Dojo domReady vs ready

One of the biggest misconceptions in Dojo is what module to choose to create a document-on-load event handler. In Dojo there are two modules that are commonly used, called dojo/ready and dojo/domReady. In this small tutorial I will explain you what the differences are and in what situation you should use them.

Syntax

If we only look at the syntax, you may notice that they’re being used differently. For example:

The dojo/ready module is just a simple module that allows you to provide a callback that will be executed when ready.

require(["dojo/ready"], function(ready) {
    ready(function() {

    });
});

The dojo/domReady! module plugin on the other hand will only complete being loaded if the DOM is loaded. Because it delays the loading process, it means you no longer need a callback, because the module loader callback itself is being delayed. A code example:

require(["dojo/domReady!"], function() {
    ...
});

You probably noticed that I called dojo/domReady a plugin in stead of a module. Well, in Dojo there are both plugins and modules. A plugin is in fact a module that allows parameters when being loaded. The parameter that you provide should be aded behind the exclamination mark. Another common plugin is dojo/text, for example:

require(["dojo/text!template.html"], function(template) {
    ...
});

As you can see clearly here, we’re adding a parameter template.html to the end of the dojo/text plugin. This plugin will then load the contents of that file. When you’re using plugins, you always have to use the exclamination mark. Even when you don’t have to provide data to the plugin, you still use it. So you will always write dojo/domReady! but never dojo/domReady.

The domReady plugin can be written in a similar way as the ready module though, the following will work as well:

require(["dojo/domReady!"], function(domReady) {
    domReady(function() {

    });
});

dojo/ready uses dojo/domReady

The title makes some things clear already. Because dojo/ready uses dojo/domReady, it means that it can do something extra/additional compared to the domReady module. The “extra things” are in fact extra preconditions that should be met before the dojo/ready callback is being executed.

The dojo/domReady plugin only requires 1 condition to be met, namely the DOM should be loaded. If the DOM is loaded, then the callback will be executed. The dojo/ready plugin on the other hand has two extra pre-conditions:

• The DOM must be loaded (same as dojo/domReady)
• (All AMD modules in that block have to be loaded)
• All other ready() callbacks with a higher priority should have been executed

The reason I added the second point between brackets is because it always happens by default. The Dojo AMD loader will only execute the callback when all modules are loaed. Also, because the dojo/domReady! plugin is executed only when the DOM is ready and the AMD loader only executed when all modules are ready, this module in fact does the same as well. For example:

require(["dojo/text!template.html", "dojo/dom", "dojo/dom-construct"], function(template, dom, domConstruct) {
    // This callback is only executed if dojo/text, dojo/dom and dojo/dom-construct are loaded
});

Priority?

So the big difference that remains is the priority you can pass to the dojo/ready module. When using the  ready() function you can in fact add more than just a callback. You can also add a priority. The lower the priority number, the faster it will be executed. For example:

require(["dojo/ready"], function(ready) {
    ready(1500, function() {
        console.log("I'm second");
    });

    ready(1200, function() {
        console.log("I'm first");
    });
});

Because the second ready() statement has a lower priority number, it will be executed before the other one, even though it’s declared after the other one. This becomes really useful when you’re working with Dojo widgets (for example using the dijit library). If you used those, then you probably know there is a module called dojo/parser to parse the DOM into widgets. You can also do that automatically by using the data-dojo-config attribute with the parseOnLoad property. The good thing is that the automatic parser makes use of the dojo/ready module to load only when the DOM is loaded. So, if you know the priority, you can execute your code before the parsing happened, or after the parsing. Something you can’t do with the dojo/domReady module.

Some priorities that might be useful to remember:

Some other benefits

You can also provide a context to the dojo/ready module, for example:

require(["dojo/ready"], function(ready) {
    var myObject = {
        test: "Hello world"
    };
    ready(myObject, function() {
        console.log(this.test); // Hello world
    });
});

By providing a context you can change the object the this context points to. In this case it means it will refer to myObject. This can be quite convenient when you want to make sure that the callback of the ready() function is in fact a part of the object, for example:

require(["dojo/ready"], function(ready) {
    var myObject = {
        test: function() {
            ...
        }
    };
    ready(myObject, 'test');
});

In this example myObject.test() will be called if the conditions from the dojo/ready module are met.

Summarized

Summarized you could say that in most cases the dojo/domReady plugin will be sufficient. Only when you’re starting to use the parseOnLoad auto parsing mechanism, then the dojo/ready module becomes convenient because you can control which code will be executed before or after the parsing happens. This is very important when you’re using the dijit/registry module to retrieve a widget instance. If you’re not using the proper module, you will probably not be able to find the widget, because the parsing did not occur yet.

With the dojo/domReady plugin there is no priority queue, all code will be executed immediately after the DOM is loaded (probably before the widgets are parsed). Many people starting with Dojo are usually having troubles with these and notice that their widgets are not behaving as they should be, simply because they’re not using dojo/ready. So make sure that when you’re encountering a similar problem that you’re using the correct module or plugin.