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

From a performance point of view, await is just an internal version of .then() (doing basically the same thing). The reason to choose one over the other doesn't really have to do with performance, but has to do with desired coding style or coding convenience. Certainly, the interpreter has a few more opportunities to optimize things internally with await, but its unlikely that should be how you decide which to use. If all else was equal, I would choose await for the reason cited above. But, I'd first choose which made the code simpler to write and understand and maintain and test.

Used properly, await can often save you a bunch of lines of code making your code simpler to read, test and maintain. That's why it was invented.

There's no meaningful difference between the two versions of your code. Both achieve the same result when the axios call is successful or has an error.

Where await could make more of a convenience difference is if you had multiple successive asynchronous calls that needed to be serialized. Then, rather than bracketing them each inside a .then() handler to chain them properly, you could just use await and have simpler looking code.

A common mistake with both await and .then() is to forget proper error handling. If your error handling desire in this function is to just return the rejected promise, then both of your versions do that identically. But, if you have multiple async calls in a row and you want to do anything more complex than just returning the first rejection, then the error handling techniques for await and .then()/.catch() are quite different and which seems simpler will depend upon the situation.

Answer 2

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 3

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 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

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 6

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.

Answer 7

As far as I understand .then() and await are not the same thing. An async function won't proceed with the next command until the promise is resolved/rejected since it's basically an implementation of generators. On the contrast, in the case of .then(), the execution of the function will proceed with the next command and the resolve/reject callback will be executed "when there's time" aka when the current event loop (not entirely sure about that part) will be completed.

tldr; on a single promise await and .then() behave similarly but when one promise needs another one to be resolved first then the two of them behave entirely different

Answer 8

Many answer have been provided to this question already. However, to point out key information in the answers above and from my understanding, note below point:

• only use await when not handling error return
• if no crucial need for error handling use await instead
• use .then .catch if returned error message or data is crucial for debugging / or proper error handling instead of try catch for await

Choose any prefer method from code sample below

const getData = (params = {name: 'john', email: 'ex@gmail.com'}) => {
  return axios.post(url, params);
}

// anywhere you want to get the return data

// using await

const setData = async () => {

  const data = await getData();
  
  }
  
  // to handle error with await
  const setData = async () => {
    try {
      const data = await getData();
      }
     catch(err) {
        console.log(err.message);
     }
  }
  
   // using .then .catch
  const setData = () => {
 
      var data; 
      getData().then((res) => { 
        data = res.data; console.log(data)
      }).catch((err) => {
        console.log(err.message);
      });

  }