Hey there so I'm trying to store the result of a promise in a variable because I have some code that doesn't work inside of the promise.
(I'm pretty new to node.js so a bit of explanation would be awesome!)
And how would I use a async await?
Heres my current code:
const rbx = require("noblox.js")
var myblurb;
rbx.getBlurb(166962499).then(function (blurb) {
myblurb = blurb;
});
console.log(myblurb);
The function that sets your variable will get called after your function completes, not during it. Therefore your variable is still undefined after calling rbx.getBlurb because it returns a promise that will complete later on.
Using async/await:
One way to get what you're after is to use await. This can only be done within a function that is declared as async, and that will mean the function now will also return a Promise. If you're okay with that, then you can do it like this:
async function doStuff() {
var myblurb;
myblurb = await rbx.getBlurb(166962499);
console.log(myblurb);
}
This tells your function to stop and wait until the promise from calling getBlurb has resolved, and assign the resolved value to your local variable.
Note that if there's an error (either a promise rejection or a thrown exception) then your function will pass that along. You could surround it all with a try/catch to handle that if you need.
Operating Within then
If you don't want to make your function return a promise then you could alternately do this:
rbx.getBlurb(166962499).then(function (blurb) {
const myblurb = blurb;
console.log(myblurb);
// myblurb is only useful within this 'then' resolve function
});
It just means that you have to do the rest of your functionality that relies on the myblurb variable within that resolve function of the then.
Note that whoever is calling this code needs to understand that it will be asynchronous, so it should either use a callback function or Promise to give its results back to the caller (if needed)
Your callback function where assignment is happening will be called after promise is called. Console.log will be executed before that. So it will always log undefined.
You have to do console.log inside function of you can also use async await so solve this.
Related
When I hover on the keyword 'function' the description says:
"(local function)(this: any, next: (err?: mongoose.CallbackError | undefined) => void): Promise<void>"
So does It return a Promise<void> or a simple <void>? I can't even understand what does this function returns? And to be honest I don't understand really well the concept of Promise<void>...
userSchema.pre('save', async function (next) {
let user = this as UserDocument;
if(!user.isModified('password')){
return next();
}
const salt = await bcrypt.genSalt(config.get<number>('saltWorkFactor'));
const hash = await bcrypt.hash(user.password, salt);
user.password = hash;
return next();
})
This question is really interesting. Your function returns a Promise<void>, which is compatible with the void return type that pre is expecting, but Mongoose is quietly smart enough to know what to do with your Promise so you don't even have to call next at all.
First some background:
void has a special meaning in TypeScript to mean that the return value could be any value; the value is frequently undefined (because that's what a function returns without a return statement) but it doesn't have to be. As in the TypeScript FAQ, this makes it convenient to accept or pass functions that return a value by indicating the return value is unused. If you need to supply a function with return type void, you could pass back a function that returns a string, Promise<void>, Promise<SomeObject>, null, undefined, or anything else.
All async functions return Promises, and this is no exception. A Promise<number> is a Promise that says that its then function will receive a number; a Promise<void> is a Promise that doesn't tell you anything about what its then function receives. The then function will still be called, unless it has an error to catch; you just don't know much about its argument.
In Mongoose's types, pre takes a PreSaveMiddlewareFunction<T> function, which is the type of the function you wrote. It accepts a function called next and returns void: Mongoose claims not to care what you return. Your middleware function is allowed to be asynchronous; when you're done you're expected to call next (with an error object, if you have one), and that call to next also returns void.
Your function passed to pre returns type Promise<void>: The function is async so it absolutely returns a promise, and your return next(); means that the Promise resolves to whatever next returns, which is defined as void. You don't know what next returns and shouldn't care about it. You don't even need to return next(), you just need to call it: It's just a callback so you can tell Mongoose your middleware is done and report any errors.
So your async function returns Promise<void>, but that works with the definition of pre: pre doesn't care what kind of return value your function has (void) as long as you call next to indicate you're done.
But wait! Reporting that your asynchronous function is done and whether or not there were errors is exactly the problem that Promises were designed to solve, and the next callback pattern is exactly the kind of pattern that Promises were designed to replace. If you're returning a Promise, why would you need to call next at all when Mongoose can just watch the promise you return?
In fact, in Mongoose 5.x or later, that's exactly what happens: If the function you pass into pre returns a Promise, then you can use that instead of calling next. You can still call next manually for compatibility's sake, but in your case you could delete return next() and everything would keep working. See the middleware docs:
In mongoose 5.x, instead of calling next() manually, you can use a function that returns a promise. In particular, you can use async/await.
schema.pre('save', function() {
return doStuff().
then(() => doMoreStuff());
});
// Or, in Node.js >= 7.6.0:
schema.pre('save', async function() {
await doStuff();
await doMoreStuff();
});
The docs further explain why return next() is a pattern at all:
If you use next(), the next() call does not stop the rest of the code in your middleware function from executing. Use the early return pattern to prevent the rest of your middleware function from running when you call next().
const schema = new Schema(..);
schema.pre('save', function(next) {
if (foo()) {
console.log('calling next!');
// `return next();` will make sure the rest of this function doesn't run
/*return*/ next();
}
// Unless you comment out the `return` above, 'after next' will print
console.log('after next');
});
In summary, the expected return type of void is compatible with the fact that you're returning a Promise<void>, but it hides the fact that recent versions of Mongoose are smart enough to check whether you're returning a Promise and do the right thing without needing a call to next. They're two different styles that both work.
