at 12:58

Denys Séguret

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2 Comments

rodorgas Over a year ago

Thanks! Isn't the in ambiguous? It says await is only valid in async function, but "function" can be the context or the function which I'm calling.

Denys Séguret Over a year ago

@rodorgas It is tricky, most of us have been tricked once, but I don't think it's ambiguous if you're cautious enough. And I don't see any obviously better message.

messerbill · Accepted Answer · 2018-10-05 20:28:52Z

8

SyntaxError: await is only valid in async function - just like the error tells you, you may only use await inside a function which is marked as async. So you cannot use the await keyword anywhere else.

https://basarat.gitbooks.io/typescript/docs/async-await.html

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-1-7.html

examples:

function test() {
  await myOtherFunction() // NOT working
}

async function test() {
  await myOtherFunction() //working
}

You can also make anonymous callback functions async:

myMethod().then(async () => {
  await myAsyncCall()
})

edited Oct 5, 2018 at 20:28

answered Feb 8, 2018 at 12:58

messerbill

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Comments

Oliver · Accepted Answer · 2018-02-08 13:00:01Z

3

The await operator can only be used in an async function.

answered Feb 8, 2018 at 13:00

Oliver

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Comments

Redu · Accepted Answer · 2018-02-08 17:38:44Z

2

You may not need async await abstraction really. Why don't you just simply promisify dbc.solve() function with a promisifier like;

function promisify(f){
  return data => new Promise((v,x) => f(data, (err, id, sol) => err ? x(err) : v({i:id, s:solution})));
}

You will have a promisified version of your dbc.solve() and if it doesn't fire an error you will be returned with an object like {i:id, s: solution} at it's then stage.

answered Feb 8, 2018 at 17:38

Redu

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