Filter Observable AngularFire results by multiple attributes dynamically with a Form

course-manager.service.ts

...
  getCourses(): Observable<Course[]> {
    return this.db
      .collection<Course>(this.path)
      .valueChanges({ idField: 'id' }); // id field is nice to have, only
  }
...

filter-results.component.ts

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup } from '@angular/forms';
import { Course } from '../../shared/course';
import { CourseManagerService } from '../../shared/course-manager.service';

@Component({
  selector: 'app-course-read-all',
  templateUrl: './course-read-all.component.html',
  styleUrls: ['./course-read-all.component.scss']
})
export class CourseReadAllComponent implements OnInit {
  private form: FormGroup;
  courses: Course[];
  coursesFiltered: Course[];
  showSpinner = true;

  constructor(
    private manager: CourseManagerService,
    private formBuilder: FormBuilder
  ) {}

  ngOnInit() {
    this.resetForm();
    this.resetList();
  }

  resetForm() {
    this.form = this.formBuilder.group({
      term: [''],
      language: [''],
      category: ['']
    });

    this.form.valueChanges.subscribe(() => this.performFilter());
  }

  resetList() {
    // Get typed-array from Observable-array
    this.manager.getCourses().subscribe(data => {
      this.courses = data;
      this.coursesFiltered = this.courses;
      this.showSpinner = false;
    });
  }

  performFilter() {
    const filterBy = this.form.get('term').value.toLocaleLowerCase();
    const filterByLang = this.form.get('language').value.toLocaleLowerCase();
    const filterByCat = this.form.get('category').value.toLocaleLowerCase();
    this.coursesFiltered = this.courses.filter(
      (course: Course) =>
        (filterBy === '' ||
          (course.name.toLocaleLowerCase().indexOf(filterBy) !== -1 ||
            course.description.toLocaleLowerCase().indexOf(filterBy) !== -1 ||
            course.tags.toLocaleLowerCase().indexOf(filterBy) !== -1)) &&
        (filterByCat === '' ||
          course.category.toLocaleLowerCase() === filterByCat) &&
        (filterByLang === '' ||
          course.language.toLocaleLowerCase() === filterByLang)
    );
  }
}