Angular – Dynamically add/remove validators

If you are using Angular 12.2 or higher, you can use the AbstractControl methods addValidators, removeValidators, and hasValidator, as per the docs:

if(this.businessFormGroup.get('businessType').value !== 'Other'){
    this.businessFormGroup.get('description').addValidators(Validators.required);               
} else {                
    this.businessFormGroup.get('description').clearValidators();               
}

For older versions, Angular forms have a built in function setValidators() that enables programmatic assignment of Validators. However, this will overwrite your validators.

For your example you can do:

if(this.businessFormGroup.get('businessType').value !== 'Other'){
    this.businessFormGroup.controls['description'].setValidators([Validators.required, Validators.maxLength(200)]);              
} else {                
    this.businessFormGroup.controls['description'].setValidators([Validators.maxLength(200)]);               
}
this.businessFormGroup.controls['description'].updateValueAndValidity();

It is important to keep in mind that by using this method you will overwrite your existing validators so you will need to include all the validators you need/want for the control that you are resetting.