I am reading MDN on how async code works with javascript. They propose this example:
async function myFetch() {
let response = await fetch('coffee.jpg');
if (!response.ok) {
throw new Error(`HTTP error! status: ${response.status}`);
}
let myBlob = await response.blob();
let objectURL = URL.createObjectURL(myBlob);
let image = document.createElement('img');
image.src = objectURL;
document.body.appendChild(image);
}
myFetch()
.catch(e => {
console.log('There has been a problem with your fetch operation: ' + e.message);
});
Function myFetch() is defined and then it's called in a Promise-way style. How exactly is the function executed? It's an async function, so I assume that it's given to some thread to execute without interfering with the single thread that runs Javascript code?
Also in the docs they said await is stoping the code on that line until completion? So the function can't be executed by the main application thread because then it's not async code anymore?
So those are my issues and I wanna be sure I understand this mechanism before I start writing actual code. Thank you!
using async/await is pretty similar to using promises, async function returns a Promise for you, that promise can have one of three statuses (Pending, Fulfilled, Rejected)
By default, the returned Promise is Fulfilled (Resolved)
similar to:
Promise.resolve().then(_ => { //... })
using await, the Promise will be pending on the code that you are awaiting for
new Promise((resolve, reject) => {
// ... some tasks here
resolve(resolvedData);
})
.then(_ => { //... })
when an error is thrown, automatically or manually, the Promise will be rejected
new Promise((resolve, reject) => {
reject(); // Reject manually
})
.then(_ => { })
.catch(e => { //... })
By default, all JavaScript code is executing within the main thread, which is the only thread that JavaScript has
Look at the following example:
async function c() {
const t = await Array.from(Array(100000000).keys())
}
c().then(t => console.log('I am waiting for the Promise to be fulfilled!'))
console.log('Hey, I don\'t need to wait');
You will get the expected output:
"Hey, I don't need to wait"
"I am waiting for the Promise to be fulfilled!"
but both logs are waiting for filling this 100000000 array, so the main thread now is busy and blocked to complete this task first, JavaScript itself can't help with this case,
Another Example:
async function c() {
const t = await fetch('https://jsonplaceholder.typicode.com/photos')
}
c().then(_ => console.log('I am waiting for the Promise to be fulfilled!'))
console.log('Hey, I don\'t need to wait');
Here you'll get the same result, but Hey, I don't need to wait will be logged immediately, and only I am waiting for the Promise to be fulfilled! will be waiting,
Why? here we need to refer to few important things to read more about (Event Loop, Call Stack, Micro & Macro tasks )
In simple words, the promise behaves the same, it's pending on something, and the first console log in the code is awaiting the promise to be fulfilled,
the difference is that the task that we are waiting for to have a fulfilled promise now is not thread-blocking, it is not controlled by JavaScript itself
the fetching task is managed by an external API that is observed by the Event loop, in this case, the call stack will not be busy, and the main JavaScript thread can continue working, the code that will resolve the promise is added to a queue and will be pushed to the call stack by the Event loop when the fetching task is done by the external API
Function myFetch() is defined and then it's called in a Promise-way style.
It's called like any other function. All async functions wrap the return value in a Promise, thus there are then, catch and finally methods available.
How exactly is the function executed?
Using () like any other function.
It's an async function, so I assume that it's given to some thread to execute without interfering with the single thread that runs Javascript code?
There is no need to have it run on some other thread. As you say, JavaScript execution happens in a single thread.
Also in the docs they said await is stoping the code on that line until completion?
Yes, in that async function.
So the function can't be executed by the main application thread because then it's not async code anymore?
