In Angular, how to add Validator to FormControl after control is created?

To add onto what @Delosdos has posted.

Set a validator for a control in the FormGroup: this.myForm.controls['controlName'].setValidators([Validators.required])

Remove the validator from the control in the FormGroup: this.myForm.controls['controlName'].clearValidators()

Update the FormGroup once you have run either of the above lines. this.myForm.controls['controlName'].updateValueAndValidity()

This is an amazing way to programmatically set your form validation.

You simply pass the FormControl an array of validators.

Here's an example showing how you can add validators to an existing FormControl:

this.form.controls["firstName"].setValidators([Validators.minLength(1), Validators.maxLength(30)]);

Note, this will reset any existing validators you added when you created the FormControl.

Angular 12 update

Since Angular 12, if you want to add new validators to the form without removing the existing validators, you can use addValidator:

this.form.controls["firstName"].addValidators([Validators.minLength(1), Validators.maxLength(30)]);