Angular Check checkboxes on load

Question

need your help

I have an array of checkbox values

colors = ['Black', 'Red', 'White', 'Blue', 'Yellow', 'Grey'];

Implemented them in HTML

<div *ngFor="let color of colors; let i = index" class="checkbox">
        <label>
          <input type="checkbox" class="colors" [value]="color" (change)="addCheckboxData($event)" />
          {{ color }}
        </label>
      </div>

So it looks like this:

I have an array of values that should be checked when I load the page

  checkedValues = ['Black', 'White', 'Grey'];

I added them to formarray

color: this._formBuilder.array(this.checkedValues),

It works and color formarray have values like in checkedValues array:

But checkboxes are not checked so I want this values to be displayed as checked. How can I do this? Thank You

Eliseo · Accepted Answer · 2020-10-07 06:58:07Z

you need pass an array of booleans

color: this._formBuilder.array(
              this.colors.map(x=>this.checkedValues.indexOf(x)>=0)),

And manage the form like

<form [formGroup]="form">
    <input formControlName="name">
    <div formArrayName="color">
        <div *ngFor="let control of colorFormArray.controls;let i=index">
            <input type="checkbox" [formControlName]="i">{{colors[i]}}
      </div>
        </div>
</form>
<pre>
{{form?.value|json}}
</pre>

Well, you see that "color" get as value an array of booleans, e.g. [true,false,true,false,false,true]

And you want that color get an array with the colors, so the idea is has a formControl not a formArray and use [ngModel] and (ngModelChange). As we are using [ngModel] inside a FormGroup, we need use [ngModel]="{standalone:true}"

See the form and the .html

this.form2=this._formBuilder.group({
  name:[],
  color:[this.checkedValues] //<--see that is a simple FormControl
})

<form [formGroup]="form2">
        <div *ngFor="let col of colors;let i=index">
            <input type="checkbox" 
           [ngModel]="form2.get('color').value && form2.get('color').value.indexOf(col)>=0" 
           (ngModelChange)="colorChange($event,i)"
           [ngModelOptions]="{standalone:true}"
                 
      >{{colors[i]}}
        </div>
</form>

The [ngModel] is true or false according form2.get('color').value.indexOf

The function colorChange received the value of the checkbox and an index

  colorChange(checked:boolean,index:number)
  {
    const checkedColors=this.form2.get('color').value || [] //the actual value of the formControl
                                                            //if is null an empty array
    const color=this.colors[index];  //the color selected

    ///if is checked and is not in the array
    if (checked && checkedColors.indexOf(color)<0)
    {
      //concatenate to the value the new color
      //and sort the result array 

      const orderColors=[...checkedColors,color]
                  .sort((a,b)=>this.colors.indexOf(a)-this.colors.indexOf(b))

      //make a setValue
      this.form2.get('color').setValue(orderColors)
    }

    //if not checked

    if (!checked)
      this.form2.get('color').setValue(checkedColors.filter(x=>x!=color))

  }

In the stackblitz you has the two aproach

NOTE: In this secod aproach we can make that if there are no color select, the value of the control becomes null. This allow us use a Validators.required, e.g. The only is change the function colorChange

  colorChange(checked:boolean,index:number)
  {
        ...

    if (!checked)
    {
      const result=checkedColors.filter(x=>x!=color)
      this.form2.get('color').setValue(result.length>0?result:null)
    }

  }

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