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Combination of async function + await + setTimeout

I am trying to use the new async features and I hope solving my problem will help others in the future. This is my code which is working:

  async function asyncGenerator() {
    // other code
    while (goOn) {
      // other code
      var fileList = await listFiles(nextPageToken);
      var parents = await requestParents(fileList);
      // other code
    }
    // other code
  }

  function listFiles(token) {
    return gapi.client.drive.files.list({
      'maxResults': sizeResults,
      'pageToken': token,
      'q': query
    });
  }

The problem is, that my while loop runs too fast and the script sends too many requests per second to the google API. Therefore I would like to build a sleep function which delays the request. Thus I could also use this function to delay other requests. If there is another way to delay the request, please let me know.

Anyway, this is my new code which does not work. The response of the request is returned to the anonymous async function within the setTimeout, but I just do not know how I can return the response to the sleep function resp. to the initial asyncGenerator function.

  async function asyncGenerator() {
    // other code
    while (goOn) {
      // other code
      var fileList = await sleep(listFiles, nextPageToken);
      var parents = await requestParents(fileList);
      // other code
    }
    // other code
  }

  function listFiles(token) {
    return gapi.client.drive.files.list({
      'maxResults': sizeResults,
      'pageToken': token,
      'q': query
    });
  }

  async function sleep(fn, par) {
    return await setTimeout(async function() {
      await fn(par);
    }, 3000, fn, par);
  }

I have already tried some options: storing the response in a global variable and return it from the sleep function, callback within the anonymous function, etc.


R
Rob Bednark

Your sleep function does not work because setTimeout does not (yet?) return a promise that could be awaited. You will need to promisify it manually:

function timeout(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}
async function sleep(fn, ...args) {
    await timeout(3000);
    return fn(...args);
}

Btw, to slow down your loop you probably don't want to use a sleep function that takes a callback and defers it like this. I recommend:

while (goOn) {
  // other code
  var [parents] = await Promise.all([
      listFiles(nextPageToken).then(requestParents),
      timeout(5000)
  ]);
  // other code
}

which lets the computation of parents take at least 5 seconds.


Love the Promise.all approach. So simple and elegant!
what does the notation of var [parents] represent? I haven't seen it before and it's a difficult thing to google
@NateUsher It's array destructuring
@tinkerr "timeout needs to be declared async if it needs to be awaited" - Nope. A function only needs to return a promise that can be awaited (or actually, a thenable is enough). How it achieves that is up to the implementation of the function, it does not need to be an async function.
@naisanza No, async/await is based on promises. The only thing it replaces are then calls.
F
FlavorScape

The quick one-liner, inline way

 await new Promise(resolve => setTimeout(resolve, 1000));

what does it mean when you guys use "resolve" x 2 times in the same line? Like: await new Promise(resolve => setTimeout(resolve, 1000)); does it ref. to itself or what? I would do something like this instead: function myFunc(){}; await new Promise(resolve => setTimeout(myFunc, 1000));
@PabloDK that would block forever because the promise never resolves.
@PabloDK You can expand the one-liner above to this (more verbose) version, which hopefully makes it obvious why resolve appears twice. If it's still confusing, take a look at the MDN docs for Promise.
@PabloDK It could also be represented like this: await new Promise((resolve, reject) => setTimeout(resolve, 1000));. So, resolve and reject are callbacks exposed when you create a Promise. You're simply telling setTimeout to execute resolve().
this fail with eslint 8, no-promise-executor-return
H
Harry

Since Node 7.6, you can combine the functions promisify function from the utils module with setTimeout() .

Node.js

const sleep = require('util').promisify(setTimeout)

Javascript

const sleep = m => new Promise(r => setTimeout(r, m))

Usage

(async () => {
    console.time("Slept for")
    await sleep(3000)
    console.timeEnd("Slept for")
})()

In nodeJS await require('util').promisify(setTimeout)(3000) can also be achieved without require by: await setTimeout[Object.getOwnPropertySymbols(setTimeout)[0]](3000)
Interesting @Shl. I think it is less readable than my solution though. If people disagree I can add it to the solution?
The require version is clearly much better than the getOwnPropertySymbols version... if it ain't broke...!
Hey there @Harry. It appears you incorporated the one liner from FlavorScape's answer in your own answer. I don't want to presume of your intentions, but that isn't really fair to them. Could you rollback your edit? Right now it looks a bit like plagiarism..
I've removed the one-liner as the answer is right below, however I have seen many popular answers update their answers to include other new answers as most readers don't bother looking past the first few responses.
L
Leonid Beschastny

setTimeout is not an async function, so you can't use it with ES7 async-await. But you could implement your sleep function using ES6 Promise:

function sleep (fn, par) {
  return new Promise((resolve) => {
    // wait 3s before calling fn(par)
    setTimeout(() => resolve(fn(par)), 3000)
  })
}

Then you'll be able to use this new sleep function with ES7 async-await:

var fileList = await sleep(listFiles, nextPageToken)

Please, note that I'm only answering your question about combining ES7 async/await with setTimeout, though it may not help solve your problem with sending too many requests per second.

