If I have the following code
new Promise(res => res(1))
.then(val => console.log(val))
is this equivalent to
let val = await new Promise(res => res(1))
console.log(val)
I know one difference is that I have to wrap the second one in an async function, but otherwise are they equivalent?
Because your promise always resolves (never rejects), they are equivalent. You could also do:
Promise.resolve(1).then(val => console.log(val));
Keep in mind that a major difference with await (besides it needs to be wrapped in an async function) is what happens when the promise rejects. Though your example is hard-wired to resolve, not reject, lets look at what they look like with actual error handling (which you should always have):
new Promise(res => res(1))
.then(val => console.log(val))
.catch(err => console.log(err));
And:
try {
let val = await new Promise(res => res(1));
console.log(val);
} catch(e) {
console.log(err);
}
Or, if you didn't have the try/catch, then any rejects would automatically be sent to the promise that was automatically returned by the async function. This automatic propagation of errors (both synchronous exceptions and asynchronous rejections) is very useful in an async function.
It becomes more apparent when you have multiple asynchronous operations in series:
const fsp = require('fs').promises;
async function someFunc() {
let handle = await fsp.open("myFile.txt", "w");
try {
await handle.write(...);
await handle.write(...);
} finally {
await handle.close().catch(err => console.log(err));
}
}
someFunc().then(() => {
console.log("all done");
}).catch(err => {
console.log(err);
});
Here, the async wrapper catches errors form any of the three await statements and automatically returns them all to the caller. The finally statement catches either of the last two errors in order to close the file handle, but lets the error continue to propagate back to the caller.
Related
I'm using the async.eachLimit function to control the maximum number of operations at a time.
const { eachLimit } = require("async");
function myFunction() {
return new Promise(async (resolve, reject) => {
eachLimit((await getAsyncArray), 500, (item, callback) => {
// do other things that use native promises.
}, (error) => {
if (error) return reject(error);
// resolve here passing the next value.
});
});
}
As you can see, I can't declare the myFunction function as async because I don't have access to the value inside the second callback of the eachLimit function.
You're effectively using promises inside the promise constructor executor function, so this the Promise constructor anti-pattern.
Your code is a good example of the main risk: not propagating all errors safely. Read why there.
In addition, the use of async/await can make the same traps even more surprising. Compare:
let p = new Promise(resolve => {
""(); // TypeError
resolve();
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e)); // Catches it.
with a naive (wrong) async equivalent:
let p = new Promise(async resolve => {
""(); // TypeError
resolve();
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e)); // Doesn't catch it!
Look in your browser's web console for the last one.
The first one works because any immediate exception in a Promise constructor executor function conveniently rejects the newly constructed promise (but inside any .then you're on your own).
The second one doesn't work because any immediate exception in an async function rejects the implicit promise returned by the async function itself.
Since the return value of a promise constructor executor function is unused, that's bad news!
Your code
There's no reason you can't define myFunction as async:
async function myFunction() {
let array = await getAsyncArray();
return new Promise((resolve, reject) => {
eachLimit(array, 500, (item, callback) => {
// do other things that use native promises.
}, error => {
if (error) return reject(error);
// resolve here passing the next value.
});
});
}
Though why use outdated concurrency control libraries when you have await?
I agree with the answers given above and still, sometimes it's neater to have async inside your promise, especially if you want to chain several operations returning promises and avoid the then().then() hell. I would consider using something like this in that situation:
const operation1 = Promise.resolve(5)
const operation2 = Promise.resolve(15)
const publishResult = () => Promise.reject(`Can't publish`)
let p = new Promise((resolve, reject) => {
(async () => {
try {
const op1 = await operation1;
const op2 = await operation2;
if (op2 == null) {
throw new Error('Validation error');
}
const res = op1 + op2;
const result = await publishResult(res);
resolve(result)
} catch (err) {
reject(err)
}
})()
});
(async () => {
await p;
})().catch(e => console.log("Caught: " + e));
The function passed to Promise constructor is not async, so linters don't show errors.
