Determine if a JavaScript property has a getter or setter defined?

Question

Is it possible, given an object and property name to determine if that property is defined using either a getter or setter, or is it completely transparent? I only want to define a getter/setter if there is not already one defined on the property.

I need it to work in WebKit/Firefox.

Answer 1

I think you're looking for getOwnPropertyDescriptor? https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor

Answer 2

You can use Object.getOwnPropertyDescriptor(obj, prop)

For example:

var obj = { first: 1 }
obj.__defineGetter__('second', function() { return 2; });

// get descriptors
var descriptor1 = Object.getOwnPropertyDescriptor(obj, 'first');
var descriptor2 = Object.getOwnPropertyDescriptor(obj, 'second');

// check if it's a getter

descriptor2.get
// returns function () { return 2; }

descriptor1.get
// returns undefined

Answer 3

You can use Object.getOwnPropertyDescriptor(obj, prop).

function isGetter (obj, prop) {
  return !!Object.getOwnPropertyDescriptor(obj, prop)['get']
}

Usage:

obj = {
  foo: 'foo',
  get bar() { return 'bar' }
}

isGetter(obj, 'bar') // will return true
isGetter(obj, 'foo') // will return false

Answer 4

Find the sample Object template for your java Script requirement:

var Man=function(n,a){
  var name,age;

  function getName(){
       return this.name;
  }

  function getAge(){
       return this.age;
  }
  function create(){
       this.name=n;
       this.age=a;
       console.log('new object created');
  }

  function setAge(age){
       this.age=age;
  }

  function setName(name){
      this.name=name;
  }
  return{
   name:n,
   age:a,
   getName:getName,
   getAge:getAge,
   setName:setName,
   setAge:setAge
  };
}

var m1=new Man('Chaiguy',30);

// To check if Property exist for an Object
console.log(m1.hasOwnProperty('name'));

// To get the name and age of the object

console.log(m1.getName()+' is '+m1.getAge()+' years old.');

Answer 5

Object.defineProperty's get or set Method default value will be undefined if its not defined in the descriptor. This is ideal way to identify.

Let's say example:

var User = {};

Object.defineProperty(User, 'name', {
    get: function () {
        return _name;
    },
    enumerable: true
});

Object.defineProperty(User, 'designation', {
    get: function () {
        return _designation;
    },
    set: function (val) {
        _designation = val;
    },
    enumerable: true
});


var d = Object.getOwnPropertyDescriptor(User, 'name');
console.log(d.set == undefined);

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty