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How to transform an asynchronous function into a synchronous function in javascript? [duplicate]

First, this is a very specific case of doing it the wrong way on-purpose to retrofit an asynchronous call into a very synchronous codebase that is many thousands of lines long and time doesn't currently afford the ability to make the changes to "do it right." It hurts every fiber of my being, but reality and ideals often do not mesh. I know this sucks.
OK, that out of the way, how do I make it so that I could:
function doSomething() {
var data;
function callBack(d) {
data = d;
}
myAsynchronousCall(param1, callBack);
// block here and return data when the callback is finished
return data;
}
The examples (or lack thereof) all use libraries and/or compilers, both of which are not viable for this solution. I need a concrete example of how to make it block (e.g. NOT leave the doSomething function until the callback is called) WITHOUT freezing the UI. If such a thing is possible in JS.

"don't tell me about how I should just do it "the right way" or whatever"
OK. but you should really do it the right way... or whatever
" I need a concrete example of how to make it block ... WITHOUT freezing the UI. If such a thing is possible in JS."
No, it is impossible to block the running JavaScript without blocking the UI.
Given the lack of information, it's tough to offer a solution, but one option may be to have the calling function do some polling to check a global variable, then have the callback set data to the global.
function doSomething() {
// callback sets the received data to a global var
function callBack(d) {
window.data = d;
}
// start the async
myAsynchronousCall(param1, callBack);
}
// start the function
doSomething();
// make sure the global is clear
window.data = null
// start polling at an interval until the data is found at the global
var intvl = setInterval(function() {
if (window.data) {
clearInterval(intvl);
console.log(data);
}
}, 100);
All of this assumes that you can modify doSomething(). I don't know if that's in the cards.
If it can be modified, then I don't know why you wouldn't just pass a callback to doSomething() to be called from the other callback, but I better stop before I get into trouble. ;)
Oh, what the heck. You gave an example that suggests it can be done correctly, so I'm going to show that solution...
function doSomething( func ) {
function callBack(d) {
func( d );
}
myAsynchronousCall(param1, callBack);
}
doSomething(function(data) {
console.log(data);
});
Because your example includes a callback that is passed to the async call, the right way would be to pass a function to doSomething() to be invoked from the callback.
Of course if that's the only thing the callback is doing, you'd just pass func directly...
myAsynchronousCall(param1, func);

