Is there any way to get data from the database without using async/await in Nest.js? - node.js

Question

Is there any way to use callback function for getting the data from the database using typeorm in nest.js and process that data, after that I want to send a response.

Answer 1

I am not sure that I understood your question correctly, but out of the box, you can use TypeORM (assuming you use a SQL DB, Mongoose works similarly, though). The repository functions return a Promise<>, so you could use something like this (from the docs):
return this.photoRepository
.find()
.then(result => {//... your callback code goes here...
});
You could wrap this code in a function getModifiedResult(cb){} and pass the callback into it. Secondly, Remember that async/await is just syntactic sugar for promises, so the above is equivalent to:
result = await this.photoRepository.find();
cbAction = //... do something with your result here
return cbAction;
Again, you could just wrap this.
Another idea is to wrap the promise in an Observable, using the RxJS from operator (fromPromise for RxJS versions < 6). You can then put your callback into the subscription:
//... Note that this returns a subscription for you to unsubscribe.
return from(this.photoRepository
.find()
.then(result => result))
.subscribe(result => //... your callback code
);
If you go down that route, however, it may be worthwhile to modify your results using RxJS operators, like map, switchMap,....

Answer 2

You can just use the from/of operators from rxjs.
Example
create(user: UserInterface): Observable<UserInterface> {
return from(this.userRepository.save(newUser))
}
And if you want you can also pipe the outcome
create(user: UserInterface): Observable<UserInterface> {
return from(this.userRepository.save(user)).pipe(
map((user: UserInterface) => user))
)
}

Answer 3

The answer is that you can use observables and / or promises (async await). I often use an observable for the wrapper function and then promises for addition work in a pipe. I'm not sure why I don't use observables for everything but it doesn't matter.
TypeORM integrates nicely with Nestjs and the docs show how to do the basics. With Postgres there is a problem with arrays that I'm trying to figure out though. An SO post and a Github issue have gone unanswered.

Answer 4

It looks like TypeORM, along with most modern JS packages, have been built with only promises in mind. Callbacks are, for the most part, not used in many programs today unless absolutely necessary, as promises and async/await syntax make the code much cleaner and more readable than the possible callback hell you may enter when using callbacks. It does look like sequelize accepts callbacks, and there are some docs in the recipes section about how to use NestJS with Sequelize

Answer 5

This is not much to do with Nest itself, it's just how you write your code and handle whatever libraries you might have. Let's say you have your database fetching function with a callback:
function findUsersWithCallback(function callback() {
// do something with db
callback(err, results);
});
You can wrap this into a promise-like function with, say, util.promisify
const findUsersPromisified = require('util').promisify(findUsersWithCallback);
What's left is to use your standard Nest provider:
#Injectable() UsersService {
findUsers() {
return findUsersPromisified();
}
}
Now your UsersService behaves like the rest of the framework, and your old callback-based code is nicely wrapped so you can safely ignore it.