What is Unobtrusive JavaScript?

Unobtrusive JavaScript is the way of writing JavaScript language in which we properly separate Document Content and Script Content thus allowing us to make a clear distinction between them. Just as we should separate our structure and presentation by putting all CSS in a separate file and eschewing the use of style attributes or other such presentational markup, we should also separate our HTML structure and JavaScript behaviour . The reasons are the same: it separates your concerns, keeps your code clean, and allows you to work on the JavaScript without touching either HTML or CSS. So it is basically separating behavior or javascript from presentation or html. This approach is useful in so many ways as it makes our code less error prone, easy to update and to debug. The basic concept of unobtrusive programming is that JavaScript should be used as an enhancement to our web page instead of an absolute requirement. If you do not need JavaScript , then don't use it; your static content will display just fine with plain old HTML and CSS. So many developers make the mistake of importing code libraries before even figuring out if they will be even required. jQuery for example has been so much mis-used when all our basic needs can be easily fulfilled by CSS and pure JavaScript code. The idea of unobtrusive JavaScript is to turn old school design on its head: instead of making 100% JavaScript-powered, dynamic web pages make 100% regular, static web pages and then, almost as an afterthought, add an "also" layer of JavaScript. example
<input type="button" id="btn" onclick="alert('Test')" />
That is not unobstrusive javascript because behaviour and presentation are mixed. The onclick shouldn't be there in html and should be part of javascript itself not html.

With above example, you can go unobstrusive like this:

<input type="button" id="btn" />
var el = document.getElementById('btn'); el.onclick = function(){ alert('Test'); };