Unable to access service functionality beyond the constructor

I've browsed through various solutions as there were similar queries like mine, but unfortunately, I couldn't find a suitable answer. Feeling quite lost here, I've experimented with different methods since I'm fairly new to Angular.

ERROR:

books-search.component.ts:13 Uncaught TypeError: Cannot read property 'testService' of undefined

UPDATE:

I mistakenly assumed I could test the method there and encountered a new error after relocating the method call.

NEW ERROR:

compiler.js:19550 Uncaught Error: Provider parse errors: Cannot instantiate cyclic dependency! BooksService ("[ERROR ->]"): in NgModule AppModule in ./AppModule@-1:-1

UPDATE:

The error has been rectified, thank you.

books-search.component.ts

import { Component, OnInit} from '@angular/core';
import { BooksService } from '../services/books.service';
declare var jquery:any;
declare var $ :any;
@Component({
  selector: 'app-books-search',
  templateUrl: './books-search.component.html',
  styleUrls: ['./books-search.component.css']
})
export class BooksSearchComponent implements OnInit {
  constructor(private service:BooksService){};
  ngOnInit(){
//moved here after second edit
  this.service.testService();}
  searchBook(bookTitle:string){
    var searchTerm = '';
    var books = [];
    //initial api url
    var googleAPI = "https://www.googleapis.com/books/v1/volumes?q=";
    //grabbing our search term
    searchTerm = bookTitle;
    //replacing spaces with +
    searchTerm.toString();
    searchTerm = searchTerm.replace(/ /g,"+");
    //adding our searchterm to our api
    googleAPI += searchTerm;
    // gets a json object from google books api
    $.getJSON(googleAPI, function(response){
      this.service.testService();
    });
  }
}

books.service.ts

import { Injectable } from '@angular/core';

@Injectable()
export class BooksService {
  constructor(private service: BooksService){}
  public books = [];
  testService(){
    console.log('service test success!');
  }
  setData(data){
    this.books = data;
    console.log('data set : ' + this.books);
  }
  getData(){
    return this.books;
  }
}

app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { BooksService } from './services/books.service';

import { AppComponent } from './app.component';
import { BooksSearchResultComponent } from './books-search-result/books-search-result.component';
import { BookshelfComponent } from './bookshelf/bookshelf.component';
import { BooksSearchComponent } from './books-search/books-search.component';


@NgModule({
  declarations: [
    AppComponent,
    BooksSearchResultComponent,
    BookshelfComponent,
    BooksSearchComponent
  ],
  imports: [BrowserModule],
  providers: [BooksService],
  bootstrap: [AppComponent]
})
export class AppModule { }

Answer №1

Invoke testService() within the ngOnInit lifecycle hook

ngOnInit(){
this.service.testService();
}

// To fix the second error, remove injecting the service directly

@Injectable()
export class BooksService {
constructor(){}
}

Answer №2

Make sure to properly encapsulate this.service.testService(); within a method in your class. It seems like it's currently loose within the class definition. Consider placing it inside your `ngOnInit()' function instead of leaving it floating on its own.

Answer №3

Injecting BooksService into its own constructor is causing issues. Try removing it and the problem should be resolved.

import { Injectable } from '@angular/core';

@Injectable()
export class BooksService {
    constructor(){}
    //... remainder of the class
}

Answer №4

The issue might lie within the service implementation

@Injectable()
export class LibraryService {
  constructor(private service: LibraryService){}

By injecting LibraryService into its own constructor, you are causing a cyclic dependency. Try removing it and testing again.

@Injectable()
export class LibraryService {
  constructor(){}

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