How do you implement custom form validation in Angular?

Short answer: Create a validator function returning an error object or null.

Explain a bit more

✅ Example (Custom validator checking forbidden name): function forbiddenNameValidator(nameRe: RegExp): ValidatorFn { return (control: AbstractControl): ValidationErrors | null => { const forbidden = nameRe.test(control.value); return forbidden ? { forbiddenName: { value: control.value } } : null; }; } // Usage: new FormControl('', [forbiddenNameValidator(/admin/i)])

Example code

Create a validator function returning an error object or null. ✅ Example (Custom validator checking forbidden name): function forbiddenNameValidator(nameRe: RegExp): ValidatorFn { return (control: AbstractControl): ValidationErrors | null => {
const forbidden = nameRe.test(control.value);
return forbidden ? { forbiddenName: { value: control.value } } : null; }; } // Usage: new FormControl('', [forbiddenNameValidator(/admin/i)])

Real-world example (ShopNest)

ShopNest admin can be built in Angular with modules/components, services for API calls, and reactive forms for product edit.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.