Length of a JavaScript object

Length of a JavaScript object

Updated answer

Heres 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 doesnt 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

Older answer

The most robust answer (i.e. that captures the intent of what youre 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);

Theres a sort of convention in JavaScript that you dont add things to Object.prototype, because it can break enumerations in various libraries. Adding methods to Object is usually safe, though.

If you know you dont have to worry about hasOwnProperty checks, you can use the Object.keys() method in this way:

Object.keys(myArray).length

Length of a JavaScript object

Updated: If youre using Underscore.js (recommended, its lightweight!), then you can just do

_.size({one : 1, two : 2, three : 3});
=> 3

If not, and you dont want to mess around with Object properties for whatever reason, and are already using jQuery, a plugin is equally accessible:

$.assocArraySize = function(obj) {
    // http://stackoverflow.com/a/6700/11236
    var size = 0, key;
    for (key in obj) {
        if (obj.hasOwnProperty(key)) size++;
    }
    return size;
};