Javascript – Appending to an object

The most robust answer (i.e. that captures the intent of what you're trying to do while causing the fewest bugs) would be:

Object.size = function(obj) {
  var size = 0,
    key;
  for (key in obj) {
    if (obj.hasOwnProperty(key)) size++;
  }
  return size;
};

// Get the size of an object
const myObj = {}
var size = Object.size(myObj);

There's a sort of convention in JavaScript that you don't add things to Object.prototype, because it can break enumerations in various libraries. Adding methods to Object is usually safe, though.

Here's an update as of 2016 and widespread deployment of ES5 and beyond. For IE9+ and all other modern ES5+ capable browsers, you can use Object.keys() so the above code just becomes:

var size = Object.keys(myObj).length;

This doesn't have to modify any existing prototype since Object.keys() is now built-in.

Edit: Objects can have symbolic properties that can not be returned via Object.key method. So the answer would be incomplete without mentioning them.

Symbol type was added to the language to create unique identifiers for object properties. The main benefit of the Symbol type is the prevention of overwrites.

Object.keys or Object.getOwnPropertyNames does not work for symbolic properties. To return them you need to use Object.getOwnPropertySymbols.

var person = {
  [Symbol('name')]: 'John Doe',
  [Symbol('age')]: 33,
  "occupation": "Programmer"
};

const propOwn = Object.getOwnPropertyNames(person);
console.log(propOwn.length); // 1

let propSymb = Object.getOwnPropertySymbols(person);
console.log(propSymb.length); // 2

The usual way to check if the value of a property is the special value undefined, is:

if(o.myProperty === undefined) {
  alert("myProperty value is the special value `undefined`");
}

To check if an object does not actually have such a property, and will therefore return undefined by default when you try and access it:

if(!o.hasOwnProperty('myProperty')) {
  alert("myProperty does not exist");
}

To check if the value associated with an identifier is the special value undefined, or if that identifier has not been declared. Note: this method is the only way of referring to an undeclared (note: different from having a value of undefined) identifier without an early error:

if(typeof myVariable === 'undefined') {
  alert('myVariable is either the special value `undefined`, or it has not been declared');
}

In versions of JavaScript prior to ECMAScript 5, the property named "undefined" on the global object was writeable, and therefore a simple check foo === undefined might behave unexpectedly if it had accidentally been redefined. In modern JavaScript, the property is read-only.

However, in modern JavaScript, "undefined" is not a keyword, and so variables inside functions can be named "undefined" and shadow the global property.

If you are worried about this (unlikely) edge case, you can use the void operator to get at the special undefined value itself:

if(myVariable === void 0) {
  alert("myVariable is the special value `undefined`");
}