There doesn't have to be any threading involved. It is only the fetch and blob operations that are performed asynchronously. While those operations are being awaited, the event loop and the execution of code outside the async function will continue. If you have code below the call to myFetch then that will be executed while the fetch is being awaited, unless you await the call to myFetch itself.
async and await are syntactic sugar on top of Promises, so effectively your code is doing the following:
fetch('coffee.jpg').then(response => {
if (!response.ok) {
throw new Error(`HTTP error! status: ${response.status}`);
}
response.blob().then(myBlob => {
let objectURL = URL.createObjectURL(myBlob);
let image = document.createElement('img');
image.src = objectURL;
document.body.appendChild(image);
});
}).catch(e => {
console.log('There has been a problem with your fetch operation: ' + e.message);
});
Related
I would like to run this code with babel:
redisClientAsync.delAsync('key');
return await someOtherAsyncFunction();
inside an async function without await the first line. is this OK?
how else can I run something that I don't care?
Can I just fire the non-promisified function del('key',null) without a callback?
Yes, you can do that, and it will run the two asynchronous functions in parallel. You've just created a promise and thrown it away.
However, this means that when the promise is rejected you won't notice. You'll just get an unhandledRejection eventually which will crash your process if not handled.
Is this OK? How can I run something that I don't care?
Probably it's not OK. If you truly wouldn't care, you hadn't run it in the first place. So you should be clear and explicit what you care about (and what not):
do you want to wait? (for side effects)
do you need the result?
do you want to catch exceptions?
If you only want to wait and don't care for the result value, you can easily throw away the result:
void (await someAsyncFunction()); // or omit the void keyword,
// doesn't make a difference in an expression statement
If you don't care about exceptions, you can ignore them using
… someAsyncFunction().catch(function ignore() {}) …
You can throw that away, await it, do anything with it.
If you want the result, you have to await it. If you care about exceptions, but don't really want to wait, you may want to execute it in parallel with the following functions:
var [_, res] = await Promise.all([
someAsyncFunction(), // result is ignored, exceptions aren't
someOtherAsyncFunction()
]);
return res;
inside an async function without await the first line. is this OK?
Yes, there are cases where you'd want to do this which are perfectly reasonable. Especially where you don't care about the result - one example is an analytics tracking operation that should not interfere with business critical code.
how else can I run something that I don't care?
In many ways, however simply calling the promise function works. Your del without a callback would probably work in this case but some functions don't guard against not passing callbacks, so you can pass an empty function instead (.del('key', () => {})).
You do want to however make sure that you know about it failing, even if you don't want to disrupt the operation of code - so please consider adding a process.on("unhandledRejection', event handler to explicitly ignore these particular exceptions or suppress them via:
redisClient.delAsync('key').catch(()=>{});
Or preferably, something like:
redisClient.delAsync('key').catch(logErr);
From all the research I've made so far, I think it's fine to do it, as long as you guarantee that the function you are not awaiting for guarantees a way to handle its own errors in case that happens. For example, a try-catch wrapping the whole function body, like you see in the following snippet for the asyncFunction.
It doesn't matter if the function throws synchronously or asynchronously. It guarantees the your mainFunction will complete no matter what. That's the key point here.
If you don't guarantee that, you have to risks:
If it throws synchronously, your main function will not complete.
If it throws asynchronously, you'll get an unhandled excepction
// THIS IS SOME API CALL YOU DON'T WANT TO WAIT FOR
const mockAPI = () => {
console.log("From mockAPI");
return new Promise((resolve,reject) => {
setTimeout(() => reject("LATE THROW: API ERROR"), 500);
});
};
// THIS IS THE SOME ASYNC FUNCTION YOU CALL BUT NOT AWAIT FOR
const asyncFunction = async (syncThrow) => {
try {
console.log("Async function START");
if (syncThrow) throw new Error("EARLY THROW");
await mockAPI();
console.log("Async function DONE");
}
catch(err) {
console.log("From async function catch");
console.log(err.message || err);
return;
}
};
// THIS IS YOUR MAIN FUNCTION
const mainFunction = async (syncThrow) => {
try {
console.clear();
console.log("Main function START");
asyncFunction(syncThrow);
console.log("Main function DONE <<< THAT'S THE IMPORTANT PART");
}
catch(err) {
console.log("THIS WILL NEVER HAPPEN");
console.log(err);
}
};
<div>
<button onClick="mainFunction(true)">Sync throw</button>
<button onClick="mainFunction(false)">Async throw</button>
</div>
Not in Node.js.
