decode ways javascript

1/**
2 * @param {string} s
3 * @return {number}
4 */
5var numDecodings = function(s) {
6  var len = s.length;
7  var tmp = 0;
8  var tmp1 = 1;
9  var tmp2 = 0;
10  var num1 = 0;
11  var num2 = 0;
12
13  if (s === '' || s[0] === '0') return 0;
14
15  for (var i = 1; i <= len; i++) {
16    num1 = Number(s.substr(i - 1, 1));
17    num2 = Number(s.substr(i - 2, 2));
18    if (num1 !== 0) tmp += tmp1;
19    if (10 <= num2 && num2 <= 26) tmp += tmp2;
20    tmp2 = tmp1;
21    tmp1 = tmp;
22    tmp = 0;
23  }
24
25  return tmp1;
26};
27