JavaScript reduce implementation

reduce.js

var __reduce= function (func, list, initial) {
        if (initial != null) {
          var value = initial;
          var idx = 0;
        } else if (list) {
          var value = list[0];
          var idx = 1;
        } else {
          return null;
        }
  
        for (; idx < list.length; idx++) {
          value = func(value, list[idx]);
        }
  
        return value;
      }
};