Async functions, a feature in ES2017, make async code look sync by using promises (a particular form of async code) and the await keyword. Also notice in the code examples below the keyword async in front of the function keyword that signifies an async/await function. The await keyword won't work without being in a function pre-fixed with the async keyword. Since currently there is no exception to this that means no top level awaits will work (top level awaits meaning an await outside of any function). Though there is a proposal for top-level await.
ES2017 was ratified (i.e. finalized) as the standard for JavaScript on June 27th, 2017. Async await may already work in your browser, but if not you can still use the functionality using a javascript transpiler like babel or traceur. Chrome 55 has full support of async functions. So if you have a newer browser you may be able to try out the code below.
See kangax's es2017 compatibility table for browser compatibility.
Here's an example async await function called doAsync which takes three one second pauses and prints the time difference after each pause from the start time:
function timeoutPromise (time) {
return new Promise(function (resolve) {
setTimeout(function () {
resolve(Date.now());
}, time)
})
}
function doSomethingAsync () {
return timeoutPromise(1000);
}
async function doAsync () {
var start = Date.now(), time;
console.log(0);
time = await doSomethingAsync();
console.log(time - start);
time = await doSomethingAsync();
console.log(time - start);
time = await doSomethingAsync();
console.log(time - start);
}
doAsync();
When the await keyword is placed before a promise value (in this case the promise value is the value returned by the function doSomethingAsync) the await keyword will pause execution of the function call, but it won't pause any other functions and it will continue executing other code until the promise resolves. After the promise resolves it will unwrap the value of the promise and you can think of the await and promise expression as now being replaced by that unwrapped value.
So, since await just pauses waits for then unwraps a value before executing the rest of the line you can use it in for loops and inside function calls like in the below example which collects time differences awaited in an array and prints out the array.
function timeoutPromise (time) {
return new Promise(function (resolve) {
setTimeout(function () {
resolve(Date.now());
}, time)
})
}
function doSomethingAsync () {
return timeoutPromise(1000);
}
// this calls each promise returning function one after the other
async function doAsync () {
var response = [];
var start = Date.now();
// each index is a promise returning function
var promiseFuncs= [doSomethingAsync, doSomethingAsync, doSomethingAsync];
for(var i = 0; i < promiseFuncs.length; ++i) {
var promiseFunc = promiseFuncs[i];
response.push(await promiseFunc() - start);
console.log(response);
}
// do something with response which is an array of values that were from resolved promises.
return response
}
doAsync().then(function (response) {
console.log(response)
})
The async function itself returns a promise so you can use that as a promise with chaining like I do above or within another async await function.
The function above would wait for each response before sending another request if you would like to send the requests concurrently you can use Promise.all.
// no change
function timeoutPromise (time) {
return new Promise(function (resolve) {
setTimeout(function () {
resolve(Date.now());
}, time)
})
}
// no change
function doSomethingAsync () {
return timeoutPromise(1000);
}
// this function calls the async promise returning functions all at around the same time
async function doAsync () {
var start = Date.now();
// we are now using promise all to await all promises to settle
var responses = await Promise.all([doSomethingAsync(), doSomethingAsync(), doSomethingAsync()]);
return responses.map(x=>x-start);
}
// no change
doAsync().then(function (response) {
console.log(response)
})
If the promise possibly rejects you can wrap it in a try catch or skip the try catch and let the error propagate to the async/await functions catch call. You should be careful not to leave promise errors unhandled especially in Node.js. Below are some examples that show off how errors work.
function timeoutReject (time) {
return new Promise(function (resolve, reject) {
setTimeout(function () {
reject(new Error("OOPS well you got an error at TIMESTAMP: " + Date.now()));
}, time)
})
}
function doErrorAsync () {
return timeoutReject(1000);
}
var log = (...args)=>console.log(...args);
var logErr = (...args)=>console.error(...args);
async function unpropogatedError () {
// promise is not awaited or returned so it does not propogate the error
doErrorAsync();
return "finished unpropogatedError successfully";
}
unpropogatedError().then(log).catch(logErr)
async function handledError () {
var start = Date.now();
try {
console.log((await doErrorAsync()) - start);
console.log("past error");
} catch (e) {
console.log("in catch we handled the error");
}
return "finished handledError successfully";
}
handledError().then(log).catch(logErr)
// example of how error propogates to chained catch method
async function propogatedError () {
var start = Date.now();
var time = await doErrorAsync() - start;
console.log(time - start);
return "finished propogatedError successfully";
}
// this is what prints propogatedError's error.
propogatedError().then(log).catch(logErr)
If you go here you can see the finished proposals for upcoming ECMAScript versions.
An alternative to this that can be used with just ES2015 (ES6) is to use a special function which wraps a generator function. Generator functions have a yield keyword which may be used to replicate the await keyword with a surrounding function. The yield keyword and generator function are a lot more general purpose and can do many more things then just what the async await function does. If you want a generator function wrapper that can be used to replicate async await I would check out co.js. By the way co's function much like async await functions return a promise. Honestly though at this point browser compatibility is about the same for both generator functions and async functions so if you just want the async await functionality you should use Async functions without co.js.
(I recommend just using async/await it's pretty widely supported in most environments that the above strikethrough is supported in.)
Browser support is actually pretty good now for Async functions (as of 2017) in all major current browsers (Chrome, Safari, and Edge) except IE.

