How to add an element in nested FormArrays?

Question

I have an array in another array like that:

<div class="col-sm-6">
  <div formArrayName="site_source_pages">
    <div *ngFor="let site_source_page of siteForm.controls.site_source_pages.controls; let x=index">
      <div [formGroupName]="x">
        <div class="card card-accent-success">
          <div class="card-header">
            <strong>Yeni Kaynak Sayfa</strong>
          </div>
          <div class="card-block">
            <div class="row">
              <!--Site Page Source Form Group-->
              <div class="form-group">
                <div class="form-group col-sm-6">
                  <label for="page_url">Sayfa Url</label>
                  <!--<label *ngIf="!site_source_page.controls[x].controls.page_url.valid" class="text-danger">*</label>-->
                  <input type="text" class="form-control" id="page_url" placeholder="Badge-Manager" formControlName="page_url">
                </div>
              </div>
              <div class="form-group col-sm-6">
                <label for="postal-code">Tarama Aralığı</label>
                <input type="text" class="form-control" id="postal-code" placeholder="Badge-Email">
              </div>
            </div>
            <div class="row">
              <div formArrayName="page_containers">
                <div *ngFor="let page_container of site_source_page.controls.page_containers.controls; let i=index">
                  <div [formGroupName]="i">
                    <div class="col-sm-4">
                      <div class="form-group">
                        <label for="ccnumber">Container {{i + 1}} Type</label>
                        <label [hidden]="site_source_page.controls.page_containers.controls[i].controls.container_type.valid" class="text-danger">*</label>
                        <input type="text" class="form-control" id="container_type" placeholder="Container Type" formControlName="container_type">
                      </div>
                    </div>
                    <div class="col-sm-4">
                      <div class="form-group">
                        <label for="ccnumber">Container {{i + 1}} Selector Type</label>
                        <input type="text" class="form-control" id="selector_type" placeholder="Container Selector Type" formControlName="selector_type">
                      </div>
                    </div>
                    <div class="col-sm-4">
                      <div class="form-group">
                        <label for="ccnumber">Container {{i + 1}} Css Query</label>
                        <span class="glyphicon glyphicon-remove pull-right" *ngIf="site_source_page.controls.page_containers.controls.length > 1" (click)="removePageContainer(i)">x</span>
                        <input type="text" class="form-control" id="container_css_query" placeholder="Container Css Query" formControlName="selector_css_query">
                      </div>
                    </div>
                    <div class="col-sm-12">
                      <button type="button" class="btn btn-secondary btn-block" (click)="addPageContainer(x)">Container Ekle</button>
                    </div>
                  </div>
                </div>
              </div>
            </div>
          </div>
        </div>
      </div>
    </div>
  </div>
</div>

I want to add item to second array via typescript. I try to reach second array (page_containers) but get this error:

Cannot read property 'controls' of undefined

My function is like that:

addPageContainer(x: number) {
  // add container to the list    
  const controlSp = <FormArray>this.siteForm.controls['site_source_pages'];
  const controlPc = <FormArray>controlSp.controls['page_containers'];
  controlPc.push(this.initPageContainer());
}

Why can't I reach second array with this line?

const controlPc = <FormArray>controlSp.controls['page_containers'];

Any idea?

Answer 1

The problem is that controlSp is a FormArray, so to access page_containers, as any other common array, you have to specify the index.

You can do it using FormArray#at method:

const controlSp = <FormArray>this.siteForm.controls['site_source_pages'];
const controlPc = <FormArray>controlSp.at(x).controls['page_containers'];

or even better using AbstractControl#get method:

addPageContainer(index: number) {
  const controlPc = this.siteForm.get(`site_source_pages.${index}.page_containers`) as FormArray;
  controlPc.push(this.initPageContainer());
}

Also, as you may have noticed I changed the type cast from <variable> to as variable.

Why? Because while the as syntax is fully compatible with JSX/TSX syntax, the other way isn't.

Note that if you don't have any problem with this, you can use whatever you prefer.

---

That said, as an advice, you can simplify your HTML a lot.

Instead of (lines in order):

*ngFor="let site_source_page of siteForm.controls.site_source_pages.controls"

[hidden]="site_source_page.controls.page_containers.controls[i].controls.container_type.valid

*ngIf="site_source_page.controls.page_containers.controls.length > 1

You can have (lines in order):

*ngFor="let site_source_page of siteForm.get('site_source_pages').controls"

*ngIf="page_container.get('container_type').invalid

*ngIf="site_source_page.get('page_containers').length > 1"

---

Also, just a minor note:

The following line is actually commented, but it's invalid:

<label *ngIf="!site_source_page.controls[x].controls.page_url.valid" class="text-danger">*</label>

It should be:

<label *ngIf="!site_source_page.controls.page_url.valid" class="text-danger">*</label>

or even better:

<label *ngIf="site_source_page.get('page_url').invalid" class="text-danger">*</label>

Answer 2

Try to use get method,

controlSp = <FormArray> this.siteForm.get('site_source_pages');

FOr more details you can refer to the Angular document