Javascript IsUnique String

gistfile1.js

function isUnique( s ) {
  var chars = {}
  var dup = true;
  for (var i = 0; i < s.length; ++i) {
    if ((s[i] in chars)) {
        return false;
    }
    chars[s[i]] = 1;
  }

  return dup;
}

console.log(isUnique("abc")); // true
console.log(isUnique("abcb")); // false