Requirejs domReady plugin vs Jquery $(document).ready()?

It seems like all the key points were already hit, but a few details fell through the cracks. Mainly:

domReady

It is both a plugin and a module. If you include it in the the requirements array w/ a trailing ! your module won't execute until it's "safe" to interact w/ the DOM:

define(['domReady!'], function () {
    console.info('The DOM is ready before I happen');
});

Note that loading and executing are different; you want all your files to load as soon as possible, it's the execution of the contents that is time sensitive.

If you omit the !, then it's just a normal module that happens to be a function, which can take a callback that won't execute before the DOM is safe to interact with:

define(['domReady'], function (domReady) {
    domReady(function () {
        console.info('The DOM is ready before I happen');
    });
    console.info('The DOM might not be ready before I happen');        
});

Advantage when using domReady as a plugin

Code that depends on a module that in turn depends on domReady! has a very significant advantage: It does not need to wait for the DOM to be ready!

Say that we have a block of code, A, that depends on a module, B, that depends on domReady!. Module B will not finish loading before the DOM is ready. In turn, A will not run before B has loaded.

If you were to use domReady as a regular module in B, it would also be necessary for A to depend on domReady, as well as wrapping its code inside a domReady() function call.

Furthermore, this means that domReady! enjoys that same advantage over $(document).ready().

Re the differences between domReady and $(document).ready()

Both sniff out whether/when the DOM is ready in essentially the same way.

Re fear of jQuery firing at the wrong time

jQuery will fire any ready callback even if the DOM loads before jQuery does (your code shouldn't care which happens first.).

An attempt at answering your main question:

Why does requirejs provides a domReady plugin when we can have jquery's $(document).ready();?

They do two different things, really. RequireJS' domReady dependency signifies that this module requires the DOM to be completely loaded before it can be run (and can therefore be found in any number of modules in your application if you so desire), while $(document).ready() instead fires off its callback functions when the DOM is done loading.

The difference may seem subtle, but think of this: I have a module that needs to be coupled to the DOM in some way, so I can either depend on domReady and couple it at module definition time, or put down a $(document).ready() at the end of it with a callback to an init function for the module. I'd call the first approach cleaner.

Meanwhile, if I have an event that needs to happen right as the DOM is ready, the $(document).ready() event would be the go-to, since that does not in particular depend on RequireJS being done loading modules, provided the dependencies of the code you're calling it from are met.

It's also worth considering that you do not necessarily use RequireJS with jQuery. Any library module that needs DOM access (but does not rely on jQuery) would then still be useful by using domReady.

Answering your bullets in order of appearance:

• They both do accomplish the same thing
• If you have reservations about jquery for whatever reason then use domReady
• Correct, so just use jQuery
• Because not everyone uses jQuery
• I agree, just use jQuery
• Plugins by definition 'feed a need'.
• Cross Browser ajax isn't a thing. Cross Domain? There probably is, and if there isn't then there is no need to feed.
• , -, -, - Ok

When it comes down to it, you are overthinking this. It's a mechanism to execute javascript on domReady. If you didn't have jQuery I would advocate the domReady plugin. Since you have jQuery then don't load more scripts to do something that is already available.

Clarity Update

The domReady plugin collects functions to call when the document is 'ready'. If it is already loaded then they execute immediately.

JQuery collects functions and binds a deferred object to the dom being 'ready'. When the dom is ready the deferred object will be resolved and the functions will fire. If the dom is already 'ready' then the deferred will already be resolved so the function will execute immediately.

This means that effectively they do exactly the same thing.