All of the async functions can be called in sequential order using await.
Custom errors can be added to validate the results of async operations
The error is caught nicely eventually.
A drawback though is that you have to remember putting try/catch and attaching it to reject.
BELIEVING IN ANTI-PATTERNS IS AN ANTI-PATTERN
Throws within an async promise callback can easily be caught.
(async () => {
try {
await new Promise (async (FULFILL, BREAK) => {
try {
throw null;
}
catch (BALL) {
BREAK (BALL);
}
});
}
catch (BALL) {
console.log ("(A) BALL CAUGHT", BALL);
throw BALL;
}
}) ().
catch (BALL => {
console.log ("(B) BALL CAUGHT", BALL);
});
or even more simply,
(async () => {
await new Promise (async (FULFILL, BREAK) => {
try {
throw null;
}
catch (BALL) {
BREAK (BALL);
}
});
}) ().
catch (BALL => {
console.log ("(B) BALL CAUGHT", BALL);
});
I didn't realized it directly by reading the other answers, but what is important is to evaluate your async function to turn it into a Promise.
So if you define your async function using something like:
let f = async () => {
// ... You can use await, try/catch, throw syntax here (see answer of Vladyslav Zavalykhatko) ..
};
your turn it into a promise using:
let myPromise = f()
You can then manipulate is as a Promise, using for instance Promise.all([myPromise])...
Of course, you can turn it into a one liner using:
(async () => { code with await })()
static getPosts(){
return new Promise( (resolve, reject) =>{
try {
const res = axios.get(url);
const data = res.data;
resolve(
data.map(post => ({
...post,
createdAt: new Date(post.createdAt)
}))
)
} catch (err) {
reject(err);
}
})
}
remove await and async will solve this issue. because you have applied Promise object, that's enough.
in an async function i, want to main function to return only in a specific point and not to return void at the end of the function
const fun = () => {
const list = [];
let streamFinished = 0;
let streamCount = files.length;
await fs.readdir(JSON_DIR, async(err, files) => {
await files.forEach((filename) => {
const readStream = fs.createReadStream(path.join("directory", filename));
const parseStream = json.createParseStream();
await parseStream.on('data', async(hostlist: Host[]) => {
hostlist.forEach(async host => {
list.push(host);
});
});
parseStream.on('end', () => {
streamFinished++;
if (streamFinished === streamCount) {
// End of all streams...
return list; //this should be the only point when the function return something
}
})
readStream.pipe(parseStream);
})
});
};
There are lots of things wrong with this code. It's a big mix of event-driven streams, callback-driven functions and promises. The first order of business is to make the main business logic just be promise-driven (as you can see in the new parseData() function and switch all control flow outside of that to just use promises. Here's one way to do it:
const fsp = fs.promises;
function parseData(filename) {
return new Promise((resolve, reject) => {
const readStream = fs.createReadStream(path.join("directory", filename));
const parseStream = json.createParseStream();
const list = [];
parseStream.on('data', (hostlist: Host[]) => {
list.push(...hostlist);
}).on('end', () => {
resolve(list);
}).on('error', reject);
readStream.pipe(parseStream);
});
}
const fun = async () => {
const list = [];
const files = await fsp.readdir(JSON_DIR);
for (let filename of files) {
const listData = await parseData(filename);
list.push(...listData);
}
return list;
};
fun().then(result => {
console.log(result);
}).catch(err => {
console.log(err);
});
A few thoughts here:
The easiest way to "wait" for a stream operation to complete is to encapsulate it in a promise which you can then use await or .then() with. That's the purpose of the parseData() function. In addition, this also enables error handling by hooking stream errors to the promise rejection.
For processing a loop of asynchronous operations, you can either do them one at a time, using await on each asynchronous operation in the loop or you can run them in parallel by collecting an array of promises and using let data = await Promise.all(arrayOfPromises); on that array of promises.
It is only useful to use await if the thing you're awaiting is a promise that is connected to your asynchronous operation. So, things like await parseStream.on('data', ...) and await files.forEach(...) are pointless because neither of those return promises. Do NOT just stick await in places unless you KNOW you are awaiting a promise.