Take a look at JQuery Promises:
http://api.jquery.com/promise/
http://api.jquery.com/jQuery.when/
http://api.jquery.com/deferred.promise/
Refactor the code:
var dfd = new jQuery.Deferred();
function callBack(data) {
dfd.notify(data);
}
// do the async call.
myAsynchronousCall(param1, callBack);
function doSomething(data) {
// do stuff with data...
}
$.when(dfd).then(doSomething);

You can force asynchronous JavaScript in NodeJS to be synchronous with sync-rpc.
It will definitely freeze your UI though, so I'm still a naysayer when it comes to whether what it's possible to take the shortcut you need to take. It's not possible to suspend the One And Only Thread in JavaScript, even if NodeJS lets you block it sometimes. No callbacks, events, anything asynchronous at all will be able to process until your promise resolves. So unless you the reader have an unavoidable situation like the OP (or, in my case, are writing a glorified shell script with no callbacks, events, etc.), DO NOT DO THIS!
But here's how you can do this:
./calling-file.js
var createClient = require('sync-rpc');
var mySynchronousCall = createClient(require.resolve('./my-asynchronous-call'), 'init data');
var param1 = 'test data'
var data = mySynchronousCall(param1);
console.log(data); // prints: received "test data" after "init data"
./my-asynchronous-call.js
function init(initData) {
return function(param1) {
// Return a promise here and the resulting rpc client will be synchronous
return Promise.resolve('received "' + param1 + '" after "' + initData + '"');
};
}
module.exports = init;
LIMITATIONS:
These are both a consequence of how sync-rpc is implemented, which is by abusing require('child_process').spawnSync:
This will not work in the browser.
The arguments to your function must be serializable. Your arguments will pass in and out of JSON.stringify, so functions and non-enumerable properties like prototype chains will be lost.

There is one nice workaround at http://taskjs.org/
It uses generators which are new to javascript. So it's currently not implemented by most browsers. I tested it in firefox, and for me it is nice way to wrap asynchronous function.
Here is example code from project GitHub
var { Deferred } = task;
spawn(function() {
out.innerHTML = "reading...\n";
try {
var d = yield read("read.html");
alert(d.responseText.length);
} catch (e) {
e.stack.split(/\n/).forEach(function(line) { console.log(line) });
console.log("");
out.innerHTML = "error: " + e;
}
});
function read(url, method) {
method = method || "GET";
var xhr = new XMLHttpRequest();
var deferred = new Deferred();
xhr.onreadystatechange = function() {
if (xhr.readyState === 4) {
if (xhr.status >= 400) {
var e = new Error(xhr.statusText);
e.status = xhr.status;
deferred.reject(e);
} else {
deferred.resolve({
responseText: xhr.responseText
});
}
}
};
xhr.open(method, url, true);
xhr.send();
return deferred.promise;
}

What you want is actually possible now. If you can run the asynchronous code in a service worker, and the synchronous code in a web worker, then you can have the web worker send a synchronous XHR to the service worker, and while the service worker does the async things, the web worker's thread will wait. This is not a great approach, but it could work.

let result;
async_function().then(r => result = r);
while (result === undefined) // Wait result from async_function
require('deasync').sleep(100);

In Node.js it's possible to write synchronous code which actually invokes asynchronous operations.
node-fibers allows this. It's a 3rd party native extension provided as an npm module.
It implements fibers/coroutines, so when a specific fiber is blocked waiting for asynchronous operation, the whole program events loop doesn't block - another fiber (if exists) continues its job.
With fibers your code would look like this:
var Fiber = require('fibers');
function doSomething() {
var fiber = Fiber.current;
function callBack(data) {
fiber.run(data);
}
myAsynchronousCall(param1, callBack);
// execution blocks here
var data = Fiber.yield();
return data;
}
// The whole program must be wrapped with Fiber
Fiber(function main() {
var data = doSomething();
console.log(data);
}).run();
Note, that you should avoid it and use async/await instead. See below a note from the project readme https://github.com/laverdet/node-fibers:
NOTE OF OBSOLESCENCE -- The author of this project recommends you avoid its use if possible. The original version of this module targeted nodejs v0.1.x in early 2011 when JavaScript on the server looked a lot different. Since then async/await, Promises, and Generators were standardized and the ecosystem as a whole has moved in that direction.
I'll continue to support newer versions of nodejs as long as possible but v8 and nodejs are extraordinarily complex and dynamic platforms. It is inevitable that one day this library will abruptly stop working and no one will be able to do anything about it.
I'd like to say thank you to all the users of fibers, your support over the years has meant a lot to me.

