Angular custom validator - restricted words

customValidator.ts

  private restrictedWords(words) {
    return (control: FormControl): {[key: string]: any} => {
      if (!words ) { return null; }
      const invalidWords = words
        .map(word => control.value.includes(word) ? word : null)
        .filter(word => word != null);
      return invalidWords && invalidWords.length > 0
        ? { restrictedWords: invalidWords.join(', ')}
        : null;
    };
  }