Difference between obtrusive and unobtrusive javascript

What is the difference between obtrusive and unobtrusive javascript - in plain english. Brevity is appreciated. Short examples are also appreciated.

Answers:

Answer

No javascript in the mark up is unobtrusive:

<div id="informationHeader">Information</div>

obtrusive:

<div onclick="alert('obstrusive')">Information</div>
Answer
  1. Separation of HTML and JavaScript (define your JavaScript in external JavaScript files)
  2. Graceful degradation (important parts of the page still work with JavaScript disabled).

For a long-winded explanation, checkout the Wikipedia page on the subject.

Answer

To expand on Mike's answer: using UJS behavior is added "later".

<div id="info">Information</div>

... etc ...

// In an included JS file etc, jQueryish.
$(function() {
    $("#info").click(function() { alert("unobtrusive!"); }
});

UJS may also imply gentle degradation (my favorite kind), for example, another means to get to the #info click functionality, perhaps by providing an equivalent link. In other words, what happens if there's no JavaScript, or I'm using a screen reader, etc.

Answer

unobtrusive - "not obtrusive; inconspicuous, unassertive, or reticent."

obtrusive - "having or showing a disposition to obtrude, as by imposing oneself or one's opinions on others."

obtrude - "to thrust (something) forward or upon a person, especially without warrant or invitation"

So, speaking of imposing one's opinions, in my opinion the most important part of unobtrusive JavaScript is that from the user's point of view it doesn't get in the way. That is, the site will still work if JavaScript is turned off by browser settings. With or without JavaScript turned on the site will still be accessible to people using screen readers, a keyboard and no mouse, and other accessibility tools. Maybe (probably) the site won't be as "fancy" for such users, but it will still work.

If you think in term's of "progressive enhancement" your site's core functionality will work for everybody no matter how they access it. Then for users with JavaScript and CSS enabled (most users) you enhance it with more interactive elements.

The other key "unobtrusive" factor is "separation of concerns" - something programmers care about, not users, but it can help stop the JavaScript side of things from obtruding on the users' experience. From the programmer's point of view avoiding inline script does tend to make the markup a lot prettier and easier to maintain. It's generally a lot easier to debug script that isn't scattered across a bunch of inline event handlers.

Answer

I don't endorse this anymore as it was valid in 2011 but perhaps not in 2018 and beyond.

Separation of concerns. Your HTML and CSS aren't tied into your JS code. Your JS code isn't inline to some HTML element. Your code doesn't have one big function (or non-function) for everything. You have short, succinct functions.

Modular. This happens when you correctly separate concerns. Eg, Your awesome canvas animation doesn't need to know how vectors work in order to draw a box.

Don't kill the experience if they don't have JavaScript installed, or aren't running the most recent browsers-- do what you can to gracefully degrade experience.

Don't build mountains of useless code when you only need to do something small. People endlessly complicate their code by re-selecting DOM elements, goofing up semantic HTML and tossing numbered IDs in there, and other strange things that happen because they don't understand the document model or some other bit of technology-- so they rely on "magic" abstraction layers that slow everything down to garbage-speed and bring in mountains of overhead.

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