I use pg-promise and i tried to make a request to database in user model to get an array of all users like this:
exports.getAllUsers = function () {
let users = db.any("SELECT * FROM users;").then(function (data) {
return data; // in debug mode data is my expected array
});
return users;
};
But when i make get request from controller, it returns promise Promise { <pending> }, not an array of json objects that I expected.
There is a controller:
exports.getUsers = function (req, res) {
let users = userModel.getAllUsers();
console.log(users); // here it outputs Promise { <pending> }
res.json(users);
};
How to get array instead of promise?
Let's break down your code. You are exporting a function of your design, getUsers, which is supposed to return a sequence of user objects. But it does not -- db.any(...) returns a Promise. How else do you think you can call then on the returned object? then is a member function of the Promise class, and you pass another function to then to work with the actual data (passed as parameter to the function you pass to then). Meaning that indeed data, as passed to the then call invoked on the return value of db.any(...) call, is the sequence of users you're after.
The mistake you are making is to assume that if you return data from the callback passed to then, it will become the return value of the then(...) call. It will not. then always returns a Promise -- whatever you return from the callback to then will become the resolved value of the returned promise, but it is the promise that is returned and that is why your users variable is a Promise.
You need to read more about promises, and how they are resolved asynchronously (between running your scripts), and how to synchronise your code on them resolving.
Hint: Use the await keyword to wait for and use the value given a promise, like the value you return from the callback that you pass to your then(...) call, or accept the design of your getUsers function returning a promise and adapt the rest of your code to work with that, without use of await.
Your getAllUsers function can be reduced to:
exports.getAllUsers = function () {
return db.any("SELECT * FROM users;");
}
...and with await you can then use it like:
let users = await getUsers();
The statement above has to be part of a function that is tagged as async though -- ECMAScript requires marking these explicitly in order to allow use of await expressions:
async function whatever() {
let users = await getUsers();
/// ...
}
Execution of a script that invokes an async function such as one containing the statement above, will be interrupted by the JavaScript interpreter at each await expression, and resumed when the promise expressed after the await keyword, resolves to a value, with the value being assigned to the users variable.
Using async and await does require a version of Node.js that can support them, but current release does (as of the time of writing this).
Otherwise you can still keep the getUsers function, but you need to use it differently since you can't use await -- having to use promises as one did prior to introduction of await and async, like:
getUsers().then(users => {
/// Do something with the `users` array.
});
At any rate, it looks like you have some holes in your understanding of how promises work, and I recommend you fill these holes by reading about them this time and not just going straight through to the pg-promise API, which builds on them.
Like you already found out userModel.getAllUsers() will return a promise and not an array. You need to wait for this promise to be resolved. This can be done using the an async function.
exports.getUsers = async function (req, res) {
let users = await userModel.getAllUsers();
console.log(users);
res.json(users);
};
Note that an async function will always return a promise.
Or you can make use of then method of the returned promise.
exports.getUsers = function (req, res) {
userModel.getAllUsers().then(users => {
console.log(users);
res.json(users);
});
};
Related
I have 3 async functions:
ToDoItem.deleteMany({}); // deletes entire collection
ToDoItem.insertMany(itemArray); // adds new items to collection
ToDoItem.find({}); // finds all the items in the collection
This code alone doesn't work well, as they do not follow a consistent order. I.e. the insertion might happen before deletion, which I do not want.
I can use callbacks to chain them together (callback hell), and I can also use .then to chain them together, as they return promises. However, I would like to use async/await.
Additionally, these functions can be given optional callbacks, for instance:
ToDoItem.find({}, (data) => {
console.log(data);
});
This is useful as I want to see all the data in my DB that matches the query {} (which is all items).
However I can't figure out how to access these callbacks by using async and await. I can do it via callbacks or .then, but the code is more messy. Is there a way to do this?
Edit:
As per Bergi's reply, I have edited my code as such:
async function setupDatabase() {
const deleteResult = await ToDoItem.deleteMany({});
console.log("Items deleted. Delete result:")
console.log(deleteResult);
const insertResult = await ToDoItem.insertMany(defaultItems);
console.log("Items have been added successfully");
console.log(insertResult);
const findResult = await ToDoItem.find({});
console.log("Here are the items:")
console.log(findResult);
}
Am I correct in thinking that:
deleteResult will now evaluate to be either the deletion confirmation (if successful) or the error (if rejected). And similarly with insertResult and findResult?
