Reflection in JavaScript

While working with JavaScript you may come across a scenario when you need to reflect type of a particular property. Let us say you have an object Student like following,


If you want to find type of students object property like name and marks you can get them as following,


Now go ahead and fetch type of calculateGrades.


You will get expected output as following,


If you slightly modify above code as below you will get undefined as output. We have put braces in finding type of function



While reflecting getting these two values as output may not be good idea

  1. Function
  2. Undefined

You may want to avoid these values in your code or can use other method hasOwnProperty . You can use this method to find that whether object contains that particular property or not. Following code will alert True since name is property of object Student.


Now let us go ahead and pass function name calculateGrades as value to hasOwnProperty. You will find true as output again.


Interestingly in above code you see that when we pass constructor as input output is false. Reason behind this is hasOwnProperty method does not look for the method in whole property chain. It will only look for the input parameter in current object. As clearly we see that Student object does not have constructor property of its own hence hasOwnProperty is giving false for this.

Better way of doing reflection in JavaScript could be as follows,


By using hasOwnProperty method and typeof operator you can work with reflection in JavaScript. I for your reference code discussed in this post is given below,

<script type="text/javascript" >
 function GetData() {

var Student = {
 name: "DJ",
 marks: 100,
 calculateGrades: function () {
 var typeofmarks = typeof (Student.marks);


if (Student.hasOwnProperty('name')) {
 var typeofname = typeof (;



I hope you find this post useful. Thanks for reading.

Published by

Dhananjay Kumar

Dhananjay Kumar is Developer Evangelist for Infragistics. He is a 8 times Microsoft MVP and well respected Developer Advocate in India.He is the author of 900+ Blog Posts, and can often be found speaking around India at conferences and hosting free workshops for programmers across the country. So far, he has hosted 60 free workshops on various topics like JavaScript, Angular, WCF, ASP.NET MVC, C#, Azure etc. Follow him on twitter @debug_mode for all the updates about his blog posts and workshops. You can send him email at debugmode [at] outlook [dot] com

3 thoughts on “Reflection in JavaScript”

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s