Does await guarantee execution order without assignment in JavaScript?

Question

Subject. Can I say that two pieces of code below are equal:

await someFunc() // no assignment here
doSomethingAfterSomeFunc()

and:

someFunc().then(() => 
  doSomethingAfterSomeFunc()
)

I tried and it looks like they are equal but there is a doubt(e.g. some optimization)

Answer 1

To expand on Dan D's answer (because it took me a while to figure out myself), I'll say a few more things about execution flow. Indeed, using await blocks the flow of the method it's called in until it resolves. Let's say we have this async function:

const someFunc = (str) => {
    return new Promise(resolve => {
        setTimeout(() => {
            console.log('resolving promise')
            resolve()
        }, 1500)
    })
}

So if we call with await, like this:

console.log('before calling')
await someFunc()
console.log('after calling')

we get the following result:

before calling
resolving promise
after calling

However, when we use .then():

console.log('before then')
someFunc().then(() => console.log('resolved'))
console.log('after then')

this happens:

before then
after then
resolving promise
resolved

This is because .then() does not stops the execution flow and runs next function in the chain only when the previous promise is finished. Sometimes you want this to happen, sometimes you don't, sometimes it doesn't matters. But if you don't know about this, it can take some time to figure it out. So I hope this example will help you understand it.

Answer 2

Yes they are exactly the same, it's more or less syntactic sugar. The await causes execution to pause until the awaited Promise is resolved.

See Javascript async the section on rewriting a promise chain for more information.