What do I do if I want to return the collection found by .find({}), as the function setupDatabase is now async and returns a promise.
If 1) is correct, how do I separate out when I'm getting an error and when I'm getting a result?
As per Konrad's response, I have done the following:
async function setupDatabase() {
const deleteResult = await ToDoItem.deleteMany({});
console.log("Items deleted. Delete result:")
console.log(deleteResult);
const insertResult = await ToDoItem.insertMany(defaultItems);
console.log("Items have been added successfully");
console.log(insertResult);
const findResult = await ToDoItem.find({});
console.log("Here are the items:")
console.log(findResult);
return findResult;
}
app.get("/", function(req, res) {
(async function() {
const objectList = await setupDatabase();
let dataList = [];
for (element of objectList) {
dataList.push(element.todo);
}
res.render("list", {listTitle: "Today", newListItems: dataList});
}());
My idea was to return the findResult inside the setupDatabase function. But this is actually a promise since the function is async, so I wrapped it in an IIFE inside the .get. I then iterated over this list and created dataList which has the actual data I want to render.
You don't use async/await with callbacks.
You use await with promises, and you should not mix those with callbacks - in particular, MongoDB does not return a promise when you pass a callback. You need to write
await ToDoItem.deleteMany({}); // deletes entire collection
await ToDoItem.insertMany(itemArray); // adds new items to collection
const data = await ToDoItem.find({});
Am I correct in thinking that [the await expression] will now evaluate to be either the deletion confirmation (if successful) or the error (if rejected)?
No. The result of the await is the successful promise fulfillment value. In case of a promise rejection, the error is thrown as an exception, you can handle it with a try/catch block.
What do I do if I want to return the collection found by .find({}), as the function setupDatabase is now async and returns a promise.
The async/await syntax doesn't change anything. The function was asynchronous before, and it's impossible to immediately return the result, regardless whether you'd use callbacks or promise .then() chaining syntax or await syntax. Returning a promise is actually desirable - the caller just has to wait for it!
I have the following endpoint in a class called UserApi.js:
const controller = 'User';
...
export async function getEmployeeInfo(employeeId)
{
const query = createQueryFromObject({employeId});
const response = await get(`/${controller}/EmployeeInfo?${query}`);
return retrieveResponseData(response, []);
}
This is going to get the required information from an action method in the backend of UserController.cs.
Now, say that I want to display this information in EmployeeView.vue class, do I have to await it again? Why or why not? Initially, I would say no, you don't, as you already dealt with the await/async in the UserApi.js class, but what about the Promise.resolve? Please explain.
methods: {
async setReportData(
employeeId
) {
this.isBusy = true;
Promise.resolve(getEmployeeInfo(
employeeId
)).then((resultsEmployeeInfo) => {
this.reportDatatableEmployeeInfo = resultsEmployeeInfo;
})
.catch(() => {
this.alerts.error('An error has occurred while fetching the data');
})
.finally(() => {
this.isBusy = false;
});
},
Update:
....
* #param {Object} response
* #param {any} defaultData
* #param {Function} predicate
* #returns {Promise}
*/
export function retrieveResponseData(response, defaultData = null, predicate = (predicateResponse) => predicateResponse) {
const data = predicate(response) ? response.data : null;
return data || defaultData;
}
You need to await it since a function declared with async keyword ALWAYS returns a Promise, even if you do only synchronous stuff inside of it, hence you need to await or "thenize" it to access the value it resolved to. That depends from the implementation details of async functions which are just generators that yield promises.
If this concerns you because you work inside sync modules and don't like to use async functions just to execute more async functions, there's a good news, TOP-LEVEL await MODULES proposal is at stage 4, so it'll very soon be shipped with the next ECMA version. This way you will be able to await inside modules as if they were wrapped by async functions !
https://github.com/tc39/proposal-top-level-await
I can't tell if you need to await it again, because I can't tell what retrieveResponseData does. It might take the resolved value and wrap it in a fresh promise, which would then require callers of getEmployeeInfo to await the result.
