From MDN:
Promise.all is rejected if any of the elements are rejected. For example, if you pass in four promises that resolve after a timeout and one promise that rejects immediately, then Promise.all will reject immediately.
Let's discuss the following code snippet:
(async () => {
try {
const awaited = await Promise.all([
(() => {
console.log('1');
return Promise.reject('2');
})(),
(() => {
console.log('3');
return Promise.resolve('4');
})(),
]);
console.log(awaited);
} catch (error) {
console.log(error);
} finally {
console.log('5');
}
})();
All the promises in the code above are immediately resolved/rejected. I think that the code should execute this way:
First console.log is executed, thus logging '1';
Promise is rejected with the value '2' thus fast-failing Promise.all();
'Catch' clause is executed, logging the rejected value of '2';
'Finally' clause is executed regardless, logging '5';
Instead, I see a '3' also being logged. Why is it so?
Instead, I see a '3' also being logged. Why is it so?
All the other answers try to explain how promises or the event loop work, but in fact it has nothing to do with those.
The argument you pass to Promise.all is an array that contains the results of two function calls. Therefore both functions will be called before Promise.all is called.
Both functions console.log a value and then return a promise.
Promise.all receives an array containing those promises. So before Promise.all does anything, both functions have already been executed and logged 1 and 3 respectively.
Maybe it's easier to see you restructure your code to create the promises beforehand:
(async () => {
try {
const promise1 = (() => {
console.log('1');
return Promise.reject('2');
})();
const promise2 = (() => {
console.log('3');
return Promise.resolve('4');
})()
const awaited = await Promise.all([promise1, promise2]);
console.log(awaited);
} catch (error) {
console.log(error);
} finally {
console.log('5');
}
})();
Just like with any other function, all function arguments are evaluated before the function itself is called. The order of events is rather this:
First console.log is executed, thus logging '1';
Rejected promise with 2 is created.
Second console.log is executed, thus logging '3';
Fulfilled promise with 4 is created.
Array containing both promises is created.
Promise.all is called with array of promises as parameter.
Promise.all processes promises.
'Catch' clause is executed, logging the rejected value of '2';
'Finally' clause is executed regardless, logging '5';
Here is a simpler example: Note that nothing is "done" with the second argument passed to the function and yet the value is still logged, precisely because all arguments are evaluated first:
function process(arg) {
return 'Received ' + arg;
}
const result = process(
(() => {
console.log('1');
return 1;
})(),
(() => {
console.log('3');
return 2;
})(),
);
console.log(result)
This is due to the way that Promises are handled in JavaScript. It is asynchronous in nature and there is no security as to the execution order. Please note that this is not an error but rather a feature of the language.
I would suggest you read more on the issue in this SO question and about the event loop.
The Promise.all() does fast-fail. However, it doesn't abort the other Promises from finishes. This example with timeouts illustrates it a little better.
Promise.all([
Promise.reject(),
new Promise((resolve, reject) => setTimeout(() => { console.log('a'); resolve(); }, 500)),
new Promise((resolve, reject) => setTimeout(() => { console.log('b'); resolve(); }, 2000))
])
.then(() => console.log('done'))
.catch(() => console.log('fail'));
You can see that the Promise.all() calls the catch() almost immediately, and stops waiting for the results of the other, because it won't change the outcome. However, the other two Promises aren't aborted in any way.
If you want to abort them, you'd have to implement your own logic to do that.
Related
Given the code samples below, is there any difference in behavior, and, if so, what are those differences?
return await promise
async function delay1Second() {
return (await delay(1000));
}
return promise
async function delay1Second() {
return delay(1000);
}
As I understand it, the first would have error-handling within the async function, and errors would bubble out of the async function's Promise. However, the second would require one less tick. Is this correct?
This snippet is just a common function to return a Promise for reference.
function delay(ms) {
return new Promise((resolve) => {
setTimeout(resolve, ms);
});
}
Most of the time, there is no observable difference between return and return await. Both versions of delay1Second have the exact same observable behavior (but depending on the implementation, the return await version might use slightly more memory because an intermediate Promise object might be created).
However, as #PitaJ pointed out, there is one case where there is a difference: if the return or return await is nested in a try-catch block. Consider this example
async function rejectionWithReturnAwait () {
try {
return await Promise.reject(new Error())
} catch (e) {
return 'Saved!'
}
}
async function rejectionWithReturn () {
try {
return Promise.reject(new Error())
} catch (e) {
return 'Saved!'