Node does not wait for ever-pending Promises. If other tasks are already completed and there is nothing left in the event loop, the Node process will be terminated even though there exists pending promise.
For the following script, if someOtherAsyncFunction() get resolved in 5 seconds, but redisClientAsync.delAsync('key') takes 10 seconds to execute, the Node process will be terminated after 5 seconds in theory, before the first line is resolved.
async function doSomething() {
redisClientAsync.delAsync('key');
return await someOtherAsyncFunction();
}
await doSomething();
I am new to JS and was learning promises. So, let's say we have this code:
new Promise(function(resolve, reject) {
setTimeout(() => resolve(1), 1000); // (*)
}).then(function(result) { // (**)
alert(result); // 1
return result * 2;
})
As you can see the code above, when promise is invoked, setTimeout is run via callback queue. The question is When setTimeOut is sent to a browser, will JS engine omit .then() and continues running the rest of the code until the promise resolves? Secondly, async/await example:
async function showAvatar() {
// read our JSON
let response = await fetch('/article/promise-chaining/user.json');
let user = await response.json();
// read github user
let githubResponse = await fetch(`https://api.github.com/users/${user.name}`);
let githubUser = await githubResponse.json();
// show the avatar
let img = document.createElement('img');
img.src = githubUser.avatar_url;
img.className = "promise-avatar-example";
document.body.append(img);
// wait 3 seconds
await new Promise((resolve, reject) => setTimeout(resolve, 3000));
img.remove();
return githubUser;
}
showAvatar();
When showAvatar function is called, JS engine will encounter let response = await fetch('/article/promise-chaining/user.json'); and sends fetch to the browser to handle. The second question is Will JS engine wait until fetch gets resolved or Will JS engine continue executing let user = await response.json(); and the rest of the code inside showAvatar function? If so, how can JS engine handle response.json() since response is not received? Hope you got my point))).
Your first example works like this:
new Promise runs, calling the function you pass it (the executor function) synchronously
Code in the executor function calls setTimeout, passing in a function to call 1000ms later; the browser adds that to its list of pending timer callbacks
new Promise returns the promise
then is called, adding the function you pass into it to the promise's list of fulfillment handlers and creating a new promise (which your code doesn't use, so it gets thrown away).
1000ms or so later, the browser queues a call to the setTimeout callback, which the JavaScript engine picks up and runs
The callback calls the resolve function to fulfill the promise with the value 1
That triggers the promise's fulfillment handlers (asynchronously, but it doesn't really matter for this example), so the handler attached in Step 4 gets called, showing the alert and then returning result * 2 (which is 1 * 2, which is 1). That value is used to fulfill the promise created and thrown away in Step 4.
Will JS engine wait until fetch gets resolved or Will JS engine continue executing let user = await response.json();...
It waits. The async function is suspended at the await in await fetch(/*...*/), waiting for the promise fetch returned to settle. While it's suspended, the main JavaScript thread can do other things. Later, when the promise settles, the function is resumed and either the fulfillment value is assigned to response (if the promise is fulfilled) or an exception will get thrown (if it is rejected).
More generally: async functions are synchronous up until the first await or return in their code. At that point, they return their promise, which is settled later based on the remainder of the async function's code.
In a comment you asked:
when async function is suspended at each await, will async function is removed from the call stack and is put back to the call stack again when the promise being awaited settles?
