Is javascript async await good for web applications - javascript

While reading up on some async/await examples with the new javascript syntax, I found this code example:
const axios = require('axios'); // promised based requests - like fetch()
function getCoffee() {
return new Promise(resolve => {
setTimeout(() => resolve('☕'), 2000); // it takes 2 seconds to make coffee
});
}
async function go() {
try {
// but first, coffee
const coffee = await getCoffee();
console.log(coffee); // ☕
// then we grab some data over an Ajax request
const wes = await axios('https://api.github.com/users/wesbos');
console.log(wes.data); // mediocre code
// many requests should be concurrent - don't slow things down!
// fire off three requests and save their promises
const wordPromise = axios('http://www.setgetgo.com/randomword/get.php');
const userPromise = axios('https://randomuser.me/api/');
const namePromise = axios('https://uinames.com/api/');
// await all three promises to come back and destructure the result into their own variables
const [word, user, name] = await Promise.all([wordPromise, userPromise, namePromise]);
console.log(word.data, user.data, name.data); // cool, {...}, {....}
} catch (e) {
console.error(e); // 💩
}
}
go();
One thing that is not clear (in the linked example the whole script waits for the the coffee function to return), it appears to be that async/await is a blocking action? If this is the case does this not mean it is a bad idea for a node web application?
I have just been thrown into a project using AWS DynomoDB whose abstraction class for all actions is behind async/await... if this is blocking, this will surely destroy performance?

await does block! ..
await indeed makes your code wait.
There is no magic that will let the code that comes after the await run while you're still waiting. In the example, those ajax requests are not going to be fired until the coffee is done.
..but it can wait for multiple asynchronous jobs..
However, you can await multiple asynchronous processes. The example that you post at some point fires three simultaneous Ajax requests and waits for those only after they have all been fired. Those requests will be executed in parallel.
..you only have to wait when you need answers
The point is that you start something asynchronously, which will eventually end (promise!). You can call await when (and preferably only when) you really need to wait on the result of that call. So if you call every async method using await, then the effect is gone, but if you call them, store the promise in a variable, and then await them at the end, then you're running your long actions in parallel. The example shows the "bad" use (getting coffee and not getting to work until it's done), and then the good use (doing three requests at the same time).
You could 'optimize' the example by only awaiting the coffee and the first request at the end, just like it waits for the other requests, so it does 5 things in parallel instead of 3. This only works when you don't need the result of the first async function in the next.
.. and await blocks only you, not your caller
I said before that calling await right away for a single synchronous function is bad. This is not always the case, because you can only use await in a function that is asynchronous itself, so the caller of your function isn't blocked (unless it also calls await).
So for example you could make a simple function for fetching some data, that wraps a more complicated Ajax call using await AjaxShizzleGoesHere(). Even though your function just calls one other thing and awaits it, it is still possible to run it in parallel. Your function has to wait for the Ajax call to return (so it uses await, and is blocked), but the caller of your function doesn't have to await the result right away, and it can start other asynchronous functions first.

Related

Can I use setTimeout() on JavaScript as a parallel processed function

I know JS is single threaded. But I have a function which takes time for the calculation. I would like it to work paralleled, so this function would not freeze next statement. Calculation inside of function will take approximately 1-2 seconds.
I used to create it using promise, but it still freeze the next statement.
console.log("start");
new Promise((res, rej) => {
/* calculations */
}).then((res) => console.log(res));
console.log("end");
Then I used setTimeout function with time interval 0. LoL
console.log("start");
setTimeout(() => {
/* calculations */
console.log(res);
}, 0);
console.log("end");
Outputs:
start
end
"calculation result"
Both cases shows similar result, but using promise prevented to show console.log("end") before calculation finishes. Using setTimeout works as I wanted, and shows console.log("end") before calculation, so it was not freeze till calculation done.
I hope it was clear enough. Now for me using setTimeout is the best solution, but I would be happy to hear your ideas or any other method calculating concurrently without setTimeout.
The code you write under new Promise(() => {..code here..}) is not asynchronous. The is a very common misconception that everything under the Promise block would run asynchronously.
Instead, this JS API just let's get us a hook of some deferred task to be done once the promise is resolved. MDN
Promises are a comparatively new feature of the JavaScript language that allow you to defer further actions until after a previous action
has completed, or respond to its failure. This is useful for setting
up a sequence of async operations to work correctly.
new Promise(() => {
// whatever I write here is synchromous
// like console.log, function call, setTimeout()/fetch()/async web apis
// if there are some async tasks like fetch, setTimeout.
// they can be async by themselves but their invocation is still sync
//
})
setTimeout is not the correct option either. Code under setTimeout would run when the event stack is empty and once it enters, it would block the main thread again.
The right approach to this would be to use Web Workers.

