ΑΜΚΑ validator for angular2

amka.validator.ts

import {AbstractControl} from '@angular/forms';

export class AmkaValidator {

  public static validate(c:AbstractControl) {

    let isElevenDigitNumber = /^\d{11}$/.test(c.value),
        string_amka = c.value.toString(),
        sumIsValid = false,
        sum = 0;
    
    for (let i=1; i<= string_amka.length; i++) {
        let digit = parseInt(string_amka[i-1], 10);
        digit = i % 2 === 0 ? digit * 2 : digit;  
        sum += digit > 9 ? (digit - 9) : digit;        
    }

    sumIsValid = sum % 10 === 0 && sum > 0;

    return sumIsValid && isElevenDigitNumber ? null : {
      validateAmka: {
        valid: false
      }
    };
  }
}