Using Node 16's worker threads actually makes this possible, The following example the main thread is running the asynchronous code while the worker thread is waiting for it synchronously.
Not that is is very useful, but it at least does vaguely what the original question asked by waiting for asynchronous code synchronously.
const {
Worker, isMainThread, parentPort, receiveMessageOnPort
} = require('worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
worker.on('message', async () => {
worker.postMessage(await doAsyncStuff());
});
} else {
console.log(doStuffSync());
}
function doStuffSync(){
parentPort.postMessage({fn: 'doStuff'});
let message;
while (!message) {
message = receiveMessageOnPort(parentPort)
}
return message;
}
function doAsyncStuff(){
return new Promise((resolve) => setTimeout(() => resolve("A test"), 1000));
}

One thing people might not consider: If you control the async function (which other pieces of code depend on), AND the codepath it would take is not necessarily asynchronous, you can make it synchronous (without breaking those other pieces of code) by creating an optional parameter.
Currently:
async function myFunc(args_etcetc) {
// you wrote this
return 'stuff';
}
(async function main() {
var result = await myFunc('argsetcetc');
console.log('async result:' result);
})()
Consider:
function myFunc(args_etcetc, opts={}) {
/*
param opts :: {sync:Boolean} -- whether to return a Promise or not
*/
var {sync=false} = opts;
if (sync===true)
return 'stuff';
else
return new Promise((RETURN,REJECT)=> {
RETURN('stuff');
});
}
// async code still works just like before:
(async function main() {
var result = await myFunc('argsetcetc');
console.log('async result:', result);
})();
// prints: 'stuff'
// new sync code works, if you specify sync mode:
(function main() {
var result = myFunc('argsetcetc', {sync:true});
console.log('sync result:', result);
})();
// prints: 'stuff'
Of course this doesn't work if the async function relies on inherently async operations (network requests, etc.), in which case the endeavor is futile (without effectively waiting idle-spinning for no reason).
Also this is fairly ugly to return either a value or a Promise depending on the options passed in.
("Why would I have written an async function if it didn't use async constructs?" one might ask? Perhaps some modalities/parameters of the function require asynchronicity and others don't, and due to code duplication you wanted a monolithic block rather than separate modular chunks of code in different functions... For example perhaps the argument is either localDatabase (which doesn't require await) or remoteDatabase (which does). Then you could runtime error if you try to do {sync:true} on the remote database. Perhaps this scenario is indicative of another problem, but there you go.)

This ability of promises includes two key features of synchronous operations as follows (or then() accepts two callbacks).
When you get the result, call resolve() and pass the final result.
In case of error, call reject().
The idea is that the result is passed through the chain of .then() handlers.
const synchronize = (() => {
let chain = Promise.resolve()
return async (promise) => {
return chain = chain.then(promise)
}
})()

I wondered the same thing and noticed that the currently best answer contains the right idea in my mind for most use cases, but forgets to mention a couple of things. When using a global variable to lock execution, we're talking about Semaphores, and there are some packages which implement those (my recommendation: async-sema). I think this makes it a little simpler and cleaner.
import { Sema } from 'async-sema'
const sema = new Sema(1) // allow only one concurrent process
async function doSomething() {
var data;
await sema.acquire();
// only one process gets inside here
data = await myAsynchronousCall(param1);
sema.release();
return data;
}
The advantage is obviously that the rest of your program can still do other things asynchronously, only the single block is kind of forced to be synchronously. Disadvantage is that you have to be careful what and where to lock, try/catch/finally possible errors, etc.

You can also convert it into callbacks.
function thirdPartyFoo(callback) {
callback("Hello World");
}
function foo() {
var fooVariable;
thirdPartyFoo(function(data) {
fooVariable = data;
});
return fooVariable;
}
var temp = foo();
console.log(temp);

The idea that you hope to achieve can be made possible if you tweak the requirement a little bit
The below code is possible if your runtime supports the ES6 specification.
More about async functions
async function myAsynchronousCall(param1) {
// logic for myAsynchronous call
return d;
}
function doSomething() {
var data = await myAsynchronousCall(param1); //'blocks' here until the async call is finished
return data;
}

How do I get data from pending resolved promise without async/await?