Async await - does await block other code from running?

In javascript, does await block code? For example, let's say we have the below code:
async function queryDB() {
const addUser = await promisePool.execute("INSERT INTO Users (User) VALUES ('username')")
const selectUser = await promisePool.execute("SELECT User FROM Users")
}
Will "selectUser" wait to run until addUser is finished so that we can select the user that is added?
Also, let's say that we add some code between the awaits that is not a promise, something like this:
async function queryDB() {
const addUser = await promisePool.execute("INSERT INTO Users (User) VALUES ('username')")
setTimeout(() => console.log('Do something that takes 3 seconds'), 3000);
const selectUser = await promisePool.execute("SELECT User FROM Users")
}
Will "selectUser" wait for addUser but not the setTimeout? If so, how would you write the above code to make addUser run first, then setTimeout and then selectUser?
I would also like to add that I have been studying and reading on both stackoverflow and other resources, but I need some clarification.
Will "selectUser" wait to run until addUser is finished so that we can
select the user that is added?
From MDN Docs - await:
The await expression causes async function execution to pause until a
Promise is settled (that is, fulfilled or rejected), and to resume
execution of the async function after fulfillment. When resumed, the
value of the await expression is that of the fulfilled Promise.
Execution of queryDB function will be paused while it waits for the first promise to settle. Second line of code will only execute after first one has completed successfully.
Will "selectUser" wait for addUser but not the setTimeout?
Yes, that is correct.
If so, how would you write the above code to make addUser run first, then
setTimeout and then selectUser?
You could wrap setTimeout in a function that returns a promise and then await that promise to make sure last line executes after first two have completed.
Following is an example of a wrapper function that wraps setTimeout
function waitForTimeout(seconds) {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log("Hello World");
resolve();
}, seconds * 1000);
});
}
Once you have a wrapper function, you can await it as shown below:
async function queryDB() {
const addUser = await promisePool.execute(
"INSERT INTO Users (User) VALUES ('username')"
);
await waitForTimeout(3); // wait for 3 seconds
const selectUser = await promisePool.execute("SELECT User FROM Users")
}
Important note: blocks is probably a bad word here. A better word is it waits. Even better would be probably to just say that operations that are done using await syntax are guaranteed to be executed sequentially.
The Big Picture
The main reason for using asynchronous code in Node is that we don't block the whole app while we wait for some asynchronous operation like a database request, a network request, a file operation etc. - we only block this particular execution context.
Even though Node.js only uses one thread for executing user code, but I/O operations are asynchronous and they're non-blocking.
So imagine you have an endpoint with the code that you presented above, that is linked to some button "Add user" on the front end.
Bill presses the button, you start handling the request, start waiting for addUser operation
At this point John also presses same button. Your code will still be executed and will also start to wait until addUser finishes.
Let's say that Users table has gotten into a deadlock and any db operations with it will be super slow, and we have a third user, Jane, who is just browsing your site but she doesn't need to sign in/sign up, so she doesn't touch Users table. What happens? Jane is totally fine, her requests will go through without any issues.
How to await setTimeout
setTimeout is coming from "previous era" so it uses callback instead of async/await syntax. But it is always possible to convert callback style to async/await, if you need it.
function delay(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
await delay(5000);
In short, await only blocks the code(s) that's after the await statement in the current async function, not the whole thread. Once the await has been resolved the rest of the code gets executed. In your example, since you have 2 await statements, the second one won't run until the first one has finished, because the first one blocks the function that both await statements are inside of. For the second example, since setTimeout() doesn't return a promise and you aren't awaiting it either, the function will treat it as a 'synchronous' operation. That is, it will throw the setTimeout() callback into the call stack till the next event loop and continue executing the next line which is another await statement that will block the function again until it is resolved.
In order to execute all three lines one by one, you'll have to put the setTimeout() into a promise and await it so that JS treats it as an asynchronous operation and 'waits' for it to finish before moving on to the next line.
Will "selectUser" wait to run until addUser is finished so that we can select the user that is added?
Yes.
Will "selectUser" wait for addUser but not the setTimeout? If so, how would you write the above code to make addUser run first, then setTimeout and then selectUser?
"selectUser" will be executed after "addUser", but not the setTimeout callback. If you want to run "selectUser" after setTimeout, you can add it inside the setTimeout callback function. So it should look like:
async function queryDB() {
const addUser = await promisePool.execute("INSERT INTO Users (User) VALUES ('username')")
setTimeout(async () => {
const selectUser = await promisePool.execute("SELECT User FROM Users");
}, 3000);
}
Short answer is yes, it blocks. The purpose of await is, to make sure you have the result available in the next line. So if you needed the value of addUser to make the next query, you would have it.
It was introduced so you don't have to write then() at the end of the first line and put the second call into a callback, which makes the code hard to read.

