Unit Testing Reactive Form Validators under Invalid Inputs

import { FormControl, ValidationErrors, ValidatorFn } from '@angular/forms';
 
export function forbiddenKeywordValidator(forbiddenWord: RegExp): ValidatorFn {
  return (control: FormControl): ValidationErrors | null => {
    if (!control.value || typeof control.value !== 'string') {
      return null;
    }
    const isForbidden = forbiddenWord.test(control.value);
    return isForbidden ? { forbiddenKeyword: { value: control.value } } : null;
  };
}
 
describe('forbiddenKeywordValidator', () => {
  const validator = forbiddenKeywordValidator(/admin/i);
 
  it('should return null when control contains valid text', () => {
    const control = new FormControl('johndoe');
    expect(validator(control)).toBeNull();
  });
 
  it('should return a validation error object when forbidden keyword is present', () => {
    const control = new FormControl('SuperAdmin123');
    const result = validator(control);
 
    expect(result).toEqual({
      forbiddenKeyword: { value: 'SuperAdmin123' }
    });
  });
});