Angular - Pure vs Impure Validator Functions

pure-validator-functions.ts

// impure function: (this.allowedStates may return undefined)
private validState(): ValidatorFn {
  return (control: AbstractControl): { [key: string]: any } | null => {
    const currentState = control.value;
    const foundState = this.allowedStates.find(state => currentState === state);
    return foundState ? null : { state: 'State not allowed' };
  };
}

// pure function
private validState(allowedStates: string[]): ValidatorFn {
  return (control: AbstractControl): { [key: string]: any } | null => {
    const currentState = control.value;
    const foundState = allowedStates.find(state => currentState === state);
    return foundState ? null : { state: 'State not allowed' };
  };
}

// construct the form and attach the validator
...
allowedStates = ['FL','CA','NC'];
...
buildForm() {
  this.form = this.fb.group({
    state: ['', this.validState(this.allowedStates)],
  });
}