Here's the why:
A Promise is a wrapper around a value
await unwraps a Promise. So does the .then() handler you can register with a Promise (but the value is only unwrapped within the function you provide to .then()).
Just like a gift in the real world, once something has been unwrapped, you don't need to unwrap it again. However, very conveniently for us, you can still use await on a value that is not wrapped in a Promise, and it will just give you the value.
You can wrap any value in a Promise, like so:
let wrappedFive = Promise.resolve(5)
//> wrappedFive is a Promise that must be unwrapped to access the 5 inside it
// this does _exactly_ the same thing as the above
let wrappedFive = new Promise(resolve => {
resolve(5)
})
Sometimes you end up in a situation where you can't use await, because you're in a function that cannot be marked async. The lifecycle methods of front-end frameworks like React (and possibly Vue) are like that: the framework needs each lifecycle method to do its job and be done immediately. If you mark the lifecycle method as async, you can often prevent it from having the intended effect.
In cases like that, you often need to use chained .then() handlers, which is a little uglier, but it works:
componentDidMount() {
// this API call is triggered immediately by lifecycle,
// but it basically starts a separate thread -- the rest
// of this function does not wait for the call to finish
API.getUserInfo()
.then(userInfo => {
// this happens after the API call finishes, but
// componentDidMount has already finished, so what happens
// in here cannot affect that function
this.setState({ username: userInfo.username })
})
// this happens immediately after the API call is triggered,
// even if the call takes 30 seconds
return 5
}
Note that a Promise does not actually start a separate thread -- these all happen in the same thread that executes the lifecycle method, i.e. the browser's renderer thread. But if you think of the codepath that executes, a Promise that you don't wait for basically introduces a fork into that codepath: one path is followed immediately, and the other path is followed whenever the Promise resolves. Since browserland is pretty much a single-threaded context, it doesn't really hurt you to think of creating a Promise as spawning a separate thread. This is a nuance you can ignore until you are comfortable with asychronous patterns in JS.
Update: retrieveResponseData does not appear to return a Promise. I could be wrong, if predict returns a Promise, but if that were true, then the ternary would always return response.data because unwrapped Promises are truthy values (even Promise.resolve(false) is truthy).
However, anyone who calls getEmployeeInfo will have to wait it, because that function is marked async, and that means it returns a Promise even if nothing inside it is is asynchronous. Consider this extreme example:
// this function returns a number
function gimmeFive() {
return 5
}
// this function returns a Promise wrapped around a number
async function gimmeFive() {
return 5
}
Async function getEmployeeInfo awaits the result of the get call in order to return the value returned by a call to retrieveResponeData.
Assuming neither get nor retrieveResponeData errors, the value syntactically returned in the body of getEmployeeInfo is used to resolve the promise object actually returned by calling getEmployeeInfo.
Promise.resolve is not needed to convert the result of calling getEmployeeInfo into a promise because, given async functions return promises, it already is.
It doesn't matter if retrieveResponseData returns a promise or not: standard async function processing waits for a returned promise value to be settled before resolving the promise returned when calling the async function.
Async function setRreportData is declared as async but written using chained promise handler methods to process data and error conditions in order - the async declaration could be omitted, but may be useful if modifications are made.
The results can only be used to update the page at a time when the data has finished being obtained and extracted, shown as a comment in the following code:
setReportData( employeeId) {
this.isBusy = true;
getEmployeeInfo(
employeeId
).then((resultsEmployeeInfo) => {
this.reportDatatableEmployeeInfo = resultsEmployeeInfo;
// At this point the result in this.reportDatatableEmployeeInfo can be used to update the page
})
.catch(() => {
this.alerts.error('An error has occurred while fetching the data');
})
.finally(() => {
this.isBusy = false;
});
},
Displaying the data using EmployeeView.vue class must wait until the data is available. The simplest place to insert updating the page (in the posted code) is within the then handler function inside setReportData.
Displaying the data
As posted setReportData does not notify its caller of when report data is available, either by means of a callback or by returning a promise. All it does is save the result in this.reportDatatableEmployeeInfo and dies.
