javascript - What is an AngularJS directive?

Question

I have spent quite a lot of time reading through AngularJS documentation and several tutorials, and I have been quite surprised at how unapproachable the documentation is.

I have a simple, answerable question that may also be useful to others looking to pick up AngularJS:

What is an AngularJS directive?

There should be a simple, precise definition of a directive somewhere, but the AngularJS website offers these surprisingly useless definitions:

On the home page:

Directives are a unique and powerful feature available in AngularJS. Directives let you invent new HTML syntax, specific to your application.

In the developer documentation:

Directives are a way to teach HTML new tricks. During DOM compilation directives are matched against the HTML and executed. This allows directives to register behavior, or transform the DOM.

And there is a series of talks about directives which, ironically, seem to assume the audience already understands what they are.

Would anyone be able to offer, for clear reference, a precise definition of what a directive is that explains:

1. What it is (see the clear definition of jQuery as an example)
2. What practical problems and situations it is intended to address
3. What design pattern it embodies, or alternatively, how it fits into the purported MVC/MVW mission of AngularJS.

Answer 1

What it is (see the clear definition of jQuery as an example)?

A directive is essentially a function^† that executes when the Angular compiler finds it in the DOM. The function(s) can do almost anything, which is why I think it is rather difficult to define what a directive is. Each directive has a name (like ng-repeat, tabs, make-up-your-own) and each directive determines where it can be used: element, attribute, class, in a comment.

^† A directive normally only has a (post)link function. A complicated directive could have a compile function, a pre-link function, and a post-link function.

What practical problems and situations is it intended to address?

The most powerful thing directives can do is extend HTML. Your extensions are a Domain Specific Language (DSL) for building your application. E.g., if your application runs an online shopping site, you can extend HTML to have "shopping-cart", "coupon", "specials", etc. directives -- whatever words or objects or concepts are more natural to use within the "online shopping" domain, rather than "div"s and "span"s (as @WTK already mentioned).

Directives can also componentize HTML -- group a bunch of HTML into some reusable component. If you find yourself using ng-include to pull in lots of HTML, it is probably time to refactor into directives.

What design pattern does it embody, or alternatively, how does it fit into the purported MVC/MVW mission of angularjs

Directives are where you manipulate the DOM and catch DOM events. This is why the directive's compile and link functions both receive the "element" as an argument. You can

• define a bunch of HTML (i.e., a template) to replace the directive
• bind events to this element (or its children)
• add/remove a class
• change the text() value
• watch for changes to attributes defined in the same element (actually it is the attributes' values that are watched -- these are scope properties, hence the directive watches the "model" for changes)
• etc.

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In HTML we have things like <a href="...">, <img src="...">, <br>, <table><tr><th>. How would you describe what a, href, img, src, br, table, tr, and th are? That's what a directive is.