Status

HTMLBars is still very much a work in progress. At the moment, the parser does not have any special logic for void tags (<img>) or special contexts (<table>).

I'll keep this section updated as it progresses.

Goals

The goals of HTMLBars are to have a compiler for Handlebars that builds a DOM rather than a String.

This means that helpers can have special behavior based on their context (they know if they are inside an <a> tag, inside an attribute, etc.)

Ultimately, the goal is to have a good data binding setup for Handlebars that can work directly against DOM nodes and doesn't need special tags in the String for the data binding code can find (a major limitation in Ember).

Usage

The main way to use HTMLBars is to override the RESOLVE and RESOLVE_IN_ATTR helpers:

HTMLBars.registerHelper('RESOLVE', function(parts, options) {
	var prop = parts.join("."),
			context = this,
			parent = options.element,
			text = document.createTextNode(context.get(prop));

	addObserver(this, prop, function() {
		var newNode = document.createTextNode(context.get(prop));
		parent.insertBefore(newNode, text);
		text = newNode;
	});

	parent.appendChild(text);
});

In this example, RESOLVE is overridden to insert a text node for the current property path, and to add an observer so that the text node can be replaced when the property path changes.

This is an intentionally simplistic example:

• It doesn't cover cleaning up the observer, which would probably be managed by the view layer of a system like Backbone or Ember
• It doesn't cover unescaped content. If options.escaped is false, this code would want to create and update a DocumentFragment instead of a TextNode.

That said, it offers the broad strokes of how to build data binding with HTMLBars.

Helpers in content also receive options.element, so helpers can also generate nodes and hold references to them for use later via data binding approaches.

Attributes work similarly:

HTMLBars.registerHelper('RESOLVE_IN_ATTR', function(parts, options) {
	var context = this,
			path = parts.join(".");

	addObserver(this, parts.join("."), function() {
		options.rerender();
	});

	return this.get(parts.join("."));
});

The main difference is that attributes cannot have separate text nodes, so a change to any attribute lookup will need to re-render the entire attribute value. The options.rerender() method will re-execute the contents of the attribute and update its value.

Helpers executed inside an attribute will also receive options.rerender:

<a href="http://{{normalize-href url}}">{{title}}</a>

In this example, the normalize-href helper will get an options.rerender that it can use to re-evaluate the contents of href when the url changes.