Without using callbacks, setReportData is left with two choices
Return a promise that is fulfilled with, say,resultsEmployeeInfo or undefined if an error occurs:
setReportData( employeeId) {
this.isBusy = true;
// return the final promise:
return getEmployeeInfo(
employeeId
)
.then( (resultsEmployeeInfo) =>
(this.reportDatatableEmployeeInfo = resultsEmployeeInfo)
)
.catch(() => {
this.alerts.error('An error has occurred while fetching the data');
// return undefined
})
.finally(() => {
this.isBusy = false;
});
},
which could be used in a calling sequence using promises similar to
if(!this.busy) {
this.setReportData(someId).then( data => {
if( data) {
// update page
}
}
If you wanted to make the call in an async context you could use await:
if(!this.busy) {
const data = await this.setReportData(someId);
if( data) {
// update page
}
}
Update the page from within setReportData after the data becomes available ( as shown as a comment in the first part of this answer). The method should probably be renamed from setReportData to getReportData or similar to reflect its purpose.
I have a Vuex store and I am trying to fetch data from the Firebase Realtime Database. I am initially fetching the user information, however afterwards I would like to fetch some other information that relies upon the initial data fetched.
As you can see from the code, I am trying to do this using async / await, however whenever firing the two actions in my created() hook, the user's information isn't initialised, and therefore the second action fails.
My user store
async fetchCreds({ commit }) {
try {
firebase.auth().onAuthStateChanged(async function(user) {
const { uid } = user
const userDoc = await users.doc(uid).get()
return commit('SET_USER', userDoc.data())
})
} catch (error) {
console.log(error)
commit('SET_USER', {})
}
}
My club action which relies upon the above call
async fetchClubInformation({ commit, rootState }) {
try {
const clubIDForLoggedInUser = rootState.user.clubId
const clubDoc = await clubs.doc(clubIDForLoggedInUser).get()
return commit('SET_CLUB_INFO', clubDoc.data())
} catch (error) {
console.log(error)
}
}
}
The methods being called within my component's created() method.
created: async function() {
await this.fetchCreds();
await this.fetchClubInformation();
this.loading = false;
}
I have a feeling I'm fundamentally misunderstanding async / await, but I can't understand what in the code is incorrect - any help or advice would be greatly appreciated.
I'm not particularly familiar with Firebase but after a bit of digging through the source code I think I can shed a little light on your problems.
Firstly, consider the following example:
async function myFn (obj) {
obj.method(function () {
console.log('here 1')
})
console.log('here 2')
}
await myFn(x)
console.log('here 3')
Question: What order will you see the log messages?
Well here 2 will definitely come before here 3 but it's impossible to tell from the code above when here 1 will show up. It depends on what obj.method does with the function it's been passed. It might never call it at all. It might call it synchronously (e.g. Array's forEach method), in which case here 1 will appear before the other messages. If it's asynchronous (e.g. timers, server calls) then here 1 may not show up for some time, long after here 3.
The async modifier will implicitly return a Promise from the function if it doesn't return a Promise itself. The resolved value of that Promise will be the value returned from the function and the Promise will resolve at the point the function returns. For a function without a return at the end that's equivalent to it finishing with return undefined.
So, to stress the key point, the Promise returned by an async function will only wait until that function returns.
The method onAuthStateChanged calls its callback asynchronously, so the code in that callback won't run until after the surrounding function has completed. There's nothing to tell the implicitly returned Promise to wait for that callback to be invoked. The await inside the callback is irrelevant as that function hasn't even been called yet.
Firebase makes extensive use of Promises, so typically the solution would just be to return or await the relevant Promise:
// Note: This WON'T work, explanation follows
return firebase.auth().onAuthStateChanged(async function(user) {
// Note: This WON'T work, explanation follows
await firebase.auth().onAuthStateChanged(async function(user) {
This won't work here because onAuthStateChanged doesn't actually return a Promise, it returns an unsubscribe function.
You could, of course, create a new Promise yourself and 'fix' it that way. However, creating new Promises using new Promise is generally considered a code smell. Typically it's only necessary when wrapping code that doesn't support Promises properly. If we're working with a library that has proper Promise support (as we are here) then we shouldn't need to create any Promises.
So why doesn't onAuthStateChanged return a Promise?