Javascript - wait for async call to finish before returning from function, without the use of callbacks

I want to preface by saying I've viewed a lot of stackoverflow questions regarding this topic, but I haven't found any 'duplicates' per se since none of them contain solutions that would solve this specific case.
I've mainly looked at How do I return the response from an asynchronous call?, and the section on 'Promises with async/await' would work inside an asynchronous function, but the function I'm working on is not async and is part of an existing codebase that I can't change easily. I wouldn't be able to change the function to async.
The section on callbacks wouldn't work either, which is explained further below. Anyway, I'll jump into my question:
I'm editing a function (standard function, not async) in JavaScript by adding an asynchronous function call. I want to wait until the async call finishes before I return from the function (since the result of the async call needs to be included in the return value). How would I do so?
I looked into using callbacks, which would allow me to write code which is guaranteed to run only after the async call completes. However, this wouldn't interrupt the flow of the program in the original function, and the original function could still return before the callback is run. A callback would allow me to execute something sequentially after the async function, but it wouldn't allow me to wait for asynchronous call to complete at the highest level.
Example code, which wouldn't return the desired result:
function getPlayers() {
... other code ...
let outfieldPlayers = asyncGetOutfieldPlayersCall()
... other code ...
allPlayers.add(outfieldPlayers)
return allPlayers // the returned value may or may not include outfield players
}
The actual problem I'm facing is a little more complicated - I'm calling the async function in each iteration of a for loop, and need to wait until all calls have completed before returning. But, I think if I can solve this simpler problem, I can solve the problem with a for loop.
Sadly, it is pretty much impossible to wait for async code in a synchronous way. This is because there is no threading in JS (most JS runtimes, but some are). So code is either synchronous or asynchronous.
Asynchronous code is possible because of the event loop. The event loop is part of the javascript runtime. It works by keeping a stack of callback functions that run when events trigger them - usually either timeout events (which you can set with setTimeout()) or IO events (which happen when you make disk or HTTP requests, or on user interaction). However, these callbacks only run when no other code is running, so only when the program is idle and all functions have returned.
This means that techniques like "spin loops" (where you just run a loop until a condition is changed by another thread) that work in threaded environments don't work because the async code won't run until the spin loop finishes.
More Info: https://medium.com/front-end-weekly/javascript-event-loop-explained-4cd26af121d4
If you are using NodeJS, this is possible through execSync.
This requires you to place your asynchronous code in a separate file, spawn a separate process using execSync, which will wait until it exits.
For example, consider the following async function which prints an array.
// task.js
(async function() {
await new Promise((resolve) => setTimeout(() => {
console.log(JSON.stringify([3,4,5]));
resolve();
}, 1000));
})();
Now, you can invoke this from your main process:
function asyncGetOutfieldPlayersCall() {
const execSync = require('child_process').execSync;
return JSON.parse(execSync("node task.js"));
}
function getPlayers() {
let allPlayers = [1,2];
// ... other code ...
let outfieldPlayers = asyncGetOutfieldPlayersCall();
// ... other code ...
allPlayers = allPlayers.concat(outfieldPlayers)
return allPlayers;
}

