How to use an object as the default value in Angular FormControl (without using a reference)?

Question

I am creating a select element, and want to set an object as the default value when the application starts. In my example I have a list of animals, and a FormControl used in an HTML template. I tried adding an object (not a reference to an object) directly to the formControl like this:

animalControl = new FormControl({name:'Dog', sound: 'Woof!'}, [Validators.required]); // this does not work

This does not work. However, referencing an object from the actual list works, like this:

this.animalControl.setValue(this.animals[1]); // this works

Is there another way of setting a value to the formControl without having to find the correct reference, only passing an object to it instead?

Working example: https://stackblitz.com/edit/angular-object-default-select?file=app/select-hint-error-example.ts

TS:

 export class SelectHintErrorExample {
  animalControl = new FormControl({name:'Dog', sound: 'Woof!'}, [Validators.required]); // this does not work

  animals = [
    {name: 'Dog', sound: 'Woof!'},
    {name: 'Cat', sound: 'Meow!'},
    {name: 'Cow', sound: 'Moo!'},
    {name: 'Fox', sound: 'Wa-pa-pa-pa-pa-pa-pow!'},
  ];
  constructor(){
    // this.animalControl.setValue(this.animals[1]); // this works
  }

}

HTML:

<mat-form-field>
  <mat-select placeholder="Favorite animal" [formControl]="animalControl" required>
    <mat-option>--</mat-option>
    <mat-option *ngFor="let animal of animals" [value]="animal">
      {{animal.name}}
    </mat-option>
  </mat-select>
  <mat-error *ngIf="animalControl.hasError('required')">Please choose an animal</mat-error>
  <mat-hint>{{animalControl.value?.sound}}</mat-hint>
</mat-form-field>

AVJT82 · Accepted Answer · 2018-02-16 11:12:17Z

2

If you don't want to create a reference, you can use compareWith in your select, just like on any other select tag:

<mat-select [compareWith]="compareAnimals" ...

and TS:

compareAnimals(a1: any, a2: any): boolean {
  return a1 && a2 ? a1.name === a2.name : a1 === a2;
}

Your StackBlitz: https://stackblitz.com/edit/angular-object-default-select-ervu6r?file=app%2Fselect-hint-error-example.ts

answered Feb 16, 2018 at 11:12

AVJT82

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1 Comment

John Over a year ago

Thank you for your answer. I will give it a try next week. I am just concerned about efficiency. Will the ‘compareAnimals’ method run after each change cycle? In that case it might be better to find the reference once after all.

Eliseo · Accepted Answer · 2018-02-16 09:43:24Z

0

Use FormBuilder https://angular.io/guide/reactive-forms#introduction-to-formbuilder. You must decided that you want to use a FormArray or not

//If a only animal
createForm(data:any)
{
  this.animalcontrol=this.fb.group({
      name: [data.name?data.name:'',Validators.Required],
      sound:[data.sound=data.sound:'']
    });
}

if you want use a formArray, check too Angular 5 populate data with FormArray

answered Feb 16, 2018 at 9:43

Eliseo

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1 Comment

John Over a year ago

Thank you for your answer. This approach means that my animalcontrol must be of type FormGroup, I want it to be of type FormControl. Also, if I have an object with a lot more properties, would I still need to copy all of them? Isn't it an approach to pass the object itself, instead of copying it?

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