How to validate that at least one checkbox should be selected?

The accepted answer abuses stuff to use in a way they are not meant to be. With reactive forms the best, easiest and probably right way is to use a FormGroup that holds your grouped checkboxes and create a validator to check if at least one(or more) checkbox is checked within that group.

To do so just create another FormGroup inside your existing FormGroup and attach a validator to it:

form = new FormGroup({    // ...more form controls...    myCheckboxGroup: new FormGroup({      myCheckbox1: new FormControl(false),      myCheckbox2: new FormControl(false),      myCheckbox3: new FormControl(false),    }, requireCheckboxesToBeCheckedValidator()),    // ...more form controls...  });

And here is the validator. I made it so you can even use it to check if at least X checkboxes are checked, e.g. requireCheckboxesToBeCheckedValidator(2):

import { FormGroup, ValidatorFn } from '@angular/forms';export function requireCheckboxesToBeCheckedValidator(minRequired = 1): ValidatorFn {  return function validate (formGroup: FormGroup) {    let checked = 0;    Object.keys(formGroup.controls).forEach(key => {      const control = formGroup.controls[key];      if (control.value === true) {        checked ++;      }    });    if (checked < minRequired) {      return {        requireCheckboxesToBeChecked: true,      };    }    return null;  };}

In your template don't forget to add the directive 'formGroupName' to wrap your checkboxes. But don't worry, the compiler will remind you with an error-message if you forget. You can then check if the checkbox-group is valid the same way you do on FormControl's:

<ng-container [formGroup]="form">   <!-- ...more form controls... -->   <div class="form-group" formGroupName="myCheckboxGroup">      <div class="custom-control custom-checkbox">        <input type="checkbox" class="custom-control-input" formControlName="myCheckbox1" id="myCheckbox1">        <label class="custom-control-label" for="myCheckbox1">Check</label>      </div>      <div class="custom-control custom-checkbox">        <input type="checkbox" class="custom-control-input" formControlName="myCheckbox2" id="myCheckbox2">        <label class="custom-control-label" for="myCheckbox2">At least</label>      </div>      <div class="custom-control custom-checkbox">        <input type="checkbox" class="custom-control-input" formControlName="myCheckbox3" id="myCheckbox3">        <label class="custom-control-label" for="myCheckbox3">One</label>      </div>      <div class="invalid-feedback" *ngIf="form.controls['myCheckboxGroup'].errors && form.controls['myCheckboxGroup'].errors.requireCheckboxesToBeChecked">At least one checkbox is required to check</div>    </div>    <!-- ...more form controls... -->  </ng-container>

*This template is very static. Of course you could create it dynamically by using an additional array that holds the the form-data(key of FormControl, label, required, etc.) and create the template automatically by use of ngFor.

Please don't abuse hidden FormControl's like in the accepted answer. A FormControl is not meant to store data like id, label, help-text etc. and doesnt even have a name/key. All this, and much more, should be stored separate, e.g. by a regular array of objects. A FormControl only holds an input-value and provides all this cool state's and functions.

I created a working example you can play with: https://stackblitz.com/edit/angular-at-least-one-checkbox-checked

consider creating a FormGroup which contains your check-box group and bind the group's checked value to a hidden formcontrol with a required validator.

Assume that you have three check boxes

items = [  {key: 'item1', text: 'value1'},      // checkbox1 (label: value1)  {key: 'item2', text: 'value2'},      // checkbox2 (label: value2)  {key: 'item3', text: 'value3'},      // checkbox3 (label: value3)];

Step1: define FormArray for your check boxes

let checkboxGroup = new FormArray(this.items.map(item => new FormGroup({  id: new FormControl(item.key),      // id of checkbox(only use its value and won't show in html)  text: new FormControl(item.text),   // text of checkbox(show its value as checkbox's label)  checkbox: new FormControl(false)    // checkbox itself})));

*easy to show via ngFor

Step2: create a hidden required formControl to keep status of checkbox group

let hiddenControl = new FormControl(this.mapItems(checkboxGroup.value), Validators.required);// update checkbox group's value to hidden formcontrolcheckboxGroup.valueChanges.subscribe((v) => {  hiddenControl.setValue(this.mapItems(v));});

we only care about hidden control's required validate status and won't show this hidden control in html.

Step3: create final form group contains below checkbox group and hidden formControl

this.form = new FormGroup({  items: checkboxGroup,  selectedItems: hiddenControl});

Html Template:

<form [formGroup]="form">  <div [formArrayName]="'items'" [class.invalid]="!form.controls.selectedItems.valid">    <div *ngFor="let control of form.controls.items.controls; let i = index;" [formGroup]="control">      <input type="checkbox" formControlName="checkbox" id="{{ control.controls.id.value }}">      <label attr.for="{{ control.controls.id.value }}">{{ control.controls.text.value }}</label>    </div>  </div>  <div [class.invalid]="!form.controls.selectedItems.valid" *ngIf="!form.controls.selectedItems.valid">    checkbox group is required!  </div>  <hr>  <pre>{{form.controls.selectedItems.value | json}}</pre></form>

refer this demo.

I had the same problem and this is the solution I ended up using with Angular 6 FormGroup because I had few checkboxes.

HTMLNote: I'm using Angular Material for styling, change as needed.

<form [formGroup]="form">  <mat-checkbox formControlName="checkbox1">First Checkbox</mat-checkbox>  <mat-checkbox formControlName="checkbox2">Second Checkbox</mat-checkbox>  <mat-checkbox formControlName="checkbox3">Third Checkbox</mat-checkbox></form>

TypeScript

form: FormGroup;constructor(private formBuilder: FormBuilder){}ngOnInit(){  this.form = this.formBuilder.group({    checkbox1: [''],    checkbox2: [''],    checkbox3: [''],  });  this.form.setErrors({required: true});  this.form.valueChanges.subscribe((newValue) => {    if (newValue.checkbox1 === true || newValue.checkbox2 === true || newValue.checkbox3 === true) {      this.form.setErrors(null);    } else {      this.form.setErrors({required: true});    }  });}

Basically, subscribe to any changes in the form and then modify the errors as needed according to the new form values.