Recently I found myself getting a bit confused writing some JavaScript code with async/await. I worked through in some detail what happens when we await, and I thought it might be helpful to publish an article about it (for my future self as much as other readers!).

tl;dr: await is similar to yield but there are some differences.

The following code will hopefully clarify what happens with async/await in JavaScript. Can you figure out what it will do?

const asyncTask = () => {
    console.log('asyncTask started')

    const promise = new Promise(resolve => {
        setTimeout(() => { 
            console.log('asyncTask resolving promise')
            resolve('1000')
        }, 2000)
    })

    console.log('asyncTask returning promise')

    return promise
}

const asyncFunction = async () => {
    console.log('asyncFunction started')

    const promise = asyncTask()

    const awaitResult = await promise

    console.log('returning from asyncFunction, awaitResult = "' 
        + awaitResult + '"')

    return 'I am returning with "' + awaitResult + '"'
}

const timer = () => setInterval(()=>console.log('tick'), 500)

//start of main

const t = timer()

const mainPromise = asyncFunction()

console.log('mainPromise =  ' + mainPromise)

mainPromise.then((result) => {
    console.log('mainPromise has resolved, result = ' + result)

    //stop timer
    clearInterval(t)
})

console.log('end of main code')

Here is the output:

C:\dev>node promises.js
asyncFunction started
asyncTask started
asyncTask returning promise
mainPromise =  [object Promise]
end of main code
tick
tick
tick
asyncTask resolving promise
returning from asyncFunction, awaitResult = "1000"
mainPromise has resolved, result = I am returning with "1000"

JavaScript does some tricky things behind the scenes with await so I think it may be helpful to carefully go over this code in order to see what happens at each step:

• In the main code, we start a timer.
• Next, we call asyncFunction.
• In asyncFunction, we call asyncTask.
• asyncTask creates a promise.
• The promise initiates a setTimeout.
• asyncTask returns the promise to asyncFunction.
• In asyncFunction, we now await the promise returned from asyncTask.
• This part is important: await is very similar to yield in a generator function. What happens here is that asyncFunction is temporarily suspended and "returns" early back to the “main” code. If asyncFunction were a generator function, then we could resume it in our own code by calling its next method. However, we will see that is not quite what happens in this case.
• What is yielded when asyncFunction is suspended? It turns out that the JavaScript runtime creates a new promise at this point and that's what is assigned to the mainPromise variable. It's important to realize this promise is different from the one that asyncTask returns.
• Now the rest of the "main" code runs and we see "end of main code" printed to the console. However, the JavaScript runtime doesn't exit because it still has work to do! After all, we still have a setTimeout pending (as well as our timer's setInterval) .
• Once two seconds have gone by (we can see this happening via our timer's "ticks"), setTimeout‘s callback function is invoked.
• This callback function in turn resolves the promise that is currently being awaited by asyncFunction.
• When the promise is resolved, the JavaScript runtime resumes asyncFunction from where it was suspended by await. This is very similar to calling next on a generator function, but here the runtime does it for us.
• Since there are no more await statements, asyncFunction now runs to completion and actually properly returns.
• What happens when asyncFunction returns? After all, it was already suspended earlier, and at that point, it yielded a promise that was assigned to the mainPromise variable.
• What happens is that the JavaScript engine intercepts the return and uses whatever value is in the return statement to fulfill the promise it created earlier.
  • We can see that this happens, because now the callback supplied to mainPromise.then is actually executed.
  • We returned a string from asyncFunction that included the value of the resolved promise from asyncTask: Therefore that's the string that is passed as result to the callback in mainPromise.then((result) => { console.log('mainPromise has resolved, result = ' + result) })

Since this stuff can easily get confusing, let's summarize:

• await in an async function is very similar to yield in a generator function: In both cases the function is suspended and execution returns to the point from which it was called.
• However, await is different in the following ways:
  • The JavaScript runtime will create a new promise and that's what is yielded when the function is suspended.
  • When the promise that is being awaited is fulfilled, the JavaScript runtime will automatically resume the async function
  • When the async function returns normally, the JavaScript runtime will use the function's return value to fulfill the promise that the runtime created earlier.

References:

Aync function
Await
Generator function
Iterators and generators

Related:

• Lazy Evaluation in JavaScript with Generators, Map, Filter, and Reduce
• How to Serialize Concurrent Operations in JavaScript: Callbacks, Promises, and Async/Await
• The Iterators Are Coming! [Symbol.iterator] and [Symbol.asyncIterator] in JavaScript
• Asynchronous Generators and Pipelines in JavaScript