ERROR TypeError: Cannot read properties of undefined (reading 'push')

Question

I'm following https://www.positronx.io/ionic-firebase-list-tutorial/ which creates a CRUD list app that writes and reads from Firebase. I'm using an Ionic framework with Angular. When I try to write data to firebase, this is the console error I get:

ERROR TypeError: Cannot read properties of undefined (reading 'push')
    at UserService.createUser (user.service.ts:17:26)
    at MakePage.onFormSubmit (make.page.ts:33:23)
    at MakePage_Template_form_ngSubmit_6_listener (template.html:9:30)
    at executeListenerWithErrorHandling (core.mjs:14963:16)
    at Object.wrapListenerIn_markDirtyAndPreventDefault [as next] (core.mjs:15001:22)
    at SafeSubscriber.__tryOrUnsub (Subscriber.js:183:1)
    at SafeSubscriber.next (Subscriber.js:122:1)
    at Subscriber._next (Subscriber.js:72:1)
    at Subscriber.next (Subscriber.js:49:1)
    at EventEmitter_.next (Subject.js:39:1)

Here is the code in user.service.ts:

import { Injectable } from '@angular/core';
import { User } from './users';
import { AngularFireDatabase, AngularFireList, AngularFireObject } from '@angular/fire/compat/database';

@Injectable({
  providedIn: 'root'
})

export class UserService {
  userList: AngularFireList<any>;
  userRef: AngularFireObject<any>;

  constructor(private db: AngularFireDatabase) { }

  // Create
  createUser(user: User) {
    return this.userList.push({
      name: user.name,
      email: user.email,
      mobile: user.mobile,
    })
  }

and here is make.page.ts:

import { Component, OnInit } from '@angular/core';
import { Router } from '@angular/router';
import { FormGroup, FormBuilder } from "@angular/forms";
import { UserService } from '../shared/user.service';

@Component({
  selector: 'app-make',
  templateUrl: './make.page.html',
  styleUrls: ['./make.page.scss'],
})

export class MakePage implements OnInit {
  form: FormGroup;

  constructor(
    private apiService: UserService,
    private router: Router,
    public fb: FormBuilder
  ) { }

  ngOnInit() {
    this.form = this.fb.group({
      name: [''],
      email: [''],
      mobile: ['']
    })
  }

  onFormSubmit() {
    if (!this.form.valid) {
      return false;
    } else {
      this.apiService.createUser(this.form.value).then(res => {
        this.form.reset();
        this.router.navigate(['/home']);
      })
        .catch(error => console.log(error));
    }
  }
}

Answer 1

It seems you've not yet defined the property userList in class UserService. The tutorial you linked to does it using the getUserList method, which is being called in the init function of the HomePage.

The tutorial probably presents the code in a way it's easier to explain, but doesn't always keep the code working, so just follow the tutorial further and you should get your program working in the end.