Efficiently counting the number of keys / properties of an Object in JavaScript

This question is almost identical to How to efficiently count the number of keys/properties of an object in JavaScript?.

I want to know one extra piece of information: what is a "constant-time" way of determining the number of keys in an Object? I am mostly concerned with doing this in Node.JS, as most Objects on the browser aren't too large to be of great concern.

EDIT: It appears that Object.keys(obj).length returns in linear time O(n) in Google Chrome and in Node.JS (i.e. dependent on the number of keys in obj). Is there a better O(1) method?

I did some testing in Node.JS (source is below)

var tests = [10e3, 10e4, 10e5, 10e6]
for(j in tests) {
    var obj = {};
    for(i = 0; i < tests[j]; i++)
        obj[i] = i;
    console.time('test' + tests[j]);
    Object.keys(obj).length;
    console.timeEnd('test' + tests[j]);
}

For n = 10e3, 10e4, 10e5, 10e6... results are:

test10000: 5ms
test100000: 20ms
test1000000: 371ms
test10000000: 4009ms