Is there a way to return a value with async/await instead of a Promise ? As a synchronous function do

Firstly I am familiar with the concept of asynchronous/synchronous function.
There is also a lot of questions related to mine. But I can't find my answer anywhere.
So the question is:
Is there a way to return a value instead of a Promise using async/await ? As a synchronous function do.
For example:
async doStuff(param) {
return await new Promise((resolve, reject) => {
setTimeout(() => {
console.log('doStuff after a while.');
resolve('mystuffisdone'+param);
}, 2000);
});
}
console.log(doStuff('1'));
The only way to get the value of this function is by using the .then function.
doStuff('1').then(response => {
console.log(response); // output: mystuffisdone1
doOtherStuffWithMyResponse(response);
// ...
});
Now, what I want is:
const one = doStuff('1');
console.log(one) // mystuffisdone1
const two = doStuff('2');
console.log(two) // mystuffisdone2
To explain myself, I have an asynchronous library full of callbacks. I can turn this asynchronous behavior to a synchronous behavior by using Promises and async/await to faking a synchronous behavior.
But there is still a problem, it is still asynchronous in the end; outside of the scope of the async function.
doStuff('1').then((r) => {console.log(r)};
console.log('Hello wolrd');
It will result in: Hello world then mystuffisdone1. This is the expected behavior when using async/await functions. But that's not what I want.
Now my question would be: Is there a way to do the same thing as await do without the keyword async ? To make the code being synchronous ? And if not possible, why ?
Edit:
Thank you for all you answers, I think my question is not obsvious for all. To clear up what I think here is my comment to #Nikita Isaev answer.
"I understand why all I/O operations are asynchronously done; or done in parallel. But my question is more about the fact that why the engine doesn't block the caller of the sync function in an asynchronous manner ? I mean const a = doStuff(...) is a Promise. We need to call .then to get the result of this function. But why JavaScript or Node engine does not block the caller (just the block where the call is made). If this is possible, we could do const a = doStuff(...), wait and get the result in a without blocking the main thread. As async/await does, why there is no place for sync/wait ?"
Hope this is more clear now, feel free to comment or ask anything :)
Edit 2:
All precisions of the why of the answer are in the comments of the accepted answer.
There are some hacky ways to do what is desired, but that would be an anti-pattern. I’ll try to explain. Callbacks is one of the core concepts in javascript. When your code launches, you may set up event listeners, timers, etc. You just tell the engine to schedule some tasks: “when A happens, do B”. This is what asynchrony is. But callbacks are ugly and difficult to debug, that’s why promises and async-await were introduced. It is important to understand that this is just a syntax sugar, your code still is asynchronous when using async-await. As there are no threads in javascript, waiting for some events to fire or some complicated operations to finish in a synchronous way would block your entire application. The UI or the server would just stop responding to any other user interactions and would keep waiting for a single event to fire.
Real world cases:
Example 1.
Let’s say we have a web UI. We have a button that downloads the latest information from the server on click. Imagine we do it synchronously. What happens?
myButton.onclick = function () {
const data = loadSomeDataSync(); // 0
useDataSomehow(data);
}
Everything’s synchronous, the code is flat and we are happy. But the user is not.
