index.js
function checkSum(num) {
    if (num == null) {
      return 0;
    }
    var doubled = _splitDigits(num).reverse().map(function(v, idx) {
      return idx % 2 === 0 ? v * 2 : v;
    });
    var sum = doubled.reduce(function(tmp, v) {
      return tmp + _sum(v);
    }, 0);
    var check = 10 - (sum % 10);
    return check === 10 ? 0 : check;
  }
function checkSum(num) {
    if (num == null) {
      return 0;
    }
    var doubled = _splitDigits(num).reverse().map(function(v, idx) {
      return idx % 2 === 0 ? v * 2 : v;
    });
    var sum = doubled.reduce(function(tmp, v) {
      return tmp + _sum(v);
    }, 0);
    var check = 10 - (sum % 10);
    return check === 10 ? 0 : check;
  }
function checkSum ( num ) { if ( num == null ) { return 0 ; } var doubled = _splitDigits ( num ) . reverse ( ) . map ( function ( v , idx ) { return idx % 2 === 0 ? v * 2 : v ; } ) ; var sum = doubled . reduce ( function ( tmp , v ) { return tmp + _sum ( v ) ; } , 0 ) ; var check = 10 - ( sum % 10 ) ; return check === 10 ? 0 : check ; }
create check sum.

@param {string|number} num
@return {number} check sum value
create check sum .