}
}
In the first version, the async function awaits the rejected promise before returning its result, which causes the rejection to be turned into an exception and the catch clause to be reached; the function will thus return a promise resolving to the string "Saved!".
The second version of the function, however, does return the rejected promise directly without awaiting it within the async function, which means that the catch case is not called and the caller gets the rejection instead.
As other answers mentioned, there is likely a slight performance benefit when letting the promise bubble up by returning it directly — simply because you don’t have to await the result first and then wrap it with another promise again. However, no one has talked about tail call optimization yet.
Tail call optimization, or “proper tail calls”, is a technique that the interpreter uses to optimize the call stack. Currently, not many runtimes support it yet — even though it’s technically part of the ES6 Standard — but it’s possible support might be added in the future, so you can prepare for that by writing good code in the present.
In a nutshell, TCO (or PTC) optimizes the call stack by not opening a new frame for a function that is directly returned by another function. Instead, it reuses the same frame.
async function delay1Second() {
return delay(1000);
}
Since delay() is directly returned by delay1Second(), runtimes supporting PTC will first open a frame for delay1Second() (the outer function), but then instead of opening another frame for delay() (the inner function), it will just reuse the same frame that was opened for the outer function. This optimizes the stack because it can prevent a stack overflow (hehe) with very large recursive functions, e.g., fibonacci(5e+25). Essentially it becomes a loop, which is much faster.
PTC is only enabled when the inner function is directly returned. It’s not used when the result of the function is altered before it is returned, for example, if you had return (delay(1000) || null), or return await delay(1000).
But like I said, most runtimes and browsers don’t support PTC yet, so it probably doesn’t make a huge difference now, but it couldn’t hurt to future-proof your code.
Read more in this question: Node.js: Are there optimizations for tail calls in async functions?
Noticeable difference: Promise rejection gets handled at different places
return somePromise will pass somePromise to the call site, and await somePromise to settle at call site (if there is any). Therefore, if somePromise is rejected, it will not be handled by the local catch block, but the call site's catch block.
async function foo () {
try {
return Promise.reject();
} catch (e) {
console.log('IN');
}
}
(async function main () {
try {
let a = await foo();
} catch (e) {
console.log('OUT');
}
})();
// 'OUT'
return await somePromise will first await somePromise to settle locally. Therefore, the value or Exception will first be handled locally. => Local catch block will be executed if somePromise is rejected.
async function foo () {
try {
return await Promise.reject();
} catch (e) {
console.log('IN');
}
}
(async function main () {
try {
let a = await foo();
} catch (e) {
console.log('OUT');
}
})();
// 'IN'
Reason: return await Promise awaits both locally and outside, return Promise awaits only outside
Detailed Steps:
return Promise
async function delay1Second() {
return delay(1000);
}
call delay1Second();
const result = await delay1Second();
Inside delay1Second(), function delay(1000) returns a promise immediately with [[PromiseStatus]]: 'pending. Let's call it delayPromise.
async function delay1Second() {
return delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
}
Async functions will wrap their return value inside Promise.resolve()(Source). Because delay1Second is an async function, we have:
const result = await Promise.resolve(delayPromise);
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
Promise.resolve(delayPromise) returns delayPromise without doing anything because the input is already a promise (see MDN Promise.resolve):
const result = await delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
await waits until the delayPromise is settled.
IF delayPromise is fulfilled with PromiseValue=1:
const result = 1;
ELSE is delayPromise is rejected:
// jump to catch block if there is any
return await Promise
async function delay1Second() {
return await delay(1000);
}
call delay1Second();
const result = await delay1Second();
Inside delay1Second(), function delay(1000) returns a promise immediately with [[PromiseStatus]]: 'pending. Let's call it delayPromise.
async function delay1Second() {
return await delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
}
Local await will wait until delayPromise gets settled.
Case 1: delayPromise is fulfilled with PromiseValue=1:
async function delay1Second() {
return 1;
}
const result = await Promise.resolve(1); // let's call it "newPromise"
const result = await newPromise;
// newPromise.[[PromiseStatus]]: 'resolved'
// newPromise.[[PromiseValue]]: 1
const result = 1;
Case 2: delayPromise is rejected:
// jump to catch block inside `delay1Second` if there is any
// let's say a value -1 is returned in the end
const result = await Promise.resolve(-1); // call it newPromise
const result = await newPromise;
// newPromise.[[PromiseStatus]]: 'resolved'
// newPromise.[[PromiseValue]]: -1
const result = -1;
Glossary:
Settle: Promise.[[PromiseStatus]] changes from pending to resolved or rejected
This is a hard question to answer, because it depends in practice on how your transpiler (probably babel) actually renders async/await. The things that are clear regardless:
Both implementations should behave the same, though the first implementation may have one less Promise in the chain.