Long answer short: It return a Promise<void>
Callbacks
To understand why, here are some details.
First one must understand Callbacks in node.js. Callbacks are one of the basic structure/feature of how node.js works.
You could say that node.js is basically an Event-Driven Programming "framework" (most people will frown to the framework word...). That means that you tell node that in the event of a certain thing happening, it should do a certain action/function (callback).
For node to understand us, we normally give the callback function as a parameter to another function that will do the work of "listening to the event" and executing the callback that we give it. So it is not "us" that execute the callback, it is the event listener.
In your case,
userSchema.pre('save', async function (next) {
pre is the function (a method in Mongoose's userSchema), save is the event that one must react to, async function (next) { is the callback or what must be done after the event.
You will note that your callback is returning next(), but next() returns void, which mean that your callback is returning void.
So why is it returning Promise<void>?
The fact is that in your case, your callback is an async function. And every async functions will return a promise. It is an async function because it is awaiting another promise (two promises even) inside of it. They are hidden because of the await
const salt = await bcrypt.genSalt(config.get<number>('saltWorkFactor'));
const hash = await bcrypt.hash(user.password, salt);
Note: The bcrypt methods are very expensive in terms of CPU and time (also a security feature among other things).
It also means that normally in your code
const hash = await bcrypt.hash(user.password, salt);
user.password = hash;
you couldn't have available "right away" the hash value for the user.password and, worse, you couldn't even know when it would come. Will your program stop and wait until bcrypt finish its business?
If you have many async functions, your program will be a great favourite for the slowest champion in the Olympics.
What is going on with those promises and how can we not be labelled as a geriatric program?
Promises
Here is a quick/long comment to try to explain the concept of promises.
In "normal" code, each lines of code is executed and "finished" before the next one. Ex: (with cooking)
Combine the Butter and Sugar,
Add Eggs One at a Time, etc.
Or in your code:
let user = this as UserDocument;
if(!user.isModified('password')){
return next();
}
A promise is a certain code that is executed but not finished before the next line of code. Ex:
while the cake is in the oven (promise),
you prepare the frosting,
but you can't put it until the cake in baked (the "then" action of promises).
Note: Your code is using await so there is no "explicit" then method.
You will have many example of "promises" things in everyday life. you may have heard of asynchronous code = not one after the other, not in sync, ...
Turning on an alarm to wake you in the morning, then you make the promise that you will not ignore it;
putting a reminder on the calendar then you make the promise that you will go to that job interview; etc.
All the while, you continue with your life after making those promises.
In code, a function that returns a promise will have a then method where you tell the computer what to do when when the "alarms goes off".
It is usually written like this
mypromise().then(doThisThingFunction)
const continueWithMyLife = true
In this way the then method is very similar to the callback of node.js. It is just expressed in a different way in the code and is not specific to node (callbacks are also not specific to node...).
One very important difference between them is that callbacks are something that the listener "do" and promises is something that resolves (hopefully) to a returning value.
Async/Await
Nowadays it is common to use async/await. Fortunately/unfortunately it basically hides the asynchronous behaviour. Better flow of reading the code, but also much worse understanding of promises for new programmers.
After a await, there is no then method (Or you could say that the following line of code is the then action). There is no "continuing with your life". There is only "waiting until the alarms goes off", So the next line after the await is essentially the "get out of the bed action".
That is why, in your code, the hash value is available in the next line. Basically in the "old way" to write promises
user.password = hash;
would be inside the then function.
And that is also why it is returning Promise<void>
But still, all these analogies won't really help. The best is to try it in everyday code. There is nothing like experience to understand anything.
I'm using instagram-scraping module to display all posts with a specific hash tag but I get an issue using a function to return finded items.
// in my middleware
function getInstagramPostsByHashTag(hashtag) {
return ig.scrapeTag(hashtag).then(result => {
console.log(result); // all posts are displayed in my console
return result;
});
}
// in my ejs template i send the function getInstagramPostsByHashTag()
<%=getInstagramPostsByHashTag("instagram")%> // nothing is displayed
You can't return the posts from the function because the function has already returned at this point. That's because .then() is asynchronous. It executes the provided callback when the work (fetching the posts) is done, but the function continues to run after the call to .then(), and because you return nothing, you get nothing.
If you want a function to return the result of an asynchronous operation, you have to return a promise from the function itself. To help developers with that, there's the async function that automatically returns a promise. In an async function, you can wait for other promises with the await keyword. Your function would look like this as an async function:
async function getInstagramPostsByHashTag(hashtag) {
return await ig.scrapeTag(hashtag)
}
But this is redundant because it returns exactly the same as a direct call to ig.scrapeTag(hashtag) would do. I don't really know ejs but I think the best thing you could do is something like this (only pseudocode, again, I don't know how rendering with ejs works):
posts = insta.scrapeTag("blablabla")
then
ejs.render("<%=posts%>", {posts})
I am trying to add an asynchronous function to the prototype of an object to call it further. Thus:
const ContractObject = Object;
ContractObject.prototype['getBalance'] = async function(userId: number) {
const result = await getBalanceTx(userId);
return result;
};
Internally, I await the result from another function via await. I use it like this:
const result = await myObj.getBalance(25);
As a result, two problems arise:
1 - Function ReturnTxResult, starts to be called every tick
2 - getBalance function does not wait for a response, and is still executed ...
