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

Question

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: w

Answer 1

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.

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