How do I count a JavaScript object's attributes? [duplicate]

You can do that by using this simple code:

Object.keys(myObject).length

There's no easy answer, because Object — which every object in JavaScript derives from — includes many attributes automatically, and the exact set of attributes you get depends on the particular interpreter and what code has executed before yours. So, you somehow have to separate the ones you defined from those you got "for free."

Here's one way:

var foo = {"key1": "value1", "key2": "value2", "key3": "value3"};
Object.prototype.foobie = 'bletch'; // add property to foo that won't be counted

var count = 0;
for (var k in foo) {
    if (foo.hasOwnProperty(k)) {
       ++count;
    }
}
alert("Found " + count + " properties specific to foo");

The second line shows how other code can add properties to all Object derivatives. If you remove the hasOwnProperty() check inside the loop, the property count will go up to at least 4. On a page with other JavaScript besides this code, it could be higher than 4, if that other code also modifies the Object prototype.

Use underscore library, very useful: _.keys(obj).length.