What can you do about it?
P.S By calling the ReturnTxResult function separately, everything works fine. Everywhere there is await, all promises are resolved
Why does your function call itself? It looks like you have unbounded recursion to me.
Because it's an async function, your recursion doesn't cause a stack overflow. Instead, as you pointed out, it's called every tick.
In my finis callback, I have:
finis((code, signal, error) => {
await myprocess.stop()
console.log(`finis(${code}, ${signal}, ${error})`)
})
However, I'm getting the error:
Can not use keyword 'await' outside an async function
So how to do await within finis' callback?
PS. finis()'s callback function will be run just before the node process exits, and I want to gracefully shutdown myprocess by myprocess.stop() before program exist.
The syntax error can be fixed by declaring your callback to be async.
But, it appears that you have bigger issues than that because you want the caller of your callback (the function finis) to wait for your asynchronous operation to complete before it does the rest of its work. await inside your callback by itself will not do that.
Here are a few notes about what await does:
await only does anything useful if you await a promise. It has no magic powers to await some mythical asynchronous operation. It only awaits a promise. So, in your particular code, myprocess.stop() must return a promise that it resolved when the underlying asynchronous operation in myprocess.stop() completes for the await in await myprocess.stop() to do anything useful.
await only blocks execution within the local async function. The containing function still returns as soon as you hit the first await. So, it won't block the caller at all.
An async function (that you can use await in) returns a promise so the only way the caller of that function (the caller of your callback in your case which is finis() would actually wait for your async operation to complete is if it was expecting the callback to return a promise and if it used either .then() or await on that callback itself.
If you control the code for the finis() function, then we could probably help you modify it to do what you want (to look for a returned promise before it shuts things down), but if you don't control that function, then you're probably out of luck.
Try the following.
finis(async (code, signal, error) => {
await myprocess.stop()
console.log(`finis(${code}, ${signal}, ${error})`)
})
I'm writing a Windows 8 app in HTML / JS and have a button on a form with a click event handler. When clicked the first thing the button does is this:
WinJS.Promise.then(openDB()).done(console.log("PROMISE DONE"));
The openDB function looks like this:
function openDB() {
console.log("openDb...");
var req = indexedDB.open("MyDB", 1);
req.onsuccess = function(evt) {
var data = evt.target.result;
console.log("openDb DONE");
}
}
(I also have onerror and onupgradeneeded callbacks on the req object but have left these out for brevity).
I'm clearly misunderstanding how promises are supposed to work but I thought that I could chain multiple THEN calls on a promise, and the final call DONE would only fire when all the THEN calls have executed. Problem is that the console shows 'openDb ...', followed by 'PROMISE DONE', followed by 'OpenDb done'. So the DONE call is being executed before the THEN call. Can anyone explain why this is happening?
Your fundamental issue here is that the "then" method returns a new promise. If the function you called returns a promise, that's the promise that's returned (and thus chained off). If your function does not return a promise, a new (already completed) promise is returned that give you the value that's returned.
Looking at your openDB function, what does it return? Actually, it returns undefined. And more importantly, it kicks off async work and then returns immediately; the async operation doesn't complete until later. Thus, you get the behavior you're seeing.
So what you need to do is get openDB to return a promise that doesn't complete until the database open operation completes. WinJS promises are really bad at this, so the API is ugly. Hopefully they'll fill in the missing piece in Win8.1.
So, you need to create a new promise, and complete it when the async work's done. That looks something like this:
function openDB() {
var complete;
var = new Promise(function (c, e, p) {
complete = c;
});
console.log("openDb...");
var req = indexedDB.open("MyDB", 1);
req.onsuccess = function(evt) {
var data = evt.target.result;
console.log("openDb DONE");
complete(data);
}
return p;
}
The new Promise(function (c, e, p) { ... }) call creates a new promise object. The function you pass is itself passed three functions - one to call on successful completion of the object (c for complete), one to call if the promise unsuccessfully completes (e for error), and one to call to report progress (p for progress). We sock away the completion callback into a variable here for use later.
Now, in the success callback from indexedDB, note that we invoke the complete callback, passing the data we got. This will set the promise to completed state.
Finally, we return the created promise. Note that this return happens synchronously; the function returns before the onsuccess handler gets called.
This should get you what you want - it'll hold off the promise chain until the database open completes. You should probably hook up the error handler to something as well.
Now, having done this, the call to WinJS.Promise.then() is also incorrect, or at least unnecessary. You should instead just do:
openDB().done(function () { console.log('Promise Done'); });
Since openDB itself returns a promise, you don't need the extra wrapping in another promise you're doing.