Async / await vs then which is the best for performance?

Question

I have a simple code in JavaScript that execute a request in an API and return the response, simple. But in this case I will have thousands of requests. So, which one of the code options will perform better, and why. Also which one is recommended as good pratices these days?

First options is using the .then to resolve the promises and the seccond one is using async / await.

In my tests the two options had very similar results without significant differences, but I'm not sure in scale.

// Using then
doSomething(payload) {
  const url = 'https://link-here/consultas';
  return this.axios.get(url, {
    params: {
      token: payload.token,
      chave: payload.chave,
    },
   }).then(resp => resp.data);
}

// Using Async / await
async doSomething(payload) {
   const url = 'https://link-here/consultas';
   const resp = await this.axios.get(url, {
   params: {
     token: payload.token,
     chave: payload.chave,
    },
 });
 return resp.data;
}

Any explanation will be of great value.

Answer 1

There should be some corrections in this thread. await and .then are going to give very different results, and should be used for different reasons.

await will WAIT for something, and then continue to the next line. It's also the simpler of the two because it behaves mechanically more like synchronous behavior. You do step #1, wait, and then continue.

console.log("Runs first.");
await SomeFunction();
console.log("Runs last.");

.then splits off from the original call and starts operating within its own scope, and will update at a time the original scope cannot predict. If we can put semantics aside for a moment, it's "more asynchronous," because it leaves the old scope and branches off into a new one.

console.log("Runs first.");
SomeFunction().then((value) => {console.log("Runs last (probably). Didn't use await on SomeFunction().")})
console.log("Runs second (probably).");

Answer 2

As more explanation to @user280209 answer let's consider the following function which returns promise and compare its execution with .then() and async await.

function call(timeout) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log(`This call took ${timeout} seconds`);
      resolve(true);
    }, timeout * 1000);
  });
}

With .then()

(async () => {
  call(5).then((r) => {
    console.log(r);
  });
  await call(2); //This will print result first
  await call(1);
})();

When running the above call the logs will be

This call took 2 seconds
This call took 1 seconds
This call took 5 seconds
true

As we can see .then() didn't pause the execution of its below line until it completes.

With async/wait

(async () => {
  await call(5); //This will print result first
  await call(2); 
  await call(1);
})();

When run the above function logs will be

This call took 5 seconds
This call took 2 seconds
This call took 1 seconds

So I think if your promise's result won't be used in the following lines, .then() may be better.

Answer 3

Actually. Await/Async can perform more efficiently as Promise.then() loses the scope in which it was called after execution, you are attaching a callback to the callback stack.

What it causes is: The system now has to store a reference to where the .then() was called. In case of error it has to correctly point to where the error happens, otherwise, without the scope (as the system resumed execution after called the Promise, waiting to comeback to the .then() later) it isn't able to point to where the error happened.

Async/Await you suspend the exection of the method where it is being called thus preserving reference.

Answer 4

For those saying await blocks the code until the async call returns you are missing the point. "await" is syntactic sugar for a promise.then(). It is effectively wrapping the rest of your function in the then block of a promise it is creating for you. There is no real "blocking" or "waiting".

run();

async function run() {
    console.log('running');
    makePromises();
    console.log('exiting right away!');
}

async function makePromises() {
    console.log('make promise 1');
    const myPromise = promiseMe(1)
    .then(msg => {
        console.log(`What i want to run after the promise is resolved ${msg}`)
    })
    console.log('make promise 2')
    const msg = await promiseMe(2);
    console.log(`What i want to run after the promise is resolved via await ${msg}`)
}   

function promiseMe(num: number): Promise<string> {
    return new Promise((resolve, reject) => {
        console.log(`promise`)
        resolve(`hello promise ${num}`);
    })
}

The await line in makePromises does not block anything and the output is:

• running
• make promise 1
• promise
• make promise 2
• promise
• exiting right away!
• What i want to run after the promise is resolved hello promise 1
• What i want to run after the promise is resolved via await hello promise 2

Answer 5

If we just consider performance(time taken) then it actually depends on whether your operations are serial or parallel. If your tasks are serial then there will be no difference between await and .then. But if your tasks are parallel then .then will take less time. Consider the following example

let time = Date.now();

// first task takes 1.5 secs 
async function firstTask () {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(1);
        },1500)
    })
}

// second task takes 2 secs 
async function secondTask () {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(2);
        },2000)
    })
}

// using await
async function call(){
    const d1 = await firstTask();
    const d2 = await secondTask();
    console.log(Date.now()-time, d1+d2)
}
call()


// using .then
async function call2(){
    let d1=null,d2=null;
    firstTask().then(data => {
        d1=data;
        if(d2){
            console.log(Date.now()-time, d1+d2);
        }
    })
    secondTask().then(data => {
        d2=data;
        if(d1){
            console.log(Date.now()-time, d1+d2);
        }
    })
}
call2()

Here are the two tasks, first takes 1.5 secs and second takes 2 secs. Call function uses await where as call2 function uses .then . The output is as follows

From call2  2012 3
From call  3506 3

I hope it helps.