@title Concepts: Sigils and Metadata
@group concepts

Explains Javelin's sigils and metadata.

= Overview =

Javelin introduces two major concepts, "sigils" and "metadata", which are core
parts of the library but don't generally exist in other Javascript libraries.
Both sigils and metadata are extra information you add to the DOM which Javelin
can access. This document explains what they are, why they exist, and how you
use them.

= Sigils =

Sigils are names attached to nodes in the DOM. They behave almost exactly like
CSS class names: sigils are strings, each node may have zero or more sigils, and
sigils are not unique within a document. Sigils convey semantic information
about the structure of the document.

It is reasonable to think of sigils as being CSS class names in a different,
semantic namespace.

If you're emitting raw tags, you specify sigils by adding a `data-sigil`
attribute to a node:

  lang=html

      ...

      ...

However, this should be considered an implementation detail and you should not
rely on it excessively. In Javelin, use @{method:JX.Stratcom.hasSigil} to test
if a node has a given sigil, and @{method:JX.Stratcom.addSigil} to add a sigil
to a node.

Javelin uses sigils instead of CSS classes to rigidly enforce the difference
between semantic information and style information in the document. While CSS
classes can theoretically handle both, the conflation between semantic and style
information in a realistic engineering environment caused a number of problems
at Facebook, including a few silly, preventable, and unfortunately severe bugs.

Javelin separates this information into different namespaces, so developers and
designers can be confident that changing CSS classes and presentation of a
document will never change its semantic meaning. This is also why Javelin does
not have a method to test whether a node has a CSS class, and does not have CSS
selectors. Unless you cheat, Javelin makes it very difficult to use CSS class
names semantically.

This is an unusual decision for a library, and quite possibly the wrong tradeoff
in many environments. But this was a continual source of problems at Facebook's
scale and in its culture, such that it seemed to justify the measures Javelin
takes to prevent accidents with style information having inadvertent or
unrealized semantic value.

= Metadata =

Metadata is arbitrary pieces of data attached to nodes in the DOM. Metadata can
be (and generally is) specified on the server, when the document is constructed.
The value of metadata is that it allows handlers which use event delegation to
distinguish between events which occur on similar nodes. For instance, if you
have newsfeed with several "Like" links in it, your document might look like
this:

  lang=html

      ...
      Like

      ...
      Like

You can install a listener using Javelin event delegation (see @{article:
Concepts: Event Delegation} for more information) like this:

  lang=js
  JX.Stratcom.listen(
    'click',
    ['newsfeed', 'story', 'like'],
    function(e) {
      // ...
    });

This calls the function you provide when the user clicks on a "like" link, but
you need to be able to distinguish between the different links so you can know
which story the user is trying to like. Javelin allows you to do this by
attaching **metadata** to each node. Metadata is attached to a node by adding a
`data-meta` attribute which has an index into data later provided to
@{method:JX.Stratcom.mergeData}:

  lang=html

      ...
      Like

      ...
      Like

  ...

This data can now be accessed with @{method:JX.Stratcom.getData}, or with
@{method:JX.Event.getNodeData} in an event handler:

  lang=js
  JX.Stratcom.listen(
    'click',
    ['newsfeed', 'story', 'like'],
    function(e) {
      var id = e.getNodeData('story').storyID;
      // ...
    });

You can also add data to a node programmatically in Javascript with
@{method:JX.Stratcom.addData}.