Because it's a way of watching all sign-in/sign-out events. Every time the user signs in or signs out it'll call the callback. It isn't intended as a way to watch a particular sign-in. A Promise can only be resolved once, to a single value. So while a single sign-in event could be modelled with a Promise it's meaningless when watching all sign-in/sign-out events.
So fetchCreds is registering to be notified about all sign-in/sign-out events. It doesn't do anything with the returned unsubscribe function, so presumably it'll be listening to all such events until the page is reloaded. If you call fetchCreds multiple times it'll keep adding more and more listeners.
If you're waiting for a user to finish signing in then I suggest waiting for that directly instead. firebase.auth() has various methods starting with the prefix signIn, e.g. signInWithEmailAndPassword, and these do return a Promise that resolves when the user has finished signing in. The resolved value provides access to various information, including the user. I don't know which method you're using but the idea is much the same for all of them.
However, it might be that you're really just interested in grabbing the details of the current user. If that's all you want then you don't need to use onAuthStateChanged at all. You should just be able to grab a copy using the currentUser property. Something like this:
async fetchCreds({ commit }) {
try {
const { uid } = firebase.auth().currentUser
const userDoc = await users.doc(uid).get()
commit('SET_USER', userDoc.data())
} catch (error) {
console.log(error)
commit('SET_USER', {})
}
}
As I've already mentioned, this relies on the assumption that the user is already signed in. If that isn't a safe assumption then you might want to consider waiting until after sign in has completed before creating components that need user credentials.
Update:
Questions from the comments:
If the obj.method() call was asynchronous and we did await the callback function within it, would that ensure that the outer async function (myFn) never resolves before the inner one has finished?
I'm not entirely sure what you're asking here.
Just to be clear, I'm being very careful with my use of the words async and asynchronous. A function such as setTimeout would be considered asynchronous but it is not async.
async/await is just a lot of syntactic sugar around Promises. You don't really wait for a function, you wait for a Promise. When we talk about awaiting an async function we're really talking about waiting for the Promise it returns to resolve.
So when you say await the callback function it's not really clear what that means. Which Promise are you trying to await?
Putting the async modifier on a function doesn't make it magically wait for things. It will only wait when it encounters await. You can still have other asynchronous calls within an async function and, just like with a normal function, these calls will be performed after the function has returned. The only way to 'pause' is to await a Promise.
Putting an await inside another function, even a nested function, won't make any difference to whether the outer function waits unless the outer function is already waiting for the inner function. Behind the scenes this is all just Promises chaining then calls. Whenever you write await you're just adding another then call to a Promise. However, that won't have the desired effect unless that Promise is in the same chain as the Promise returned by the outer async function. It only needs one link to be missing for the chain to fail.
So modifying my earlier example:
async function myFn (obj) {
await obj.method(async function () {
await somePromise
// ...
})
// ...
}
await myFn(x)
Note that there are 3 functions here: myFn, method and the callback passed to method. The question is, will await myFn(x) wait for somePromise?
From the code above we can't actually tell. It would depend on what method does internally. For example, if method looked like this then it still wouldn't work:
function method (callback) {
setTimeout(callback, 1000)
}
Putting async on method won't help, that'll just make it return a Promise but the Promise still won't be waiting for the timer to fire.
Our Promise chain has a broken link. myFn and the callback are both creating their parts of the chain but unless method links those Promises together it won't work.
On the other hand, if method is written to return a suitable Promise that waits for the callback to complete then we will get our target behaviour:
function method (callback) {
return someServerCallThatReturnsAPromise().then(callback)
}
We could have used async/await here instead but there was no need as we can just return the Promise directly.
Also, if in the async myFn function you're not returning anything, does that mean it'll resolve immediately and as undefined?
The term immediately is not well-defined here.
If a function isn't returning anything at the end then it's equivalent to having return undefined at the end.
The Promise returned by an async function will resolve at the point the function returns.
The resolved value for the Promise will be the value returned.
So if you aren't returning anything it will resolve to undefined. Resolving won't happen until the end of the function is reached. If the function doesn't contain any await calls then this will happen 'immediately' in the same sense as a synchronous function returning 'immediately'.
However, await is just syntactic sugar around a then call, and then calls are always asynchronous. So while the Promise might resolve 'immediately' the await still has to wait. It's a very short wait, but it isn't synchronous and other code may get the opportunity to run in the meantime.
