typescript - Angular 4: La propiedad "push" y "controls" no existen en el tipo "AbstractControl"

Implementé el código de este enlace http://plnkr.co/edit/yV94ZjypwBgHAlb0RLK2?p=preview pero estoy recibiendo errores de push y de controles.

Aquí está lo que hice y no sé qué está mal con ello.

import { Component } from '@angular/core';
import { ViewController,Platform } from 'ionic-angular';
import { FormBuilder,FormGroup,Validators,FormControl,FormArray } from '@angular/forms';

@Component({
  selector: 'filter-vendor',
  templateUrl: 'filter-vendor.html'
})

export class FilterVendorPage {




  questions = [{id:1,text:'Question 1', answers:[{id:1},{id:2}]},{id:2,text:'Question 2', answers:[{id:11},{id:22}]}]
  surveyForm:FormGroup;



  constructor(
    private viewCtrl: ViewController, 
    private formBuilder:FormBuilder
     ){


      this.surveyForm=this.formBuilder.group({
        question:formBuilder.array([])
      })

      for(var i=0;i<this.questions.length;i++){
          let question=formBuilder.group({
            question_id:[this.questions[i].id,Validators.required],
            answer_id:formBuilder.array([])
          });
          this.surveyForm.controls['questions'].push(question);
      }
}


   onChange(id, isChecked, index) {

    const answers = <FormArray>this.surveyForm.controls.questions.controls[index].controls.answer_ids

    if(isChecked) {
          answers.push(new FormControl(id))
        } else {
          let idx = answers.controls.findIndex(x => x.value == id)
          answers.removeAt(idx)
    }
  }

}

Por favor, ayúdame a resolver este problema. Muchas gracias

Respuesta #1

Typescript se queja en la comprobación de tipos. Necesitas convertir tu control a FormArray. Así que cambia

1)

this.surveyForm.controls['questions'].push(question);

a

(<FormArray>this.surveyForm.controls['questions']).push(question);

o

(this.surveyForm.controls['questions'] as FormArray).push(question);

o

(this.surveyForm.get('questions') as FormArray).push(question);

2)

const answers = <FormArray>this.surveyForm.controls.questions.controls[index].controls.answer_ids

a

const answers = this.surveyForm.get(['questions', index, 'answer_ids']) as FormArray;

o

const answers = this.surveyForm.get(`questions.${index}.answer_ids`) as FormArray;

Plunker Bifurcado