Update: Modern node.js versions has a buid-in async timeout implementation, accessible via util.promisify helper:

const {promisify} = require('util');
const setTimeoutAsync = promisify(setTimeout);

You shouldn't do that, when fn throws the errror would not be caught.
@Bergi I think it bubbles up to the new Promise where you can sleep.catch it.
@Dodekeract No, it's in an asynchronous setTimeout callback and the new Promise callback has been done for long. It will bubble to the global context and be thrown as an unhandled exception.
> problem with sending too many requests per second. You want to use "debounce" perhaps to prevent things like UI firing too many ruquests.
t
t_dom93

Timers Promises API

await setTimeout finally arrived with Node.js 16, removing the need to use util.promisify():

import { setTimeout } from 'timers/promises';

(async () => {
  const result = await setTimeout(2000, 'resolved')
  // Executed after 2 seconds
  console.log(result); // "resolved"
})()

Official Node.js docs: Timers Promises API (library already built in Node)


D
Dave Bitter

If you would like to use the same kind of syntax as setTimeout you can write a helper function like this:

const setAsyncTimeout = (cb, timeout = 0) => new Promise(resolve => {
    setTimeout(() => {
        cb();
        resolve();
    }, timeout);
});

You can then call it like so:

const doStuffAsync = async () => {
    await setAsyncTimeout(() => {
        // Do stuff
    }, 1000);

    await setAsyncTimeout(() => {
        // Do more stuff
    }, 500);

    await setAsyncTimeout(() => {
        // Do even more stuff
    }, 2000);
};

doStuffAsync();

I made a gist: https://gist.github.com/DaveBitter/f44889a2a52ad16b6a5129c39444bb57


a function name like delayRun would make more sense here, since it will delay the running of the callback function by X seconds. Not a very await-ey example, IMO.
v
vignesh
var testAwait = function () {
    var promise = new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve('Inside test await');
        }, 1000);
    });
    return promise;
}

var asyncFunction = async function() {
    await testAwait().then((data) => {
        console.log(data);
    })
    return 'hello asyncFunction';
}

asyncFunction().then((data) => {
    console.log(data);
});

//Inside test await
//hello asyncFunction

G
General Grievance
await new Promise(resolve => setTimeout(() => { resolve({ data: 'your return data'}) }, 1000))

E
Erik P_yan

I leave this code snippet here for someone who wants to fetch API call (e.g. get clients) with setTimeout:

const { data } = await new Promise(resolve => setTimeout(resolve, 250)).then(() => getClientsService())
setName(data.name || '')
setEmail(data.email || '')

This should be the accepted answer.
I
IgorAlves

This is my version with nodejs now in 2020 in AWS labdas

const sleep = require('util').promisify(setTimeout)

async function f1 (some){
...
}

async function f2 (thing){
...
}

module.exports.someFunction = async event => {
    ...
    await f1(some)
    await sleep(5000)
    await f2(thing)
    ...
}

What is promisify doing to setTimeout for your custom sleep function that causes it to no longer need a function as the first argument? For example, if you run setTimeout(5000); (not having a function as the first argument) you get Uncaught TypeError [ERR_INVALID_CALLBACK]: Callback must be a function. Received 5000.
D
D. Schreier
await setTimeout(()=>{}, 200);

Will work if your Node version is 15 and above.


Doesn't work, even in Node ^16. Ref comments in stackoverflow.com/a/57390854/12417767
J
Jee Mok

Made a util inspired from Dave's answer

Basically passed in a done callback to call when the operation is finished.

// Function to timeout if a request is taking too long
const setAsyncTimeout = (cb, timeout = 0) => new Promise((resolve, reject) => {
  cb(resolve);
  setTimeout(() => reject('Request is taking too long to response'), timeout);
});

This is how I use it:

try {
  await setAsyncTimeout(async done => {
    const requestOne = await someService.post(configs);
    const requestTwo = await someService.get(configs);
    const requestThree = await someService.post(configs);
    done();
  }, 5000); // 5 seconds max for this set of operations
}
catch (err) {
  console.error('[Timeout] Unable to complete the operation.', err);
}

K
Kuya

The following code works in Chrome and Firefox and maybe other browsers.

function timeout(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}
async function sleep(fn, ...args) {
    await timeout(3000);
    return fn(...args);
}

But in Internet Explorer I get a Syntax Error for the "(resolve **=>** setTimeout..."


s
sounish nath

How to Log all the responses at once?

async function sayHello(name) {
  let greet = `Hey! ${name} very nice to meet you bud.`;
  setTimeout(() => {
    return {
      greet,
      createdAt: new Date(),
    };
  }, 1000);
}

const response1 = async () => await sayHello("sounish");
const response2 = async () => await sayHello("alex");
const response3 = async () => await sayHello("bill");

async function getData() {
  const data1 = await sayHello("sounish");
  const data2 = await sayHello("alex");
  const data3 = await sayHello("bill");
  return { data1, data2, data3 };
}

Promise.all([sayHello("sounish"), sayHello("alex"), sayHello("bill")]).then(
  (allResponses) => {
    console.log({ allResponses });
  }
);

getData().then((allData) => {
  console.log({ allData });
});

M
Mohammad Reza Shahrestani

This is a quicker fix in one-liner.

Hope this will help.

// WAIT FOR 200 MILISECONDS TO GET DATA //
await setTimeout(()=>{}, 200);

Doesn't work. This: await setTimeout(()=>{console.log('first')}, 200); console.log ('second') prints second then first
@gregn3 that is the point yes. This is a non blocking solution where code outside the function can continue to execute while a "blocking operation" is completed outside the main program flow. Although the syntax you and Rommy and Mohamad have provided isn't strictly correct due to the requirement for an await to be rapped in an async function (might be a fairly recent addition), also I'm using node.js. This is my tweaked solution. var test = async () => { await setTimeout(()=>{console.log('first')}, 1000); console.log ('second') } I've extended the timeout to show it's usefulness.