Consider the following:
const myFn = async function () {
console.log('here 3')
}
console.log('here 1')
Promise.resolve('hi').then(() => {
console.log('here 4')
})
console.log('here 2')
await myFn()
console.log('here 5')
The log messages will appear in the order they're numbered. So even though myFn resolves 'immediately' you'll still get here 4 jumping in between here 3 and here 5.
To make it short
fetchCreds({ commit }) {
return new Promise((resolve, reject) => {
try {
firebase.auth().onAuthStateChanged(async function(user) {
const { uid } = user
const userDoc = await users.doc(uid).get()
commit('SET_USER', userDoc.data())
resolve()
})
} catch (error) {
console.log(error)
commit('SET_USER', {})
resolve()
}}
}
async () => undefined // returns Promise<undefined> -> undefined resolves immediatly
asnyc () => func(cb) // returns Promise<any> resolves before callback got called
() => new Promise(resolve => func(() => resolve())) // resolves after callback got called
This question already has answers here:
How do I return the response from an asynchronous call?
(41 answers)
Closed 5 years ago.
I have an es6 class, with an init() method responsible for fetching data, transforming it, then update the class's property this.data with newly transformed data.
So far so good.
The class itself has another getPostById() method, to just do what it sounds like. Here is the code for the class:
class Posts {
constructor(url) {
this.ready = false
this.data = {}
this.url = url
}
async init() {
try {
let res = await fetch( this.url )
if (res.ok) {
let data = await res.json()
// Do bunch of transformation stuff here
this.data = data
this.ready = true
return data
}
}
catch (e) {
console.log(e)
}
}
getPostById(id){
return this.data.find( p => p.id === id )
}
}
Straightforward, except I have an async/await mechanism in the init() method.
Now, this code will work correctly:
let allPosts = new Posts('https://jsonplaceholder.typicode.com/posts')
allPosts.init()
.then( d => console.log(allPosts.getPostById(4)) )
// resulting Object correctly logged in console
but it only gets printed into the console:
How could I use allPosts.getPostById(4) as a return of a function ?
Like:
let myFunc = async () => {
const postId = 4
await allPosts.init() // I need to wait for this to finish before returning
// This is logging correct value
console.log( 'logging: ' + JSON.stringify(allPosts.getPostById( postId ), null, 4) )
// How can I return the RESULT of allPosts.getPostById( postId ) ???
return allPosts.getPostById( postId )
}
myFunc() returns a Promise but not the final value. I have read several related posts on the subject but they all give example of logging, never returning.
Here is a fiddle that includes two ways of handling init(): using Promise and using async/await. No matter what I try, I can't manage to USE the FINAL VALUE of getPostById(id).
The question of this post is: how can I create a function that will RETURN the VALUE of getPostById(id) ?
EDIT:
A lot of good answers trying to explain what Promises are in regards to the main execution loop.
After a lot of videos and other good reads, here is what I understand now:
my function init() correctly returns. However, within the main event loop: it returns a Promise, then it is my job to catch the result of this Promise from within a kinda parallel loop (not a new real thread). In order to catch the result from the parallel loop there are two ways:
use .then( value => doSomethingWithMy(value) )
use let value = await myAsyncFn(). Now here is the foolish hiccup:
await can only be used within an async function :p
thus itself returning a Promise, usable with await which should be embed in an async function, which will be usable with await etc...
This means we cannot really WAIT for a Promise: instead we should catch parallel loop indefinitely: using .then() or async/await.
Thanks for the help !
As for your comment; I'll add it as answer.
The code you write in JavaScript is run on one thread, that means that if your code could actually wait for something it will block any of your other code from getting executed. The event loop of JavaScript is explained very well in this video and if you like to read in this page.
A good example of blocking code in the browser is alert("cannot do anything until you click ok");. Alert blocks everything, the user can't even scroll or click on anything in the page and your code also blocks from executing.
Promise.resolve(22)
.then(x=>alert("blocking")||"Hello World")
.then(
x=>console.log(
"does not resolve untill you click ok on the alert:",
x
)
);
Run that in a console and you see what I mean by blocking.
