Receiving an error when attempting to inject the Router in a component constructor without using the elvis operator

Upon launching my app, I desire the route /home to be automatically displayed. Unfortunately, the Angular 2 version I am utilizing does not support the "useAsDefault: true" property in route definitions.

To address this issue, I considered implementing the following approach in app.component.ts (a common solution found in various online examples):

export class AppComponent implements OnInit {
  constructor(private _router: Router) {}

  ngOnInit() {
    this._router.navigate(['/home']);
  }
}

However, upon attempting this method, I encountered the error message:

"Error:(29, 12) TS2346: Supplied parameters do not match any signature of call target."

After incorporating the Elvis operator, the error was resolved successfully:

export class AppComponent implements OnInit {
  constructor(private _router?: Router) {}

  ngOnInit() {
    this._router.navigate(['/home']);
  }
}

If anyone could provide insight into why this is occurring, it would be greatly appreciated. Below is a snippet from my app.component.ts file for reference:

import {Component, OnInit} from '@angular/core';
import {Router, Routes, ROUTER_DIRECTIVES } from '@angular/router';
import {NavbarComponent} from "./navbar.component";
import {UsersComponent} from "./users.components";
import {PostsComponent} from "./posts.component";
import {HomeComponent} from "./home.component";

@Component({
    selector: 'my-app',
    templateUrl: 'app/app.component.html',
    directives: [NavbarComponent, ROUTER_DIRECTIVES]
})

@Routes([
  {path: '/home', component: HomeComponent},
  {path: '/users', component: UsersComponent},
  {path: '/posts', component: PostsComponent}
])

export class AppComponent implements OnInit {
  constructor(private _router?: Router) {}

  ngOnInit() {
    this._router.navigate(['/home']);
  }
}

Answer №1

Adding the Elvis operator resolves the issue without any errors

Note: The term "Elvis operator" is not accurate. It is simply a syntax used to indicate optional parameters (see https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#3922-parameter-list)

I would appreciate assistance in understanding why

The error message

Supplied parameters do not match any signature of call target
indicates that the constructor is being called without providing all the necessary parameters. Although the specific code causing this issue is not included in your question, you now have an explanation as to why adding optional parameters suppresses this error.

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