Angular 7: custom class decorator destroy component scope

Question

I have a decorator that on ngOnInit write a console.log

log.decorator.ts

export function Log(): ClassDecorator {

    // Decorator Factory
    return (target: Function) => {

        const ngOnInit: Function = target.prototype.ngOnInit;
        target.prototype.ngOnInit = ( ...args ) => {

            console.log('ngOnInit:', target.name);

            if ( ngOnInit ) {
                ngOnInit.apply(this, args);
            }
        };
    };
}

and a HelloComponent that use @Log() and import a service used in ngOnInit

hello.component.ts

import { Component, Input, OnInit } from '@angular/core';
import { Log } from './log.decorator';
import { HelloService } from './hello.service';

@Component({
  selector: 'hello',
  template: `<p>Hello! thanks for help and open the browser console for see the error!</p>`,
  styles: [``]
})
// if you remove @Log(), helloService.sayHello() works!
@Log()
export class HelloComponent implements OnInit  {

  constructor(private helloService: HelloService){}

  ngOnInit(){
    this.helloService.sayHello();
  }
}

but this causes an exception:

ERROR TypeError: Cannot read property 'sayHello' of undefined

if i remove @Log() from HelloComponent it works!

the decorator seems to destroy the component scope on:

ngOnInit.apply(this, args); // line 13: log.decorator.ts

after this call, this.helloService is undefined in the ngOnInit of HelloComponent, but without @Log(), this.helloService is a HelloService instance.

How do I fix this?

Live example on Stackblitz: https://stackblitz.com/edit/angular-7hhp5n

Answer 1

Arrow function forces context this to be the enclosing lexical context and it's execution context of Log function.

To have component context you should use simple function expression:

target.prototype.ngOnInit = function( ...args ) {
   ...
}

Forked Stackblitz