At a low level, yes; but to make debugging easier, a good, up-to-date JavaScript engine maintains an "async call stack" they use for error traces and such. For instance, if you run this on Chrome...
function delay(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
async function inner() {
await delay(80);
throw new Error("boom");
}
async function outer() {
await inner();
}
function wrapper() {
outer()
.then(result => {
console.log(result);
})
.catch(error => {
console.log(error.stack);
});
}
wrapper();
...the stack looks like this:
Error: boom
at inner (https://stacksnippets.net/js:18:11)
at async outer (https://stacksnippets.net/js:22:5)
Notice the "async" prior to "outer," and also notice that wrapper isn't mentioned anywhere. wrapper is done and has returned, but the async functions were suspended and resumed.
I would like to run this code with babel:
redisClientAsync.delAsync('key');
return await someOtherAsyncFunction();
inside an async function without await the first line. is this OK?
how else can I run something that I don't care?
Can I just fire the non-promisified function del('key',null) without a callback?
Yes, you can do that, and it will run the two asynchronous functions in parallel. You've just created a promise and thrown it away.
However, this means that when the promise is rejected you won't notice. You'll just get an unhandledRejection eventually which will crash your process if not handled.
Is this OK? How can I run something that I don't care?
Probably it's not OK. If you truly wouldn't care, you hadn't run it in the first place. So you should be clear and explicit what you care about (and what not):
do you want to wait? (for side effects)
do you need the result?
do you want to catch exceptions?
If you only want to wait and don't care for the result value, you can easily throw away the result:
void (await someAsyncFunction()); // or omit the void keyword,
// doesn't make a difference in an expression statement
If you don't care about exceptions, you can ignore them using
… someAsyncFunction().catch(function ignore() {}) …
You can throw that away, await it, do anything with it.
If you want the result, you have to await it. If you care about exceptions, but don't really want to wait, you may want to execute it in parallel with the following functions:
var [_, res] = await Promise.all([
someAsyncFunction(), // result is ignored, exceptions aren't
someOtherAsyncFunction()
]);
return res;
inside an async function without await the first line. is this OK?
Yes, there are cases where you'd want to do this which are perfectly reasonable. Especially where you don't care about the result - one example is an analytics tracking operation that should not interfere with business critical code.
how else can I run something that I don't care?
In many ways, however simply calling the promise function works. Your del without a callback would probably work in this case but some functions don't guard against not passing callbacks, so you can pass an empty function instead (.del('key', () => {})).
You do want to however make sure that you know about it failing, even if you don't want to disrupt the operation of code - so please consider adding a process.on("unhandledRejection', event handler to explicitly ignore these particular exceptions or suppress them via:
redisClient.delAsync('key').catch(()=>{});
Or preferably, something like:
redisClient.delAsync('key').catch(logErr);
From all the research I've made so far, I think it's fine to do it, as long as you guarantee that the function you are not awaiting for guarantees a way to handle its own errors in case that happens. For example, a try-catch wrapping the whole function body, like you see in the following snippet for the asyncFunction.
It doesn't matter if the function throws synchronously or asynchronously. It guarantees the your mainFunction will complete no matter what. That's the key point here.
If you don't guarantee that, you have to risks:
If it throws synchronously, your main function will not complete.
If it throws asynchronously, you'll get an unhandled excepction
// THIS IS SOME API CALL YOU DON'T WANT TO WAIT FOR
const mockAPI = () => {
console.log("From mockAPI");
return new Promise((resolve,reject) => {
setTimeout(() => reject("LATE THROW: API ERROR"), 500);
});
};
// THIS IS THE SOME ASYNC FUNCTION YOU CALL BUT NOT AWAIT FOR
const asyncFunction = async (syncThrow) => {
try {
console.log("Async function START");
if (syncThrow) throw new Error("EARLY THROW");
await mockAPI();
console.log("Async function DONE");
}
catch(err) {
console.log("From async function catch");
console.log(err.message || err);
return;
}
};
// THIS IS YOUR MAIN FUNCTION
const mainFunction = async (syncThrow) => {
try {
console.clear();
console.log("Main function START");
asyncFunction(syncThrow);
console.log("Main function DONE <<< THAT'S THE IMPORTANT PART");
}
catch(err) {
console.log("THIS WILL NEVER HAPPEN");
console.log(err);
}
};
<div>
<button onClick="mainFunction(true)">Sync throw</button>
<button onClick="mainFunction(false)">Async throw</button>
</div>
Not in Node.js.
