Using await in then callback - the keyword 'await' is reserved

Question

In node.js, I have a database transaction, where I want to call an async method in then callback, but I get error message the keyword 'await' is reserved.

This is async saveImage function:

const saveImage = async (parsedLink) => {
  AWS.config.region = config.awsSettings.region;
  AWS.config.accessKeyId = config.awsSettings.accessKeyId;
  AWS.config.secretAccessKey = config.awsSettings.secretAccessKey;
  const bucket = new AWS.S3({
    params: {
      Bucket: config.awsSettings.images_bucket_name,
    },
  });

  const currentDateString = new Date().toISOString().replace(/\:|\./g, '-');
  const bodystream = new Buffer(parsedLink.imgUrl, 'binary');
  const imageUrlDomain = parseDomain(parsedLink.linkUrl).domain;

  const params = {
    Key: `${parsedLink.id}/${imageUrlDomain}_${currentDateString}${parsedLink.imgType}`,
    ContentType: parsedLink.imageMime,
    ContentEncoding: 'base64',
    Body: bodystream,
  };

  const resultPromise = await bucket.upload(params).promise();
  return resultPromise.Location;
};

If I want to use saveImage function, I get the error message.

module.exports.addTestObject = async (ctx) => {
  const testObj = ctx.request.body;
  try {
    switch (testObj.type) {
      case interestT.getType.link: {
        const knexTestObject = TestObject.knex();
        transaction(knexTestObject, trx =>
            TestObject.query(trx)
              .insert({
                interestDate: testObj.date,
              })
              .then(newInterest => {
                // save image
                if (parsedLink.saveImg) {
                  parsedLink.imgUrl = await saveImage(testObj);
                }

                newInterest.$relatedQuery('linkInterestsRel', trx).insert({
                  linkHeader: testObj.linkHeader,
                }),
              }
              ),
          )
          .then((linkInterest) => {
            console.log(linkInterest);
          })
          .catch((err) => {
            throw err;
          });
        break;
      }
      default:
        break;
    }
    ctx.response.body = interestObj;
  } catch (err) {
    const statusCode = err.status || 400;
    ctx.throw(statusCode, err.message);
  }
};

Bucentaur

You can only use await in an async function, and your .then() callback is not async. – Boyce

-> .then(async (newInterest) => { – Lysippus

@JonasW. Thanks, can you write it as an answer? Then I can close this question. – Bucentaur

Uh, don't use then callbacks within async functions. – Sense

Answer 1

Regular functions run synchronously till they return. Therefore you cannot use await inside them as you cannot wait for an asynchronous event in a synchronous way.

JavaScript also has async functions, which look like regular functions, but are conceptually quite different: They run synchronously till they reach an await, then they stop and continue once the awaited Promise resolves. As such they cannot return their result synchronously, instead they return a Promise which then resolves when the function finished execution.

Therefore you need to convert your function into an async function:

 async function getUsername() { // <-- async keyword here
    return (await getUser()).name; // <-- await can be used inside
 }

Now this does also work inside a .then callback:

 getUser().then(async function(user) {
    const friends = await getFriends(user);
    // ...
 })

But this somewhat mixes the abstraction async functions with their underlying primitive Promise. If you would just await the Promise instead of adding a .then callback, the code gets way more readable:

 (async function() {
    const user    = await getUser();
    const friends = await getFriends(user);
 })();

---

The concrete question could be rewritten as:

 const linkInterest = await transaction(knexTestObject, async trx => {
     const newInterest = await TestObject.query(trx)
          .insert({  interestDate: testObj.date,   });

     if (parsedLink.saveImg) {
       parsedLink.imgUrl = await saveImage(testObj);
     }

    await newInterest.$relatedQuery('linkInterestsRel', trx)
       .insert({  linkHeader: testObj.linkHeader, }),
});

Lysippus

The transaction callback still would need to be an async function if one wanted to use await for the query (as one indeed should). – Sense

Clearest explanation I've seen of how to use await instead of .then – Bun