CODEWITHSUNDEEP
javascriptperformanceobjectnode.jskey
BMiner
BMiner

Reputation: 17077

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

Upvotes: 24

Views: 6809

Answers (3)

deadrunk
deadrunk

Reputation: 14135

ECMA 6 harmony introduces Map and Set classes which you can probably utilize (in the future :)

var map = new Map;
map.set('a', 'b');
console.log(map.size); // prints 1

I believe it should have complexity O(1), not tried though. You can run it in node 0.11+ via node --harmony script.js.

Another way is to use Proxy class which also has been added in harmony.

Upvotes: 2

BMiner
BMiner

Reputation: 17077

After a bit of research, there is no way to determine the number of keys in a JavaScript Object in constant time, at least not in Node... and not quite yet. Node internally keeps track of this information, but it does not expose it, since there is no method to do so in ECMA-262 5th.

It's worth noting that Harmony (ECMA version 6) may natively support Maps and Sets. Not sure what the spec for these will turn out to be.

I am told that we need to bring this up with TC39 committee.

Bug report for V8: http://code.google.com/p/v8/issues/detail?id=1800

Upvotes: 7

Andrew
Andrew

Reputation: 13853

See the source, specifically GetLocalElementKeys

v8 objects.cc

Upvotes: -2

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