You will generally NOT want to use .forEach() with an asynchronous operation in the loop. Because .forEach() has no return value and no loop control, you can't really control much. Use a regular for loop instead which gives you full control. I consider .forEach() pretty much obsolete for asynchronous programming.
Don't mix promises and callbacks and don't mix promises and streams. If you have those, then "promisify" the stream or callback so all your main logic/control flow can be promises. This will vastly simplify your code and make error handling possible/practical.
Imagine the following hypothetical, minimal implementation of a function that does a simple HTTP GET request using axios. This uses await/async as depicted in the post.
const axios = require('axios')
exports.getStatus = async id => {
const resp = await axios.get(
`https://api.example.com/status/${id}`
)
return resp
}
Is the promise not resolved using await? Is it required that the client uses await as depicted below? Is it safe to assume that anytime a client consumes an async function, that it also needs to use await when calling the function?
// client
const { getStatus } = require('./status')
const response = await getStatus(4)
Short answer, no.
Labelling a function async means 2 things:
1) you're allowed to use await inside the function.
2) The function returns a promise; i.e. you can use await on its return value, or you can use .then, or Promise.all, or even ignore the return value, or anything else you can do with promises.
E.g. you could do the following which (depending on your use case) could be more performant because it needs not wait for the response to continue.
// client
const { getStatus } = require('./status')
const response4 = getStatus(4);
const response5 = getStatus(5);
// do stuff that don't rely on the responses.
response4.then(response => myOutput.setFirstOutput(response));
response5.then(response => myOutput.setSecondOutput(response));
Btw, your first snippet is redundant with its usage of await; it's equivalent to
const axios = require('axios')
exports.getStatus = id =>
axios.get(`https://api.example.com/status/${id}`);
This is because return await promise is equivalent to return promise, both return the same promise. There is one exception regarding rejected promises. Awaiting a rejected promise will throw an exception (which you can catch with a try/catch block), whereas directly returning a rejected promise will not throw an exception but you should still handle the rejection (with a .catch clause). To illustrate:
let promise = new Promise(resolve => setTimeout(() => resolve('resolved after 2 seconds'), 2000));
let returnPromise = () => promise;
let returnAwaitPromise = async () => await promise;
returnPromise().then(value => console.log('returnPromise,', value));
returnAwaitPromise().then(value => console.log('returnAwaitPromise,', value));
No, you don't need to use await. An async function returns a Promise that should be resolved. Instead of using await, you can simply call .then() like so:
// client
const { getStatus } = require('./status');
getStatus(4).then((response) => {
//Do something with the response
}).catch((error) => {
//Handle possible exceptions
});
Below I'll address your question from the comments. The getStatus function could be rewritten like this:
exports.getStatus = (id) => {
return new Promise((resolve, reject) => {
axios.get(`https://api.example.com/status/${id}`).then((resp) => {
resolve(resp);
}).catch((error) => {
reject(error);
})
});
}
It would act exactly the same as in your version. When you call asynchronous functions in series, you have to resolve each of the returned Promises. In your case, there are two of them - the one returned by axios.get and the one returned by getStatus.
No you don’t have to. The async function executes and waits until it returns error or completes execution and return a response!
So I need to do something like:
promiseFunction1().then((result) => {
}).catch((err) => {
// handle err
});
promiseFunction2().then((result) => {
}).catch((err) => {
// handle err
});
....
promiseFunctionN().then((result) => {
}).catch((err) => {
// handle err
});
// WAIT FOR BOTH PROMISES TO FINISH
functionWhenAllPromisesFinished();
I cannot use Promise.all, as I DO NOT CARE IF ONE or ALL OF THEM FAIL. I need to be sure that ALL promises have finished. Also, the callback functions in then() are quite unique to each of the promiseFunctionX().
I am sure this is somewhat trivial, but I cannot figure it out. My initial idea was to keep a counter in the upper scope of running promises and incrementing it when one is run and decrementing it in finally(). Then I would need some async function checkIfRunningPromisesAre0() , but I am not sure how to implement this either, as it looked like recursion hell.