This creates a problem when you want to do something that takes time. In other frameworks you'd use a thread or processes but there is no such thing in JavaScript (technically there is with web worker and fork in node but that's another story and usually far more complicated than using async api's).
So when you want to make a http request you can use fetch but fetch takes some time to finish and your function should not block (has to return something as fast as possible). This is why fetch returns a promise.
Note that fetch is implemented by browser/node and does run in another thread, only code you write runs in one thread so starting a lot of promises that only run code you write will not speed up anything but calling native async api's in parallel will.
Before promises async code used callbacks or would return an observable object (like XmlHttpRequest) but let's cover promises since you can convert the more traditional code to a promise anyway.
A promise is an object that has a then function (and a bunch of stuff that is sugar for then but does the same), this function takes 2 parameters.
Resolve handler: A function that will be called by the promise when the promise resolves (has no errors and is finished). The function will be passed one argument with the resolve value (for http requests this usually is the response).
Reject handler: A function that will be called by the promise when the promise rejects (has an error). This function will be passed one argument, this is usually the error or reason for rejection (can be a string, number or anything).
Converting callback to promise.
The traditional api's (especially nodejs api's) use callbacks:
traditionalApi(
arg
,function callback(err,value){
err ? handleFail(err) : processValue(value);
}
);
This makes it difficult for the programmer to catch errors or handle the return value in a linear way (from top to bottom). It gets even more impossible to try and do things parallel or throttled parallel with error handling (impossible to read).
You can convert traditional api's to promises with new Promise
const apiAsPromise = arg =>
new Promise(
(resolve,reject)=>
traditionalApi(
arg,
(err,val) => (err) ? reject(err) : resolve(val)
)
)
async await
This is what's called syntax sugar for promises. It makes promise consuming functions look more traditional and easier to read. That is if you like to write traditional code, I would argue that composing small functions is much easier to read. For example, can you guess what this does?:
const handleSearch = search =>
compose([
showLoading,
makeSearchRequest,
processRespose,
hideLoading
])(search)
.then(
undefined,//don't care about the resolve
compose([
showError,
hideLoading
])
);
Anayway; enough ranting. The important part is to understand that async await doesn't actually start another thread, async functions always return a promise and await doesn't actually block or wait. It's syntax sugar for someFn().then(result=>...,error=>...) and looks like:
async someMethod = () =>
//syntax sugar for:
//return someFn().then(result=>...,error=>...)
try{
const result = await someFn();
...
}catch(error){
...
}
}
The examples allways show try catch but you don't need to do that, for example:
var alwaysReject = async () => { throw "Always returns rejected promise"; };
alwaysReject()
.then(
x=>console.log("never happens, doesn't resolve")
,err=>console.warn("got rejected:",err)
);
Any error thrown or await returning a rejected promise will cause the async function to return a rejected promise (unless you try and catch it). Many times it is desirable to just let it fail and have the caller handle errors.
Catching errors could be needed when you want the promise to succeed with a special value for rejected promises so you can handle it later but the promise does not technically reject so will always resolve.
An example is Promise.all, this takes an array of promises and returns a new promise that resolves to an array of resolved values or reject when any one of them rejects. You may just want to get the results of all promises back and filter out the rejected ones:
const Fail = function(details){this.details=details;},
isFail = item => (item && item.constructor)===Fail;
Promise.all(
urls.map(//map array of urls to array of promises that don't reject
url =>
fetch(url)
.then(
undefined,//do not handle resolve yet
//when you handle the reject this ".then" will return
// a promise that RESOLVES to the value returned below (new Fail([url,err]))
err=>new Fail([url,err])
)
)
)
.then(
responses => {
console.log("failed requests:");
console.log(
responses.filter(//only Fail type
isFail
)
);
console.log("resolved requests:");
console.log(
responses.filter(//anything not Fail type
response=>!isFail(response)
)
);
}
);
Your question and the comments suggest you could use a little intuition nudge about the way the event loop works. It really is confusing at first, but after a while it becomes second nature.
Rather than thinking about the FINAL VALUE, think about the fact that you have a single thread and you can't stop it — so you want the FUTURE VALUE -- the value on the next or some future event loop. Everything you write that is not asynchronous is going to happen almost immediately — functions return with some value or undefined immediately. There's nothing you can do about. When you need something asynchronously, you need to setup a system that is ready to deal with the async values when they return sometime in the future. This is what events, callbacks, promises (and async/await) all try to help with. If some data is asynchronous, you simply can not use it in the same event loop.