A javascript process can only ever execute a single line of code in a particular moment. User will not be able to click other buttons, see any animations etc, the app is stuck waiting for loadSomeDataSync() to finish. Even if this lasts 3 seconds, it’s a terrible user experience, you can neither cancel nor see the progress nor do something else.
Example 2.
We have a node.js http server which has over 1 million users. For each user, we need to execute a heavy operation that lasts 5 seconds and return the result. We can do it in a synchronous or asynchronous manner. What happens if we do it in async?
User 1 connects
We start execution of heavy operation for user 1
User 2 connects
We return data for user 1
We start execution of heavy operation for user 2
…
I.e we do everything in parallel and asap. Now imagine we do the heavy operation in a sync manner.
User 1 connects
We start execution of heavy operation for user 1, everyone else is waiting for it to accomplish
We return data for user 1
User 2 connects
…
Now imagine the heavy operation takes 5 seconds to accomplish, and our server is under high load, it has over 1 million users. The last one will have to wait for nearly 5 million seconds, which is definitely not ok.
That’s why:
In browser and server API, most of the i/o operations are asynchronous
Developers strive to make all heavy calculation asynchronous, even React renders in an asynchronous manner.
No, going from promise to async/await will not get you from async code to sync code. Why? Because both are just different wrapping for the same thing. Async function returns immediately just like a promise does.
You would need to prevent the Event Loop from going to next call. Simple while(!isMyPromiseResolved){} will not work either because it will also block callback from promises so the isMyPromiseResolved flag will never be set.
BUT... There are ways to achieve what you have described without async/await. For example:
OPTION 1: using deasync approach. Example:
function runSync(value) {
let isDone = false;
let result = null;
runAsync(value)
.then(res => {
result = res;
isDone = true;
})
.catch(err => {
result = err;
isDone = true;
})
//magic happens here
require('deasync').loopWhile(function(){return !isDone;});
return result;
}
runAsync = (value) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
// if passed value is 1 then it is a success
if(value == 1){
resolve('**success**');
}else if (value == 2){
reject('**error**');
}
}, 1000);
});
}
console.log('runSync(2): ', runSync(2));
console.log('runSync(1): ', runSync(1));
OR
OPTION 2: calling execFileSync('node yourScript.js') Example:
const {execFileSync} = require('child_process');
execFileSync('node',['yourScript.js']);
Both approaches will block the user thread so they should be used only for automation scripts or similar purposes.
Wrap the outer body in an asynchronous IIFE:
/**/(async()=>{
function doStuff(param) { // no need for this one to be async
return new Promise((resolve, reject) => { // just return the original promise
setTimeout(() => {
console.log('doStuff after a while.');
resolve('mystuffisdone'+param);
}, 2000);
});
}
console.log(await doStuff('1')); // and await instead of .then
/**/})().then(()=>{}).catch(e=>console.log(e))
The extra cleanup of the doStuff function isn't strictly necessary -- it works either way -- but I hope it helps clarify how async functions and Promises are related. The important part is to wrap the outer body into an async function to get the improved semantics throughout your program.
It's also not strictly necessary to have the final .then and .catch, but it's good practice. Otherwise, your errors might get swallowed, and any code ported to Node will whine about uncaught Promise rejections.

