Support using direct injectable instance

[shlomiassaf]

I'm submitting a...

[ ] Regression 
[x] Bug report
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

@Injtectable Classes can't return their own instance, the instance will always be
a single instance of the prototype which is later get's values assigned to it from the new
instance created.

For example, if I have class XYZ {}, when Nest init's it will do this:

component.instance = Object.create(XYZ.prototype);

Laster when XYZ is needed and instantiated it will do this:

component.instance = Object.assign(component.instance, new XYZ());

See here:
https://github.com/nestjs/nest/blob/master/packages/core/injector/injector.ts#L63-L66

Expected behavior

Allow an opt-in option to use the returned value from the constructor (i.e. new XYZ()) directly.

What is the motivation / use case for changing the behavior?

Mainly, using Proxy classes and other sort of thing requiring the return of a different value.

Here is an example of a winston wrapper:

import { Injectable, Inject } from '@nestjs/common';
import { LeveledLogMethod, Logger, LoggerOptions, createLogger } from 'winston';

const WINSTON_PROXY_HANDLER: ProxyHandler<WLogger> = {
  has: (target: WLogger, p: PropertyKey) => Reflect.has(target.winston, p),
  get: (target: WLogger, p: PropertyKey, receiver: any) => Reflect.get(target.winston, p, target.winston),
  set: (target: WLogger, p: PropertyKey, value: any, receiver: any) => Reflect.set(target.winston, p, value, target.winston),
};

@Injectable()
export class WLogger {
  readonly winston: Logger;

  // for cli and npm levels
  error: LeveledLogMethod;
  warn: LeveledLogMethod;
  help: LeveledLogMethod;
  data: LeveledLogMethod;
  info: LeveledLogMethod;
  debug: LeveledLogMethod;
  prompt: LeveledLogMethod;
  http: LeveledLogMethod;
  verbose: LeveledLogMethod;
  input: LeveledLogMethod;
  silly: LeveledLogMethod;

  // for syslog levels only
  emerg: LeveledLogMethod;
  alert: LeveledLogMethod;
  crit: LeveledLogMethod;
  warning: LeveledLogMethod;
  notice: LeveledLogMethod;

  constructor(@Inject(WINSTON_LOGGER_OPTIONS) options?: LoggerOptions) {
    this.winston = createLogger(options);
    const proxy = new Proxy(this, WINSTON_PROXY_HANDLER);
    return proxy;
  }
}

Notice the last part, the constructor...

Now, I know I can use a factory provider to solve this, but it makes to solution cumbersome and requires extra steps. Proxies are powerful but not natively usable with nest.

I know it's a big deal and a core behaviour, this is why I suggest to add it as an opt-in feature.

[kamilmysliwiec]

Fixed in 5.4.1

[willsoto]

@kamilmysliwiec Nice! Thanks for adding this. Will there be documentation added? How can we opt-in to this behavior?

[kamilmysliwiec]

It just fixed old issues (no API changes)

[lock]

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