Especially if you drop the unnecessary await, the second version would not require any extra code from the transpiler, while the first one does.
So from a code performance and debugging perspective, the second version is preferable, though only very slightly so, while the first version has a slight legibility benefit, in that it clearly indicates that it returns a promise.
In our project, we decided to always use 'return await'.
The argument is that "the risk of forgetting to add the 'await' when later on a try-catch block is put around the return expression justifies having the redundant 'await' now."
Here is a typescript example that you can run and convince yourself that you need that "return await"
async function test() {
try {
return await throwErr(); // this is correct
// return throwErr(); // this will prevent inner catch to ever to be reached
}
catch (err) {
console.log("inner catch is reached")
return
}
}
const throwErr = async () => {
throw("Fake error")
}
void test().then(() => {
console.log("done")
}).catch(e => {
console.log("outer catch is reached")
});
here i leave some code practical for you can undertand it the diferrence
let x = async function () {
return new Promise((res, rej) => {
setTimeout(async function () {
console.log("finished 1");
return await new Promise((resolve, reject) => { // delete the return and you will see the difference
setTimeout(function () {
resolve("woo2");
console.log("finished 2");
}, 5000);
});
res("woo1");
}, 3000);
});
};
(async function () {
var counter = 0;
const a = setInterval(function () { // counter for every second, this is just to see the precision and understand the code
if (counter == 7) {
clearInterval(a);
}
console.log(counter);
counter = counter + 1;
}, 1000);
console.time("time1");
console.log("hello i starting first of all");
await x();
console.log("more code...");
console.timeEnd("time1");
})();
the function "x" just is a function async than it have other fucn
if will delete the return it print "more code..."
the variable x is just an asynchronous function that in turn has another asynchronous function, in the main of the code we invoke a wait to call the function of the variable x, when it completes it follows the sequence of the code, that would be normal for "async / await ", but inside the x function there is another asynchronous function, and this returns a promise or returns a" promise "it will stay inside the x function, forgetting the main code, that is, it will not print the" console.log ("more code .. "), on the other hand if we put" await "it will wait for every function that completes and finally follows the normal sequence of the main code.
below the "console.log (" finished 1 "delete the" return ", you will see the behavior.
Given the code samples below, is there any difference in behavior, and, if so, what are those differences?
return await promise
async function delay1Second() {
return (await delay(1000));
}
return promise
async function delay1Second() {
return delay(1000);
}
As I understand it, the first would have error-handling within the async function, and errors would bubble out of the async function's Promise. However, the second would require one less tick. Is this correct?
This snippet is just a common function to return a Promise for reference.
function delay(ms) {
return new Promise((resolve) => {
setTimeout(resolve, ms);
});
}
Most of the time, there is no observable difference between return and return await. Both versions of delay1Second have the exact same observable behavior (but depending on the implementation, the return await version might use slightly more memory because an intermediate Promise object might be created).
However, as #PitaJ pointed out, there is one case where there is a difference: if the return or return await is nested in a try-catch block. Consider this example
async function rejectionWithReturnAwait () {
try {
return await Promise.reject(new Error())
} catch (e) {
return 'Saved!'
}
}
async function rejectionWithReturn () {
try {
return Promise.reject(new Error())
} catch (e) {
return 'Saved!'
}
}
In the first version, the async function awaits the rejected promise before returning its result, which causes the rejection to be turned into an exception and the catch clause to be reached; the function will thus return a promise resolving to the string "Saved!".
The second version of the function, however, does return the rejected promise directly without awaiting it within the async function, which means that the catch case is not called and the caller gets the rejection instead.
As other answers mentioned, there is likely a slight performance benefit when letting the promise bubble up by returning it directly — simply because you don’t have to await the result first and then wrap it with another promise again. However, no one has talked about tail call optimization yet.
Tail call optimization, or “proper tail calls”, is a technique that the interpreter uses to optimize the call stack. Currently, not many runtimes support it yet — even though it’s technically part of the ES6 Standard — but it’s possible support might be added in the future, so you can prepare for that by writing good code in the present.
