Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – await has no effect on this kind of expression – but clearly does

I recently added jsdocs to my project and since then I have eslint telling me that
‘await has no effect on this kind of expression’

const result = await someAsyncFunction(); // await has no effect
console.log(result.property);

Removing the await keyword however has the effect that console tries to log before result has resolved.

I know that that eslint complains because someAsncyFunction does not return a promise itself but has async functions inside of it.

async function someAsyncFunction(){
  const response = await fetch('dataset');
  const anotherResponse = await fetch('anotherDataset')
  const merged = combineDatasets(response, anotherResponse)
  return merged;
}

Am I really supposed to wrap this function in another Promise?

async function someAsyncFunction(){
  return new Promise(async(resolve) => {
    ...
    resolve(merged);
  })
}

Ignoring the warning about ‘await having no effect’ works without problems – every time. How ever it makes me think that my approach and / or my understanding of promises is way of.

Maybe you could point me in the right direction here.

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    Every Async Function returns a promise

    And so does someAsyncFunction() by returning merged

    // await unwraps the promise 
const result = await someAsyncFunction()

    So jsdoc complained because my comment was wrong

    /** @returns {Promise<customType>} */

  2. 0

    Am I really supposed to wrap this function in another Promise?

    No need to do so. This is done by your async function as it always returns a promise and resolves this promise on a successful/non-erroneous return:

    /**
 * Returns a promise that is successfully resolved as soon as "merged" is returned
 * @returns {Promise} You might want to also type the response: Promise<typeOfMerged>
 */
async function someAsyncFunction() {
  const response = await fetch("dataset");
  const anotherResponse = await fetch("anotherDataset");
  const merged = combineDatasets(response, anotherResponse);
  return merged;
}

/**
 * await is needed if you want to use "merged".
 * If you want to work with the not yet resolved promise, you remove await here.
 */
const result = await someAsyncFunction(); 
console.log(result.property);
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search