Question

between saving a key name like this

    for(var key in this.data)
        {
            var key_name = key;
            for(key in this.data[key_name].content){
                alert(this.data[key_name].content[key].score);
            }
        }

or making checkpoint objects for every parent node

    for(var key in this.data)
        {
            var obj = this.data[key];
            for(key in obj.content){
                var  inner_obj = obj.content;
                alert(inner_obj[key].score);
            }
        }

which one has better performance? any other suggestion?

Answer 1

Theoretically at least, the fastest would be to get the object reference outside the inner loop. In practice the difference might not be that big, if the browser is already doing this internally. Also, the performance might differ between browsers.

for(var key in this.data) {
  var obj = this.data[key].content;
  for(var key2 in obj){
    alert(obj[key2].score);
  }
}

Note that you should use separate variables for the loops, otherwise it will be hard to follow which values is in the variable at which point when you put some actual code in the loops.

Edit:

When I measure the performance in IE and Firefox, I find that this method is slightly faster than the methods proposed in the question. However, the difference is so small that you should not be concerned with it. Either method works just fine, and any performance concerns would be what you actually do with the values inside the inner loop.