So what do you do?
If you want a pattern where you create an instance, call init() and then some function that further process it, you simply need to setup a system that does the processing when the data arrives. There are a lot of ways to do this. Here's one way that's a variation on your class:
function someAsync() {
console.log("someAsync called")
return new Promise(resolve => {
setTimeout(() => resolve(Math.random()), 1000)
})
}
class Posts {
constructor(url) {
this.ready = false
this.data = "uninitilized"
this.url = url
}
init() {
this.data = someAsync()
}
time100() {
// it's important to return the promise here
return this.data.then(d => d * 100)
}
}
let p = new Posts()
p.init()
processData(p)
// called twice to illustrate point
processData(p)
async function processData(posts) {
let p = await posts.time100()
console.log("randomin * 100:", p)
}
init() saves the promise returned from someAsync(). someAsync() could be anything that returns a promise. It saves the promise in an instance property. Now you can call then() or use async/await to get the value. It will either immediately return the value if the promise has already resolved or it will deal with it when it has resolved. I called processData(p) twice just to illustrate that it doesn't calle the someAsync() twice.
That's just one pattern. There are a lot more — using events, observables, just using then() directly, or even callbacks which are unfashionable, but still can be useful.
NOTE: Wherever you use await it has to be inside an async function.
Check out the UPDATED FIDDLE
You need to use await myFunc() to get the value you expect from getPostById because an async function always returns a promise.
This sometimes is very frustrating as the whole chain needs to be converted into async functions but that's the price you pay for converting it to a synchronous code, I guess. I am not sure if that can be avoided but am interested in hearing from people who have more experience on this.
Try out the below code in your console by copying over the functions and then accessing final and await final.
NOTE:
An async function CAN contain an await expression.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
There is no rule that is must have await in order to even declare an async function.
The example below uses an async function without await just to show that an async function always returns a promise.
const sample = async () => {
return 100;
}
// sample() WILL RETURN A PROMISE AND NOT 100
// await sample() WILL RETURN 100
const init = async (num) => {
return new Promise((resolve, reject) => {
resolve(num);
});
}
const myFunc = async (num) => {
const k = await init(num);
return k;
}
// const final = myFunc();
// final; This returns a promise
// await final; This returns the number you provided to myFunc
I stumbled upon the following ES6 method during a rewiew:
async getUsers(res) {
User.findAll().then(users => res.json(users));
}
This seems buggy to me. I would have expected the following:
async getUsers(res) {
return User.findAll().then(users => res.json(users));
}
But then, I know that async has many tricks such as auto-wrapping promises, so is my intuition correct?
The code as it is written (without the return) is likely fine as long as the caller does not need access to the asynchronously retrieved value.
First off, async is needed as part of your function definition ONLY if you plan on using await inside the function. If you aren't using await inside (which your code does not show), then the async part of the definition is not needed. So, it could probably be just this:
getUsers(res) {
User.findAll().then(users => res.json(users));
}
Second, if a caller wants to use await on the getUsers() function, then it needs to return a promise (as you proposed). If the caller will not use await with it or otherwise need any access to a return value, then the return is not necessary. So, if you wanted to use await on this function, then it should probably be this:
getUsers(res) {
// return promise so caller can get async value
return User.findAll().then(users => {
res.json(users);
return users; // return value so caller can get access to the value
});
}
So, the original code may be just fine as long as the caller is never expecting an asynchronous return value from that function (which is possible here because the result of the async operation is being sent with res.json() and that may be the only result that is needed. As far as await/async are concerned, you only have to return a promise from functions that you plan to use await on.
To recap:
async is only needed if you plan on using await inside the function. Your function probably doesn't even need the async keyword on it at all.
A returned promise is only needed if the caller wants to be able to get access to the asynchronous response either via a traditional f().then() or via let x = await f().
A caller can await a function that is not marked as async. The await will get the return value from the function and, if the return value is not a promise it will wrap it in a resolved promise. If it is a promise, it will wait for it to resolve.
FYI, this is a very good, short and sweet review of async/await and how they interact with promises.