Node does not wait for ever-pending Promises. If other tasks are already completed and there is nothing left in the event loop, the Node process will be terminated even though there exists pending promise.
For the following script, if someOtherAsyncFunction() get resolved in 5 seconds, but redisClientAsync.delAsync('key') takes 10 seconds to execute, the Node process will be terminated after 5 seconds in theory, before the first line is resolved.
async function doSomething() {
redisClientAsync.delAsync('key');
return await someOtherAsyncFunction();
}
await doSomething();
I am learning about Javascript ES2017 async/await feature.
Been reading a lot about it, and came across an understanding that await is like yield, and allows us to wait for the promises to complete.
From https://javascript.info/async-await
async function showAvatar() {
// read our JSON
let response = await fetch('/article/promise-chaining/user.json');
let user = await response.json();
// read github user
let githubResponse = await fetch(`https://api.github.com/users/${user.name}`);
let githubUser = await githubResponse.json();
// show the avatar
let img = document.createElement('img');
img.src = githubUser.avatar_url;
img.className = "promise-avatar-example";
document.body.append(img);
// wait 3 seconds
await new Promise((resolve, reject) => setTimeout(resolve, 3000));
img.remove();
return githubUser;
}
showAvatar();
The question I have is, can I add await to every single line of code?
Or what happens if I remove the word await?
And another question is, if async/ await makes the code seems to run synchronously and in order, why don't we don't use it at all (means make everything stay synchronous in the first place?)
Thank you!
async functions are just syntactic sugar around Promises. It does nothing to change the function to be synchronous. In fact any function that is async implicitly returns a Promise object.
All await does is provide a convenient way to wait for a Promise. If you remove the await keyword, then the Promise will not be "unwrapped," which is not what you want if your function is going to operate on the result of that Promise.
To illustrate, this is the desugared version of your async function:
function showAvatar() {
let githubUser;
// read our JSON
return fetch('/article/promise-chaining/user.json').then(response => {
return response.json();
}).then(user => {
// read github user
return fetch(`https://api.github.com/users/${user.name}`);
}).then(githubResponse => {
return githubResponse.json();
}).then(user => {
githubUser = user;
// show the avatar
let img = document.createElement('img');
img.src = githubUser.avatar_url;
img.className = "promise-avatar-example";
document.body.append(img);
// wait 3 seconds
return new Promise((resolve, reject) => setTimeout(resolve, 3000));
}).then(() => {
img.remove();
return githubUser;
});
}
So await essentially just adds a .then callback to a Promise. Without await being specified, you'll just have a Promise object, not the result of the Promise.
await tells the runtime to wait for the promise returned from the expression on the right-hand side to be fulfilled.
In the following code control is stopped in the async function until the promise returned by the fetch invocation is fulfilled, and the response value sent to the fulfilled promise is printed to the console.
async function go() {
let response = await fetch('http://www.example.com');
console.log(response); // print the response after some time
}
go();
If the await keyword was omitted then control would immediately continue to the next line of the function and the the promise returned by fetch would be immediately printed to the console.
async function go() {
let response = fetch('http://www.example.com');
console.log(response); // print the promise from fetch immediately
}
go();
await is a contextual keyword that only means something special when used inside a function marked async. By marking a function async you are telling the runtime to:
treat the function as a generator function
yield a value where the user types await (usually a promise)
wrap the function in special handling logic so that progress through the function only occurs when each promise yielded by await is fulfilled
In this way, asynchronous control flows can be written in a style closer to the traditional synchronous style. ie. without nesting, callbacks or visible promises. try..catch can also be used in the normal way.
