Tanver-Hasan

curtomerForm: FormGroup;

            constructor(public fb: FormBuilder) { }

            ngOnInit() {
              this.customerForm= this.fb.group({
                 firstName: [''],
                 lastName: [''],
                 mobiles: this.fb.array([this.buildNumber()])
              });

Tanver-Hasan

 import { FormBuilder } from '@angular/forms';
                   constructor(public fb: FormBuilder) { }

Tanver-Hasan

import { ReactiveFormsModule } from '@angular/forms';
      @NgModule({
        declarations: [
          AppComponent
        ],
        imports: [
          BrowserModule,
          AppRoutingModule, ReactiveFormsModule,
          ServiceWorkerModule.register('/ngsw-worker.js', { enabled: environment.production })
        ],