Here is my sample, but consider it just a material to laugh at poor implementation:
async function RunningPromisesFinished(){
if(RunningPromises > 0){
await sleep(2000);
return await RunningPromisesFinished();
}else{
return true;
}
}
on top of that Id have to implement async function sleep(N) and in few seconds the recursion level would be high, which im sure is not good for RAM.
Collect all the promises:
const promise1 = promiseFunction1().then((result) => {
}).catch((err) => {
// handle err
});
Then you can use Promise.all on them:
await Promise.all([promise1, promise2, /*...*/ ]);
I cannot use Promise.all, as I DO NOT CARE IF ONE or ALL OF THEM FAIL
For sure you can. As you added a .catch to each promise that gets the promise chain back into the resolution branch, promise1 will never reject, therefore Promise.all will never reject too.
You could use the Promise.allSettled method:
The Promise.allSettled() method returns a promise that resolves after
all of the given promises have either resolved or rejected, with an
array of objects that each describe the outcome of each promise.
As this is relatively new, it might not be supported by the majority of browsers yet. Here is a polyfill:
function allSettled(promises) {
let wrappedPromises = promises.map(p =>
Promise.resolve(p)
.then(
val => ({ status: 'fulfilled', value: val }),
err => ({ status: 'rejected', reason: err })
)
);
return Promise.all(wrappedPromises);
}
I know that promise.all() fails when even 1 of the promise is failed. I only want to try for failed promises and don't want to run promise.all() again.
Any recommendations on how I can achieve this in minimal way?
Promises are eager construct and model a value obtained asynchronously,
a Promise is produced using some kind of producer, like fetch for instance.
If you retain a reference to this producer then you can replay the nmechanism
that produced the Promise in the first place.
// producer function
function getData (arg) {
const result = new Promise();
return result.then(value => {
return { ok:true, value };
}, error => {
return {
ok: false,
value: error,
// retry is a function which calls the producer with the same arguments
retry: () => getData(arg)
};
})
}
Then if you have something like:
const data = [];
// Promise<{ok: boolean, value: any, retry?: function}>
// No promises will fail in this array
const asyncResults = data.map(getResults);
Promise.all(asyncResults)
.then((results) => {
const successes = results.filter(res => res.ok);
const retrys = results.filter(res => !res.ok).map(res => res.retry()); // retry all failed promises
})
Memory leaks, stack overflow: because I retain a reference to original arguments in order to retry and the algorithm is recursive there could be a memory leak. However the algorithm cannot "stack overflow":
getData calls do not get "deeper" over time (see retry definition)
the asyncrhonicity of the algorithm prevent this behaviour if a promise was never resolved
old data is properly discarded when accessing the results as const resultData = results.filter(res => res.ok).map(res => res.value);
However the algorithm could take a long time to settle if a promise keep on not getting resolved and prevent access to the rest of the values.
In an alternative I suggest you take a look at another async primitive, not yet part of the language (maybe some day) : Observables which are designed for this kind of tasks: lazy, retry-able async operations.
You may use async package and wrap all promise calls with closure with done argument.
Then simply resolve results.
const async = require('async');
const promiseAll = promises => {
return new Promise((resolve) => {
// preparing array of functions which has done method as callback
const parallelCalls = promises.map(promise => {
return done => {
promise
.then(result => done(null, result)
.catch(error => {
console.error(error.message);
done();
});
}
});
// calling array of functions in parallel
async.parallel(
parallelCalls,
(_, results) => resolve(results.filter(Boolean))
);
});
};
router.get('/something', async (req, res) => {
...
const results = await promiseAll(promises);
...
});
Or we can simply do Promise.all without using async package:
router.get('/something', async (req, res) => {
...
const results = (
await Promise.all(
promises.map(promise => {
return new Promise(resolve => {
promise.then(resolve).catch(e => resolve());
});
});
)
).filter(Boolean);
...
});