I have abstraction:
function fetchDataFromAPI() {
const url = `https://api...`
return fetch(url).then(response => response.json())
}
I want to use it in my other piece of code like:
if(something){
const data = fetchDataFromAPI()
return data
}
if I console.log data what I get is resolved pending promise
Promise {<pending>}
__proto__: Promise
[[PromiseStatus]]: "resolved"
[[PromiseValue]]: Object
How do I get that Object in data instead of Promise?

You can not. Here is why:
Promise is a language construct that makes JavaScript engine to continue to execute the code without waiting the return of inner function, also known as the executor function. A promise always run inside the event loop.
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
console.log(p);
Basically a promise is a glorified syntactic sugar for a callback. We will see how but first lets have a more realistic code:
function someApiCall(){
return new Promise(function(resolve, reject){
setTimeout(()=>{
resolve('Hello');
})
})
}
let data = someApiCall();
console.log(data);
This is a so-called asynchronous code, when JavaScript engine executes it, someApiCall immediately returns a result, in this case pending promise:
> Promise {<pending>}
If you pay attention to the executor, you will see we needed to pass resolve and reject arguments aka callbacks. Yes, they are callbacks required by the language construct. When either of them called, promise will change its state and hence be settled. We don't call it resolved because resolving implies successful execution but a function also can error out.
How do we get the data? Well we need more callbacks, which will be called by the executor function once the promise is settled:
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
p.then((result) => {
console.log(result); // foo
}).catch((err) => {
console.log(err);
});
Why we need to pass separate callbacks? Because one will be fed to the resolve, and the other to the reject. Then callback will be called by the resolve function, the catch callback by the reject function.
Javascript engine will execute these callbacks later on its leisure, for a regular function it means when the event loop is cleared, for timeout when the time is up.
Now to answer your question, how do we get data out from a promise. Well we can't.
If you look closely, you will see we don't really get the data out but keep feeding callbacks. There is no getting data out, but passing callbacks in.
p.then((result) => {
console.log(result);
}).catch((err) => {
console.log(err);
});
Some say use await:
async function() {
let result = await p;
}
But there is a catch. We have to or wrap it in async function. Always. Why? Because Async/await is another level of abstraction or syntactic sugar, whichever you prefer, on top of promise api. That is why we can not use await directly but always wrap it in async statement.
To sum up, when we use promise or async/await we need to follow certain convention and write terse code with closely knitted callbacks. Either javascript engine or transpilers like babeljs or typescript converts these code to regular javascript to be run.
I can understand your confusion because people keep saying getting data out when talking about promises, but we don't get any data out but pass callback to be executed when the data is ready.
Hope everything is clear now.

No, you cannot without using promises or async/await etc because calling a REST API is an asynchronous operation and is non blocking.
When you make a call to a REST API, the code shouldn't wait until the API returns a value because it may take a lot of time, making program non-responsive, thus by design making a network request is categorized as an asynchronous operation.

To avoid async/await, you'll need to use another .then:
if (something) {
return fetchDataFromAPI()
.then((data) => data /* you can console.log(data) here */)
}

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

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.

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.

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).");

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.

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

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.

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.

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

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);
});
}

Why Does Async Always Return a Promise?

This question is theoretical - I have no concrete problem to solve.
With that said, why does the async keyword wrap the return value of an async function in a promise? What's the point? Is it ONLY because the await expression expects a promise? Or is there some meaning / use behind this decision?