Using "await" inside non-async function

I have an async function that runs by a setInterval somewhere in my code. This function updates some cache in regular intervals.
I also have a different, synchronous function which needs to retrieve values - preferably from the cache, yet if it's a cache-miss, then from the data origins
(I realize making IO operations in a synchronous manner is ill-advised, but lets assume this is required in this case).
My problem is I'd like the synchronous function to be able to wait for a value from the async one, but it's not possible to use the await keyword inside a non-async function:
function syncFunc(key) {
if (!(key in cache)) {
await updateCacheForKey([key]);
}
}
async function updateCacheForKey(keys) {
// updates cache for given keys
...
}
Now, this can be easily circumvented by extracting the logic inside updateCacheForKey into a new synchronous function, and calling this new function from both existing functions.
My question is why absolutely prevent this use case in the first place? My only guess is that it has to do with "idiot-proofing", since in most cases, waiting on an async function from a synchronous one is wrong. But am I wrong to think it has its valid use cases at times?
(I think this is possible in C# as well by using Task.Wait, though I might be confusing things here).
My problem is I'd like the synchronous function to be able to wait for a value from the async one...
They can't, because:
JavaScript works on the basis of a "job queue" processed by a thread, where jobs have run-to-completion semantics, and
JavaScript doesn't really have asynchronous functions — even async functions are, under the covers, synchronous functions that return promises (details below)
The job queue (event loop) is conceptually quite simple: When something needs to be done (the initial execution of a script, an event handler callback, etc.), that work is put in the job queue. The thread servicing that job queue picks up the next pending job, runs it to completion, and then goes back for the next one. (It's more complicated than that, of course, but that's sufficient for our purposes.) So when a function gets called, it's called as part of the processing of a job, and jobs are always processed to completion before the next job can run.
Running to completion means that if the job called a function, that function has to return before the job is done. Jobs don't get suspended in the middle while the thread runs off to do something else. This makes code dramatically simpler to write correctly and reason about than if jobs could get suspended in the middle while something else happens. (Again it's more complicated than that, but again that's sufficient for our purposes here.)
So far so good. What's this about not really having asynchronous functions?!
Although we talk about "synchronous" vs. "asynchronous" functions, and even have an async keyword we can apply to functions, a function call is always synchronous in JavaScript. An async function is a function that synchronously returns a promise that the function's logic fulfills or rejects later, queuing callbacks the environment will call later.
Let's assume updateCacheForKey looks something like this:
async function updateCacheForKey(key) {
const value = await fetch(/*...*/);
cache[key] = value;
return value;
}
What that's really doing, under the covers, is (very roughly, not literally) this:
function updateCacheForKey(key) {
return fetch(/*...*/).then(result => {
const value = result;
cache[key] = value;
return value;
});
}
(I go into more detail on this in Chapter 9 of my recent book, JavaScript: The New Toys.)
It asks the browser to start the process of fetching the data, and registers a callback with it (via then) for the browser to call when the data comes back, and then it exits, returning the promise from then. The data isn't fetched yet, but updateCacheForKey is done. It has returned. It did its work synchronously.
Later, when the fetch completes, the browser queues a job to call that promise callback; when that job is picked up from the queue, the callback gets called, and its return value is used to resolve the promise then returned.
My question is why absolutely prevent this use case in the first place?
Let's see what that would look like:
The thread picks up a job and that job involves calling syncFunc, which calls updateCacheForKey. updateCacheForKey asks the browser to fetch the resource and returns its promise. Through the magic of this non-async await, we synchronously wait for that promise to be resolved, holding up the job.
At some point, the browser's network code finishes retrieving the resource and queues a job to call the promise callback we registered in updateCacheForKey.
Nothing happens, ever again. :-)
...because jobs have run-to-completion semantics, and the thread isn't allowed to pick up the next job until it completes the previous one. The thread isn't allowed to suspend the job that called syncFunc in the middle so it can go process the job that would resolve the promise.
That seems arbitrary, but again, the reason for it is that it makes it dramatically easier to write correct code and reason about what the code is doing.
But it does mean that a "synchronous" function can't wait for an "asynchronous" function to complete.
