How to efficiently count the number of keys/properties of an object in JavaScript?

To do this in any ES5-compatible environment, such as Node, Chrome, IE 9+, Firefox 4+, or Safari 5+:

Object.keys(obj).length

• Browser compatibility
• Object.keys documentation (includes a method you can add to non-ES5 browsers)

You could use this code:

if (!Object.keys) {
    Object.keys = function (obj) {
        var keys = [],
            k;
        for (k in obj) {
            if (Object.prototype.hasOwnProperty.call(obj, k)) {
                keys.push(k);
            }
        }
        return keys;
    };
}

Then you can use this in older browsers as well:

var len = Object.keys(obj).length;

If you are using Underscore.js you can use _.size (thanks @douwe):
_.size(obj)

Alternatively you can also use _.keys which might be clearer for some:
_.keys(obj).length

I highly recommend Underscore, its a tight library for doing lots of basic things. Whenever possible they match ECMA5 and defer to the native implementation.

Otherwise I support @Avi's answer. I edited it to add a link to the MDC doc which includes the keys() method you can add to non-ECMA5 browsers.