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

You can use the following:

Double bang !! property lookup

var a = !![]; // true
var a = !!null; // false

hasOwnProperty This is something that I used to use:

var myObject = {
  name: 'John',
  address: null
};
if (myObject.hasOwnProperty('address')) { // true
  // do something if it exists.
}

However, JavaScript decided not to protect the method’s name, so it could be tampered with.

var myObject = {
  hasOwnProperty: 'I will populate it myself!'
};

prop in myObject

var myObject = {
  name: 'John',
  address: null,
  developer: false
};
'developer' in myObject; // true, remember it's looking for exists, not value.

typeof

if (typeof myObject.name !== 'undefined') {
  // do something
}

However, it doesn't check for null.

I think this is the best way.

in operator

var myObject = {
  name: 'John',
  address: null
};

if('name' in myObject) {
  console.log("Name exists in myObject");
}else{
  console.log("Name does not exist in myObject");
}

result:

Name exists in myObject

Here is a link that goes into more detail on the in operator: Determining if an object property exists