In a nutshell, TCO (or PTC) optimizes the call stack by not opening a new frame for a function that is directly returned by another function. Instead, it reuses the same frame.
async function delay1Second() {
return delay(1000);
}
Since delay() is directly returned by delay1Second(), runtimes supporting PTC will first open a frame for delay1Second() (the outer function), but then instead of opening another frame for delay() (the inner function), it will just reuse the same frame that was opened for the outer function. This optimizes the stack because it can prevent a stack overflow (hehe) with very large recursive functions, e.g., fibonacci(5e+25). Essentially it becomes a loop, which is much faster.
PTC is only enabled when the inner function is directly returned. It’s not used when the result of the function is altered before it is returned, for example, if you had return (delay(1000) || null), or return await delay(1000).
But like I said, most runtimes and browsers don’t support PTC yet, so it probably doesn’t make a huge difference now, but it couldn’t hurt to future-proof your code.
Read more in this question: Node.js: Are there optimizations for tail calls in async functions?
Noticeable difference: Promise rejection gets handled at different places
return somePromise will pass somePromise to the call site, and await somePromise to settle at call site (if there is any). Therefore, if somePromise is rejected, it will not be handled by the local catch block, but the call site's catch block.
async function foo () {
try {
return Promise.reject();
} catch (e) {
console.log('IN');
}
}
(async function main () {
try {
let a = await foo();
} catch (e) {
console.log('OUT');
}
})();
// 'OUT'
return await somePromise will first await somePromise to settle locally. Therefore, the value or Exception will first be handled locally. => Local catch block will be executed if somePromise is rejected.
async function foo () {
try {
return await Promise.reject();
} catch (e) {
console.log('IN');
}
}
(async function main () {
try {
let a = await foo();
} catch (e) {
console.log('OUT');
}
})();
// 'IN'
Reason: return await Promise awaits both locally and outside, return Promise awaits only outside
Detailed Steps:
return Promise
async function delay1Second() {
return delay(1000);
}
call delay1Second();
const result = await delay1Second();
Inside delay1Second(), function delay(1000) returns a promise immediately with [[PromiseStatus]]: 'pending. Let's call it delayPromise.
async function delay1Second() {
return delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
}
Async functions will wrap their return value inside Promise.resolve()(Source). Because delay1Second is an async function, we have:
const result = await Promise.resolve(delayPromise);
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
Promise.resolve(delayPromise) returns delayPromise without doing anything because the input is already a promise (see MDN Promise.resolve):
const result = await delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
await waits until the delayPromise is settled.
IF delayPromise is fulfilled with PromiseValue=1:
const result = 1;
ELSE is delayPromise is rejected:
// jump to catch block if there is any
return await Promise
async function delay1Second() {
return await delay(1000);
}
call delay1Second();
const result = await delay1Second();
Inside delay1Second(), function delay(1000) returns a promise immediately with [[PromiseStatus]]: 'pending. Let's call it delayPromise.
async function delay1Second() {
return await delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
}
Local await will wait until delayPromise gets settled.
Case 1: delayPromise is fulfilled with PromiseValue=1:
async function delay1Second() {
return 1;
}
const result = await Promise.resolve(1); // let's call it "newPromise"
const result = await newPromise;
// newPromise.[[PromiseStatus]]: 'resolved'
// newPromise.[[PromiseValue]]: 1
const result = 1;
Case 2: delayPromise is rejected:
// jump to catch block inside `delay1Second` if there is any
// let's say a value -1 is returned in the end
const result = await Promise.resolve(-1); // call it newPromise
const result = await newPromise;
// newPromise.[[PromiseStatus]]: 'resolved'
// newPromise.[[PromiseValue]]: -1
const result = -1;
Glossary:
Settle: Promise.[[PromiseStatus]] changes from pending to resolved or rejected
This is a hard question to answer, because it depends in practice on how your transpiler (probably babel) actually renders async/await. The things that are clear regardless:
Both implementations should behave the same, though the first implementation may have one less Promise in the chain.
Especially if you drop the unnecessary await, the second version would not require any extra code from the transpiler, while the first one does.
So from a code performance and debugging perspective, the second version is preferable, though only very slightly so, while the first version has a slight legibility benefit, in that it clearly indicates that it returns a promise.
In our project, we decided to always use 'return await'.
The argument is that "the risk of forgetting to add the 'await' when later on a try-catch block is put around the return expression justifies having the redundant 'await' now."
Here is a typescript example that you can run and convince yourself that you need that "return await"
async function test() {
try {
return await throwErr(); // this is correct
// return throwErr(); // this will prevent inner catch to ever to be reached
}
catch (err) {
console.log("inner catch is reached")
return
}
}
const throwErr = async () => {
throw("Fake error")
}
void test().then(() => {
console.log("done")
}).catch(e => {
console.log("outer catch is reached")
});
here i leave some code practical for you can undertand it the diferrence
let x = async function () {
return new Promise((res, rej) => {
setTimeout(async function () {
console.log("finished 1");
return await new Promise((resolve, reject) => { // delete the return and you will see the difference
setTimeout(function () {
resolve("woo2");
console.log("finished 2");
}, 5000);
});
res("woo1");
}, 3000);
});
};
(async function () {
var counter = 0;
const a = setInterval(function () { // counter for every second, this is just to see the precision and understand the code
if (counter == 7) {
clearInterval(a);
}
console.log(counter);
counter = counter + 1;
}, 1000);
console.time("time1");
console.log("hello i starting first of all");
await x();
console.log("more code...");
console.timeEnd("time1");
})();
the function "x" just is a function async than it have other fucn
if will delete the return it print "more code..."
the variable x is just an asynchronous function that in turn has another asynchronous function, in the main of the code we invoke a wait to call the function of the variable x, when it completes it follows the sequence of the code, that would be normal for "async / await ", but inside the x function there is another asynchronous function, and this returns a promise or returns a" promise "it will stay inside the x function, forgetting the main code, that is, it will not print the" console.log ("more code .. "), on the other hand if we put" await "it will wait for every function that completes and finally follows the normal sequence of the main code.
below the "console.log (" finished 1 "delete the" return ", you will see the behavior.
There is quite some topics posted about how async/await behaves in javascript map function, but still, detail explanation in bellow two examples would be nice:
const resultsPromises = myArray.map(async number => {
return await getResult(number);
});
const resultsPromises = myArray.map(number => {
return getResult(number);
});
edited: this if of course a fictional case, so just opened for debate, why,how and when should map function wait for await keyword. solutions how to modify this example, calling Promise.all() is kind of not the aim of this question.
getResult is an async function
The other answers have pretty well covered the details of how your examples behave, but I wanted to try to state it more succinctly.
const resultsPromises = myArray.map(async number => {
return await getResult(number);
});
const resultsPromises = myArray.map(number => {
return getResult(number);
});
Array.prototype.map synchronously loops through an array and transforms each element to the return value of its callback.
Both examples return a Promise.
async functions always return a Promise.
getResult returns a Promise.
Therefore, if there are no errors you can think of them both in pseudocode as:
const resultsPromises = myArray.map(/* map each element to a Promise */);
As zero298 stated and alnitak demonstrated, this very quickly (synchronously) starts off each promise in order; however, since they're run in parallel each promise will resolve/reject as they see fit and will likely not settle (fulfill or reject) in order.
Either run the promises in parallel and collect the results with Promise.all or run them sequentially using a for * loop or Array.prototype.reduce.
Alternatively, you could use a third-party module for chainable asynchronous JavaScript methods I maintain to clean things up and--perhaps--make the code match your intuition of how an async map operation might work:
const delay = ms => new Promise(resolve => setTimeout(resolve, ms));
const getResult = async n => {
await delay(Math.random() * 1000);
console.log(n);
return n;
};
(async () => {
console.log('parallel:');
await AsyncAF([1, 2, 3]).map(getResult).then(console.log);
console.log('sequential:');
await AsyncAF([1, 2, 3]).series.map(getResult).then(console.log)
})();
<script src="https://unpkg.com/async-af#7.0.12/index.js"></script>
async/await is usefull when you want to flatten your code by removing the .then() callbacks or if you want to implicitly return a Promise:
const delay = n => new Promise(res => setTimeout(res, n));
async function test1() {
await delay(200);
// do something usefull here
console.log('hello 1');
}
async function test2() {
return 'hello 2'; // this returned value will be wrapped in a Promise
}
test1();
test2().then(console.log);
However, in your case, you are not using await to replace a .then(), nor are you using it to return an implicit Promise since your function already returns a Promise. So they are not necessary.