There's a lot of hand-waving of details and such above. If you want to get into the nitty-gritty of it, you can delve into the spec. Pack lots of provisions and warm clothes, you'll be some time. :-)
Jobs and Job Queues
Execution Contexts
Realms and Agents
You can call an async function from within a non-async function via an Immediately Invoked Function Expression (IIFE):
(async () => await updateCacheForKey([key]))();
And as applied to your example:
function syncFunc(key) {
if (!(key in cache)) {
(async () => await updateCacheForKey([key]))();
}
}
async function updateCacheForKey(keys) {
// updates cache for given keys
...
}
This shows how a function can be both sync and async, and how the Immediately Invoked Function Expression idiom is only immediate if the path through the function being called does synchronous things.
function test() {
console.log('Test before');
(async () => await print(0.3))();
console.log('Test between');
(async () => await print(0.7))();
console.log('Test after');
}
async function print(v) {
if(v<0.5)await sleep(5000);
else console.log('No sleep')
console.log(`Printing ${v}`);
}
function sleep(ms : number) {
return new Promise(resolve => setTimeout(resolve, ms));
}
test();
(Based off of Ayyappa's code in a comment to another answer.)
The console.log looks like this:
16:53:00.804 Test before
16:53:00.804 Test between
16:53:00.804 No sleep
16:53:00.805 Printing 0.7
16:53:00.805 Test after
16:53:05.805 Printing 0.3
If you change the 0.7 to 0.4 everything runs async:
17:05:14.185 Test before
17:05:14.186 Test between
17:05:14.186 Test after
17:05:19.186 Printing 0.3
17:05:19.187 Printing 0.4
And if you change both numbers to be over 0.5, everything runs sync, and no promises get created at all:
17:06:56.504 Test before
17:06:56.504 No sleep
17:06:56.505 Printing 0.6
17:06:56.505 Test between
17:06:56.505 No sleep
17:06:56.505 Printing 0.7
17:06:56.505 Test after
This does suggest an answer to the original question, though. You could have a function like this (disclaimer: untested nodeJS code):
const cache = {}
async getData(key, forceSync){
if(cache.hasOwnProperty(key))return cache[key] //Runs sync
if(forceSync){ //Runs sync
const value = fs.readFileSync(`${key}.txt`)
cache[key] = value
return value
}
//If we reach here, the code will run async
const value = await fsPromises.readFile(`${key}.txt`)
cache[key] = value
return value
}
Now, this can be easily circumvented by extracting the logic inside updateCacheForKey into a new synchronous function, and calling this new function from both existing functions.
T.J. Crowder explains the semantics of async functions in JavaScript perfectly. But in my opinion the paragraph above deserves more discussion. Depending on what updateCacheForKey does, it may not be possible to extract its logic into a synchronous function because, in JavaScript, some things can only be done asynchronously. For example there is no way to perform a network request and wait for its response synchronously. If updateCacheForKey relies on a server response, it can't be turned into a synchronous function.
It was true even before the advent of asynchronous functions and promises: XMLHttpRequest, for instance, gets a callback and calls it when the response is ready. There's no way of obtaining a response synchronously. Promises are just an abstraction layer on callbacks and asynchronous functions are just an abstraction layer on promises.
Now this could have been done differently. And it is in some environments:
In PHP, pretty much everything is synchronous. You send a request with curl and your script blocks until it gets a response.
Node.js has synchronous versions of its file system calls (readFileSync, writeFileSync etc.) which block until the operation completes.
Even plain old browser JavaScript has alert and friends (confirm, prompt) which block until the user dismisses the modal dialog.
This demonstrates that the designers of the JavaScript language could have opted for synchronous versions of XMLHttpRequest, fetch etc. Why didn't they?
[W]hy absolutely prevent this use case in the first place?
This is a design decision.
alert, for instance, prevents the user from interacting with the rest of the page because JavaScript is single threaded and the one and only thread of execution is blocked until the alert call completes. Therefore there's no way to execute event handlers, which means no way to become interactive. If there was a syncFetch function, it would block the user from doing anything until the network request completes, which can potentially take minutes, even hours or days.
This is clearly against the nature of the interactive environment we call the "web". alert was a mistake in retrospect and it should not be used except under very few circumstances.
The only alternative would be to allow multithreading in JavaScript which is notoriously difficult to write correct programs with. Are you having trouble wrapping your head around asynchronous functions? Try semaphores!
It is possible to add a good old .then() to the async function and it will work.
Should consider though instead of doing that, changing your current regular function to async one, and all the way up the call stack until returned promise is not needed, i.e. there's no work to be done with the value returned from async function. In which case it actually CAN be called from a synchronous one.

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