I thought i'd answer this primarily because async in Javascript used to confuse the hell out of me, and all of a sudden it snapped, so i hope this analogy may help this happen for you.
You have an async event. This could be anything, getting something from a server, doing something in the browser that takes time, training a machine learning model (!), executing a function or method that uses a setTimeout etc.
The beauty of Javascript and a key reason it works so well for the browser is that it uses the processor thread it runs on in a very clever way that stops the thread from getting blocked by processes that take time (like the ones mentioned above)
Many other languages, for example Ruby run on more than one thread. It is possible to use service workers to run processes on multiple threads in javascript but that is outside the scope of this answer!
The async nature of the JS event loop allows the thread to 'go off' and do something else while it is waiting for a process to finish.
The problem with this from a programming point of view is that it is possible for something in the code that relies on the result of a blocking event to get 'undefined' as a result of the event if it doesn't wait for the event to finish before it tries to use the result of it. Take this piece of code below
let scopedVariable
console.log('the code has started')
setTimeout(() => {
scopedVariable="I am the result of some async process"
}, 5000);
console.log(scopedVariable)
When the code reaches the console log, the setTimeout hasn't yet completed. As the setTimeout sets the scopedVariable only when it completes, the variable is undefined when we log it
if however
We wrap the timeout in a promise we can await it's resolve callback (first argument of promise) and the code will 'pause' until the promise reaches the resolve callback before continuing.
When we await the promise and the setTimeout completes, the resolve function sets the variable, so that when we console log it it holds the value from the promise
let scopedVariable
const asyncEvent = new Promise ((resolve,fail) => {
setTimeout(() => {
resolve(scopedVariable="I have resolved")
}, 5000);
})
const container = async () => {
const result = await asyncEvent
console.log(scopedVariable)
}
container()
You can use await and .then interchangably
For example we could go:
let scopedVariable
const asyncEvent = new Promise ((resolve,fail) => {
setTimeout(() => {
resolve(scopedVariable="I have resolved")
}, 5000);
})
const container = async () => {
asyncEvent.then(() => console.log(scopedVariable))
}
container()
once again the code will pause at .then and then continue when the asyncEvent promise has resolved.
In fact if we use .then we don't need to enclose it in an async function so we can rewrite it like this
let scopedVariable
const asyncEvent = new Promise ((resolve,fail) => {
setTimeout(() => {
resolve(scopedVariable="I have resolved")
}, 5000);
})
asyncEvent.then(() => console.log(scopedVariable))
The great thing about .then is that the accompanying .catch allows you to catch any errors thrown by the async event (for example if retrieving something from a server when there is an error). For async await you need to wrap potentially dangerous functions in a try catch.
In order to use await you need to be inside an async function (hence the async container function above). This is not necessary with .then, but .then and .catch chains can make your code messy.
I hope this helps!

The async and await operators are just syntactic sugar that hide the underlying use of promises to implement asynchronous code.
Using async before a function definition makes the function return a promise that resolves to the function's return value, rather than returning normally.
Using await before an asynchronous function call suspends the current function until the promise that it returns is resolved. It's basically equivalent to wrapping the remainder of the function in an anonymous function, and using that as the .then() callback of the promise.
For more information between the relationship, see How to translate Promise code to async await

How to wait for a JavaScript Promise to resolve before resuming function?

I'm doing some unit testing. The test framework loads a page into an iFrame and then runs assertions against that page. Before each test begins, I create a Promise which sets the iFrame's onload event to call resolve(), sets the iFrame's src, and returns the promise.
So, I can just call loadUrl(url).then(myFunc), and it will wait for the page to load before executing whatever myFunc is.
I use this sort of pattern all over the place in my tests (not just for loading URLs), primarily in order to allow changes to the DOM to happen (e.g. mimick clicking a button, and wait for divs to hide and show).
The downside to this design is that I'm constantly writing anonymous functions with a few lines of code in them. Further, while I have a work-around (QUnit's assert.async()), the test function that defines the promises completes before the promise is run.
I'm wondering if there is any way to get a value from a Promise or wait (block/sleep) until it has resolved, similar to .NET's IAsyncResult.WaitHandle.WaitOne(). I know JavaScript is single-threaded, but I'm hoping that doesn't mean that a function can't yield.
In essence, is there a way to get the following to spit out results in the correct order?
function kickOff() {
return new Promise(function(resolve, reject) {
$("#output").append("start");
setTimeout(function() {
resolve();
}, 1000);
}).then(function() {
$("#output").append(" middle");
return " end";
});
};
function getResultFrom(promise) {
// todo
return " end";
}
var promise = kickOff();
var result = getResultFrom(promise);
$("#output").append(result);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="output"></div>

