CODEWITHSUNDEEP
javascriptpromiseasync-awaitprimitive
user3223162
user3223162

Reputation: 443

Async function returning promise, instead of value

I'm trying to understand how async/await works in conjunction together with promises.

async function latestTime() {
  const bl = await web3.eth.getBlock('latest');
  console.log(bl.timestamp); // Returns a primitive
  console.log(typeof bl.timestamp.then == 'function'); //Returns false - not a promise
  return bl.timestamp;
}
const time = latestTime(); // Promise { <pending> }

As far as I understand, await should be blocking and in the code above it seemingly blocks returning an object bl with the primitive timestamp. Then, my function returns the primitive value, however the time variable is set to a pending promise instead of that primitive. What am I missing?

Upvotes: 34

Views: 65013

Answers (3)

T.J. Crowder
T.J. Crowder

Reputation: 1074148

An async function always returns a promise. That's how it reports the completion of its asynchronous work. If you're using it in another async function, you can use await to wait for its promise to settle, but in a non-async function (often at the top level or in an event handler), you have to use the promise directly, e.g.:

latestTime()
.then(time => {
    console.log(time);
})
.catch(error => {
    // Handle/report error
});

...though if you're doing this at the top level of a JavaScript module, all modern environments now support top-level await in modules:

const time = await latestTime();

(Note that if that promise is rejected, your module will fail to load. If your module can work meaningfully even if the promise fails, be sure to wrap that in try/catch to handle promise rejection.)

It might (or might not) throw some light on things to see, in explicit promise callback terms, how the JavaScript engine handles your async function under the covers:

function latestTime() {
    return new Promise((resolve, reject) => {
        web3.eth.getBlock('latest')
        .then(bl => {
            console.log(bl.timestamp);
            console.log(typeof bl.timestamp.then == 'function');
            resolve(bl.timestamp);
        })
        .catch(reject);
    });
}

Some important notes on that:

  • The function you pass to new Promise (the promise executor function) gets called synchronously by new Promise.
    • Which is why the operation starts, web3.eth.getBlock is called synchronously to start the work.
  • Any error (etc.) thrown within the promise executor gets caught by new Promise and converted into a promise rejection.
  • Any error (etc.) thrown within a promise callback (like the one we're passing then) will get caught and converted into a rejection.

Upvotes: 33

Oleksii
Oleksii

Reputation: 1643

async function will return Promise anyway. Return value will be `Promise, so in your case it will be:

async function latestTime(): Promise<some primitive> {
  const bl = await web3.eth.getBlock('latest');
  return bl.timestamp;
}

So, further you can use it function like:

const time = await latestTime();

But for achieving general view about async/await feature it will be better to read documentation.

Upvotes: 12

Theodor Losev
Theodor Losev

Reputation: 392

Async prefix is a kind of wrapper for Promises.

async function latestTime() {
    const bl = await web3.eth.getBlock('latest');
    console.log(bl.timestamp); // Returns a primitive
    console.log(typeof bl.timestamp.then == 'function'); //Returns false - not a promise
    return bl.timestamp;
}

Is the same as

function latestTime() {
    return new Promise(function(resolve,success){
        const bl = web3.eth.getBlock('latest');
        bl.then(function(result){
            console.log(result.timestamp); // Returns a primitive
            console.log(typeof result.timestamp.then == 'function'); //Returns false - not a promise
            resolve(result.timestamp)
        })
}

Upvotes: 30

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