Parallel execution of all the Promises
If you want to run all Promises in parallel, I would suggest to simply return the result of getResult with map() and generate an array of Promises. The Promises will be started sequentially but will eventually run in parallel.
const resultsPromises = indicators.map(getResult);
Then you can await all promises and get the resolved results using Promise.all():
const data = [1, 2, 3];
const getResult = x => new Promise(res => {
return setTimeout(() => {
console.log(x);
res(x);
}, Math.random() * 1000)
});
Promise.all(data.map(getResult)).then(console.log);
Sequential execution of the Promises
However, if you want to run each Promise sequentially and wait for the previous Promise to resolve before running the next one, then you can use reduce() and async/await like this:
const data = [1, 2, 3];
const getResult = x => new Promise(res => {
return setTimeout(() => {
console.log(x);
res(x);
}, Math.random() * 1000)
});
data.reduce(async (previous, x) => {
const result = await previous;
return [...result, await getResult(x)];
}, Promise.resolve([])).then(console.log);
Array.prototype.map() is a function that transforms Arrays. It maps one Array to another Array. The most important part of its function signature is the callback. The callback is called on each item in the Array and what that callback returns is what is put into the new Array returned by map.
It does not do anything special with what gets returned. It does not call .then() on the items, it does not await anything. It synchronously transforms data.
That means that if the callback returns a Promise (which all async functions do), all the promises will be "hot" and running in parallel.
In your example, if getResult() returns a Promise or is itself async, there isn't really a difference between your implementations. resultsPromises will be populated by Promises that may or may not be resolved yet.
If you want to wait for everything to finish before moving on, you need to use Promise.all().
Additionally, if you only want 1 getResults() to be running at a time, use a regular for loop and await within the loop.
If the intent of the first code snippet was to have a .map call that waits for all of the Promises to be resolved before returning (and to have those callbacks run sequentially) I'm afraid it doesn't work like that. The .map function doesn't know how to do that with async functions.
This can be demonstrated with the following code:
const array = [ 1, 2, 3, 4, 5 ];
function getResult(n)
{
console.log('starting ' + n);
return new Promise(resolve => {
setTimeout(() => {
console.log('finished ' + n);
resolve(n);
}, 1000 * (Math.random(5) + 1));
});
}
let promises = array.map(async (n) => {
return await getResult(n);
});
console.log('map finished');
Promise.all(promises).then(console.log);
Where you'll see that the .map call finishes immediately before any of the asynchronous operations are completed.
If getResult always returns a promise and never throws an error then both will behave the same.
Some promise returning functions can throw errors before the promise is returned, in this case wrapping the call to getResult in an async function will turn that thrown error into a rejected promise, which can be useful.
As has been stated in many comments, you never need return await - it is equivalent to adding .then(result=>result) on the end of a promise chain - it is (mostly) harmless but unessesary. Just use return.
Node.JS 10.15, serverless, lambdas, invoked locally
SAMPLE A) This Works:
export async function main(event) {
const marketName = marketIdToNameMap[event.marketId];
const marketObject = marketDirectory[marketName];
const marketClient = await marketObject.fetchClient();
const marketTime = await marketObject.getTime(marketClient);
console.log(marketTime);
}
SAMPLE B) and this works:
export function main(event) {
const marketName = marketIdToNameMap[event.marketId];
const marketObject = marketDirectory[marketName];
marketObject.fetchClient().then((marketClient)=>{
marketObject.getTime(marketClient).then((result) => {
console.log('<---------------> marker 1 <--------------->');
console.log(result);
});
});
}
SAMPLE C) but this does not:
export async function main(event) {
const marketName = marketIdToNameMap[event.marketId];
const marketObject = marketDirectory[marketName];
const marketClient = await marketObject.fetchClient();
console.log('<---------------> marker 1 <--------------->');
marketObject.getTime(marketClient).then((result) => {
console.log('<---------------> marker 22 <--------------->');
console.log(result);
});
}
the guts of getTime are for all examples:
function getTime(marketClient){
return new Promise((resolve, reject) => {
return marketClient.getTime((err, result) => {
if (err) {
reject(err);
}
resolve(result);
});
}).catch(err => {
throw err;
});
}
clearly, it seems to be an issue with mixing async/awaits with classic promise then-ables. I would expect SAMPLE C to work because getTime() is returning a promise. However, the code simply finishes silently, never hitting the second marker. I have to put the first marker there just to be sure any code is run at all. It feels like i should be able to mix async/await and thenables, but i must not be considering something here.
#adrian, nope
You're neither awaiting nor returning the promise from marketObject.getTime().then(), and this will result in that promise chain executing independently, the main function returning and the process closing. Remember.. then returns a promise too.
the solution is
await marketObject.getTime(marketClient).then(...
or
return marketObject.getTime(marketClient).then(...
either way chains the promise to the main function such that whatever executes it consistently waits for all promises to resolve (or reject).
