How to check if object has any properties in JavaScript?

Question

Assuming I declare

var ad = {}; 

How can I check whether this object will contain any user-defined properties?

Answer 1

What about making a simple function?

function isEmptyObject(obj) {
  for(var prop in obj) {
    if (Object.prototype.hasOwnProperty.call(obj, prop)) {
      return false;
    }
  }
  return true;
}

isEmptyObject({}); // true
isEmptyObject({foo:'bar'});  // false

The hasOwnProperty method call directly on the Object.prototype is only to add little more safety, imagine the following using a normal obj.hasOwnProperty(...) call:

isEmptyObject({hasOwnProperty:'boom'});  // false

---

Note: _{(for the future)} The above method relies on the for...in statement, and this statement iterates only over enumerable properties, in the currently most widely implemented ECMAScript Standard (3rd edition) the programmer doesn't have any way to create non-enumerable properties.

However this has changed now with ECMAScript 5th Edition, and we are able to create non-enumerable, non-writable or non-deletable properties, so the above method can fail, e.g.:

var obj = {};
Object.defineProperty(obj, 'test', { value: 'testVal', 
  enumerable: false,
  writable: true,
  configurable: true
});
isEmptyObject(obj); // true, wrong!!
obj.hasOwnProperty('test'); // true, the property exist!!

An ECMAScript 5 solution to this problem would be:

function isEmptyObject(obj) {
  return Object.getOwnPropertyNames(obj).length === 0;
}

The Object.getOwnPropertyNames method returns an Array containing the names of all the own properties of an object, enumerable or not, this method is being implemented now by browser vendors, it's already on the Chrome 5 Beta and the latest WebKit Nightly Builds.

Object.defineProperty is also available on those browsers and latest Firefox 3.7 Alpha releases.