I'm wondering if there is any way to get a value from a Promise or
wait (block/sleep) until it has resolved, similar to .NET's
IAsyncResult.WaitHandle.WaitOne(). I know JavaScript is
single-threaded, but I'm hoping that doesn't mean that a function
can't yield.
The current generation of Javascript in browsers does not have a wait() or sleep() that allows other things to run. So, you simply can't do what you're asking. Instead, it has async operations that will do their thing and then call you when they're done (as you've been using promises for).
Part of this is because of Javascript's single threadedness. If the single thread is spinning, then no other Javascript can execute until that spinning thread is done. ES6 introduces yield and generators which will allow some cooperative tricks like that, but we're quite a ways from being able to use those in a wide swatch of installed browsers (they can be used in some server-side development where you control the JS engine that is being used).
Careful management of promise-based code can control the order of execution for many async operations.
I'm not sure I understand exactly what order you're trying to achieve in your code, but you could do something like this using your existing kickOff() function, and then attaching a .then() handler to it after calling it:
function kickOff() {
return new Promise(function(resolve, reject) {
$("#output").append("start");
setTimeout(function() {
resolve();
}, 1000);
}).then(function() {
$("#output").append(" middle");
return " end";
});
}
kickOff().then(function(result) {
// use the result here
$("#output").append(result);
});
This will return output in a guaranteed order - like this:
start
middle
end
Update in 2018 (three years after this answer was written):
If you either transpile your code or run your code in an environment that supports ES7 features such as async and await, you can now use await to make your code "appear" to wait for the result of a promise. It is still programming with promises. It does still not block all of Javascript, but it does allow you to write sequential operations in a friendlier syntax.
Instead of the ES6 way of doing things:
someFunc().then(someFunc2).then(result => {
// process result here
}).catch(err => {
// process error here
});
You can do this:
// returns a promise
async function wrapperFunc() {
try {
let r1 = await someFunc();
let r2 = await someFunc2(r1);
// now process r2
return someValue; // this will be the resolved value of the returned promise
} catch(e) {
console.log(e);
throw e; // let caller know the promise was rejected with this reason
}
}
wrapperFunc().then(result => {
// got final result
}).catch(err => {
// got error
});
async functions return a promise as soon as the first await is hit inside their function body so to the caller an async function is still non-blocking and the caller must still deal with a returned promise and get the result from that promise. But, inside the async function, you can write more sequential-like code using await on promises. Keep in mind that await only does something useful if you await a promise so in order to use async/await, your asynchronous operations must all be promise-based.

If using ES2016 you can use async and await and do something like:
(async () => {
const data = await fetch(url)
myFunc(data)
}())
If using ES2015 you can use Generators. If you don't like the syntax you can abstract it away using an async utility function as explained here.
If using ES5 you'll probably want a library like Bluebird to give you more control.
Finally, if your runtime supports ES2015 already execution order may be preserved with parallelism using Fetch Injection.

Another option is to use Promise.all to wait for an array of promises to resolve and then act on those.
Code below shows how to wait for all the promises to resolve and then deal with the results once they are all ready (as that seemed to be the objective of the question); Also for illustrative purposes, it shows output during execution (end finishes before middle).
function append_output(suffix, value) {
$("#output_"+suffix).append(value)
}
function kickOff() {
let start = new Promise((resolve, reject) => {
append_output("now", "start")
resolve("start")
})
let middle = new Promise((resolve, reject) => {
setTimeout(() => {
append_output("now", " middle")
resolve(" middle")
}, 1000)
})
let end = new Promise((resolve, reject) => {
append_output("now", " end")
resolve(" end")
})
Promise.all([start, middle, end]).then(results => {
results.forEach(
result => append_output("later", result))
})
}
kickOff()
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
Updated during execution: <div id="output_now"></div>
Updated after all have completed: <div id="output_later"></div>