As another answer mentions, async, and await are syntactic sugar that ask the runtime to use existing objects (generator functions and Promises) behind the scenes to make async code easier to read and write.
can you await every line
I would guess you can. If the expression on the right of the await results in a promise, then the async/await behaviour detailed above occurs.
If the expression on the right of the await does not result in a promise, then I would guess the value is wrapped in a resolved promise for you, and logic continues per the above, as though the value came from an immediately resolved promise. This is a guess.
First, async / await, work with promises. If you are not calling from an async function or the await sentence is not then compatible, it won't work. In most cases if you remove await, you are going to end with a Promise and not the value of the promise.
Secondly, there are some cases that you may want to continue with the execution of the code while you are resolving some asynchronous task.
I use dom-to-image.js for converting dom to png image. As dom-to-image.js uses promise, the code executes asynchronously. I want to execute .then function synchronously.
I have the following code:
domtoimage.toPng(document.getElementById("main")).then(function(dataUrl) {
console.log(dataUrl);
}).catch(function(error) {
console.error('oops, something went wrong!', error);
});
console.log("this console should be executed after console.log(dataUrl)")
I want to execute .then function first, before executing console.log("this console should be executed after console.log(dataUrl)").
Please tell me some way to achieve this.
There are of course legit reasons to force synchronous execution of a promise in js. The most obvious is when require'ing a file that needs to initialize using some asynchronous stuff, and optimization is not an overriding concern.
Seems like the package synchronized-promise seems like it ought to do the trick. In your case (warning - untested..):
const dti = () => docstoimage.toPng(document.getElementById("main"))
.then(dataUrl => console.log('url: ', dataUrl))
.catch(err => console.error('oops: ', err))
const sp = require('synchronized-promise')
const syncDti = sp(dti)
syncDti() // performs the 'synchronized version'
console.log("this console should be executed afterwards")
You didn't mention if it's done in browser or in NodeJs. For NodeJs that's possible by using execSync or spawnSync from child_process:
import { spawnSync } from "child_process";
function doSyncStuff(domObj){
const output = spawnSync(...);
console.log("this logs in server");
return output;
bit of a necro, but I'm the author of https://www.npmjs.com/package/synchronous-promise - which was around before this question was raised and does what you want (:
Async functions are a promise.
You have to use the promise syntax with them or use await (async-await syntax) on them in another async function.
An example of the regular promise syntax would be: myPromise.then(() => {})
For what you are looking for, you can use the promise syntax on them to wait for them as if they aren't a promise. Do take care though, as the code in myPromise.then(() => {}) is synchronous and can't use async-await features. I've wrapped things in a main function but you don't need to for extra code but you don't need to.
You can then run the main function through the promise method.
For e.g:
async function myFunction() {
// Do stuff
}
async function main() {
// Do stuff
await myFunction()
}
async function mySecondaryFunction() {
// Do stuff
}
function mySynchronousFunction() {
console.log("hello")
}
main().then(() => {
mySynchronousFunction()
mySecondaryFunction().then(() => {
console.log("idk")
}
})
See, we can use both types of functions!
Do know, though that this doesn't actually make promises synchronous.
It just acts like they're synchronous.
EDIT: Uhhh, I just realized you could put the console.log you want to execute after in both the try and catch functions.
For those stumbling upon this now:
If you're not concerned with IE support, you could use async and await to execute the promise as though it were synchronous. The await syntax will pause execution of the function until the promise resolves, letting you write the code as if there wasn't a promise. To catch an error, you wrap it in a try/catch block.
The only catch is that using the await syntax requires you to wrap it inside a function declared with async.
async function toPng() {
try {
let dataUrl = await domtoimage.toPng(document.getElementById("main"));
console.log(dataUrl);
}
catch (error ) {
console.error('oops, something went wrong!', error);
}
console.log("this console should be executed after console.log(dataUrl)")
}