Why is the fourth nested value way slower to access than the fifth? [closed]

Question

I was building a jsperf to illustrate time it took to access nested object members and I found this strange phenomenon. For some reason, the test run drastically slower for the fourth-nested object member than the fifth. I've tried this in Chrome and Firefox and am getting the same results.

Any ideas why this would be happening?

Answer 1

Your object is this, my comment added:

var obj = {
  "one": {
    "two": {
      "three": {
        "four": {
          "five": {
            "value": 0
          }
          /* MISSING "value": 0 */
        },
        "value": 0
      },
      "value": 0
    },
    "value": 0
  },
  "value": 0
};

};

The object at the "four" key doesn't have a "value" key, however, so apparently the JavaScript engine(s) have to do extra work to handle that case: miss the key look-up on the object, miss the key look-up on the object's prototype Object, return undefined, and then compute NaN when you add 1 to undefined.

Answer 2

The problem is that obj.one.two.three.four does not have a value attribute. In that one case, JavaScript traverses the prototype chain looking for the missing attribute before finally assigning NaN to sum.