I suspect Sample B works because the main is not async and Lambda will wait for the event-loop to complete. i.e. it will execute the promise chain even though main returned early.
https://docs.aws.amazon.com/lambda/latest/dg/nodejs-prog-model-handler.html
If you don't use callback in your code, AWS Lambda will call it
implicitly and the return value is null. When the callback is called,
AWS Lambda continues the Lambda function invocation until the event
loop is empty.
... and I suspect if you return a Promise (as you do in Sample C) then Lambda will simply terminate the process immediately once it resolves, which is does because you don't await/return the .then() chain, and thus the floating promise chain you've created will not execute.
Given the code samples below, is there any difference in behavior, and, if so, what are those differences?
return await promise
async function delay1Second() {
return (await delay(1000));
}
return promise
async function delay1Second() {
return delay(1000);
}
As I understand it, the first would have error-handling within the async function, and errors would bubble out of the async function's Promise. However, the second would require one less tick. Is this correct?
This snippet is just a common function to return a Promise for reference.
function delay(ms) {
return new Promise((resolve) => {
setTimeout(resolve, ms);
});
}
Most of the time, there is no observable difference between return and return await. Both versions of delay1Second have the exact same observable behavior (but depending on the implementation, the return await version might use slightly more memory because an intermediate Promise object might be created).
However, as #PitaJ pointed out, there is one case where there is a difference: if the return or return await is nested in a try-catch block. Consider this example
async function rejectionWithReturnAwait () {
try {
return await Promise.reject(new Error())
} catch (e) {
return 'Saved!'
}
}
async function rejectionWithReturn () {
try {
return Promise.reject(new Error())
} catch (e) {
return 'Saved!'
}
}
In the first version, the async function awaits the rejected promise before returning its result, which causes the rejection to be turned into an exception and the catch clause to be reached; the function will thus return a promise resolving to the string "Saved!".
The second version of the function, however, does return the rejected promise directly without awaiting it within the async function, which means that the catch case is not called and the caller gets the rejection instead.
As other answers mentioned, there is likely a slight performance benefit when letting the promise bubble up by returning it directly — simply because you don’t have to await the result first and then wrap it with another promise again. However, no one has talked about tail call optimization yet.
Tail call optimization, or “proper tail calls”, is a technique that the interpreter uses to optimize the call stack. Currently, not many runtimes support it yet — even though it’s technically part of the ES6 Standard — but it’s possible support might be added in the future, so you can prepare for that by writing good code in the present.
In a nutshell, TCO (or PTC) optimizes the call stack by not opening a new frame for a function that is directly returned by another function. Instead, it reuses the same frame.
async function delay1Second() {
return delay(1000);
}
Since delay() is directly returned by delay1Second(), runtimes supporting PTC will first open a frame for delay1Second() (the outer function), but then instead of opening another frame for delay() (the inner function), it will just reuse the same frame that was opened for the outer function. This optimizes the stack because it can prevent a stack overflow (hehe) with very large recursive functions, e.g., fibonacci(5e+25). Essentially it becomes a loop, which is much faster.
PTC is only enabled when the inner function is directly returned. It’s not used when the result of the function is altered before it is returned, for example, if you had return (delay(1000) || null), or return await delay(1000).
But like I said, most runtimes and browsers don’t support PTC yet, so it probably doesn’t make a huge difference now, but it couldn’t hurt to future-proof your code.
Read more in this question: Node.js: Are there optimizations for tail calls in async functions?
Noticeable difference: Promise rejection gets handled at different places
return somePromise will pass somePromise to the call site, and await somePromise to settle at call site (if there is any). Therefore, if somePromise is rejected, it will not be handled by the local catch block, but the call site's catch block.
async function foo () {
try {
return Promise.reject();
} catch (e) {
console.log('IN');
}
}
(async function main () {
try {
let a = await foo();
} catch (e) {
console.log('OUT');
}
})();
// 'OUT'
return await somePromise will first await somePromise to settle locally. Therefore, the value or Exception will first be handled locally. => Local catch block will be executed if somePromise is rejected.
async function foo () {
try {
return await Promise.reject();
} catch (e) {
console.log('IN');
}
}
(async function main () {
try {
let a = await foo();
} catch (e) {
console.log('OUT');
}
})();
// 'IN'
Reason: return await Promise awaits both locally and outside, return Promise awaits only outside
Detailed Steps:
return Promise
async function delay1Second() {
return delay(1000);
}
call delay1Second();
const result = await delay1Second();
Inside delay1Second(), function delay(1000) returns a promise immediately with [[PromiseStatus]]: 'pending. Let's call it delayPromise.
async function delay1Second() {
return delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
}
Async functions will wrap their return value inside Promise.resolve()(Source). Because delay1Second is an async function, we have:
const result = await Promise.resolve(delayPromise);
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
Promise.resolve(delayPromise) returns delayPromise without doing anything because the input is already a promise (see MDN Promise.resolve):
const result = await delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
await waits until the delayPromise is settled.
