What return this function in typescript?-CodePudding

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();

})

CodePudding user response：

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, even when that 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 returns, just that it'll do so unless it has an error to catch.
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() when it's at the end of the function: 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 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 expecting a call to next. They're two different styles that both work.

CodePudding user response：

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.