IF delayPromise is fulfilled with PromiseValue=1:
const result = 1;
ELSE is delayPromise is rejected:
// jump to catch block if there is any
return await Promise
async function delay1Second() {
return await delay(1000);
}
call delay1Second();
const result = await delay1Second();
Inside delay1Second(), function delay(1000) returns a promise immediately with [[PromiseStatus]]: 'pending. Let's call it delayPromise.
async function delay1Second() {
return await delayPromise;
// delayPromise.[[PromiseStatus]]: 'pending'
// delayPromise.[[PromiseValue]]: undefined
}
Local await will wait until delayPromise gets settled.
Case 1: delayPromise is fulfilled with PromiseValue=1:
async function delay1Second() {
return 1;
}
const result = await Promise.resolve(1); // let's call it "newPromise"
const result = await newPromise;
// newPromise.[[PromiseStatus]]: 'resolved'
// newPromise.[[PromiseValue]]: 1
const result = 1;
Case 2: delayPromise is rejected:
// jump to catch block inside `delay1Second` if there is any
// let's say a value -1 is returned in the end
const result = await Promise.resolve(-1); // call it newPromise
const result = await newPromise;
// newPromise.[[PromiseStatus]]: 'resolved'
// newPromise.[[PromiseValue]]: -1
const result = -1;
Glossary:
Settle: Promise.[[PromiseStatus]] changes from pending to resolved or rejected
This is a hard question to answer, because it depends in practice on how your transpiler (probably babel) actually renders async/await. The things that are clear regardless:
Both implementations should behave the same, though the first implementation may have one less Promise in the chain.
Especially if you drop the unnecessary await, the second version would not require any extra code from the transpiler, while the first one does.
So from a code performance and debugging perspective, the second version is preferable, though only very slightly so, while the first version has a slight legibility benefit, in that it clearly indicates that it returns a promise.
In our project, we decided to always use 'return await'.
The argument is that "the risk of forgetting to add the 'await' when later on a try-catch block is put around the return expression justifies having the redundant 'await' now."
Here is a typescript example that you can run and convince yourself that you need that "return await"
async function test() {
try {
return await throwErr(); // this is correct
// return throwErr(); // this will prevent inner catch to ever to be reached
}
catch (err) {
console.log("inner catch is reached")
return
}
}
const throwErr = async () => {
throw("Fake error")
}
void test().then(() => {
console.log("done")
}).catch(e => {
console.log("outer catch is reached")
});
here i leave some code practical for you can undertand it the diferrence
let x = async function () {
return new Promise((res, rej) => {
setTimeout(async function () {
console.log("finished 1");
return await new Promise((resolve, reject) => { // delete the return and you will see the difference
setTimeout(function () {
resolve("woo2");
console.log("finished 2");
}, 5000);
});
res("woo1");
}, 3000);
});
};
(async function () {
var counter = 0;
const a = setInterval(function () { // counter for every second, this is just to see the precision and understand the code
if (counter == 7) {
clearInterval(a);
}
console.log(counter);
counter = counter + 1;
}, 1000);
console.time("time1");
console.log("hello i starting first of all");
await x();
console.log("more code...");
console.timeEnd("time1");
})();
the function "x" just is a function async than it have other fucn
if will delete the return it print "more code..."
the variable x is just an asynchronous function that in turn has another asynchronous function, in the main of the code we invoke a wait to call the function of the variable x, when it completes it follows the sequence of the code, that would be normal for "async / await ", but inside the x function there is another asynchronous function, and this returns a promise or returns a" promise "it will stay inside the x function, forgetting the main code, that is, it will not print the" console.log ("more code .. "), on the other hand if we put" await "it will wait for every function that completes and finally follows the normal sequence of the main code.
below the "console.log (" finished 1 "delete the" return ", you will see the behavior.