Template Parts

This library is designed as a "ponyfill" library that implements the design in the [TemplateInstance proposed whatwg spec][spec] that has been proposed in order to address whatwg/html#2254.

[spec]: https://github.com/w3c/webcomponents/blob/159b1600bab02fe9cd794825440a98537d53b389/proposals/Template-Instantiation.md

This implements the minimally viable parts of the proposal, to provide something that works, but should be easy to drop if the Template Parts Proposal lands.

To reiterate the example in the above proposal, given a template such as:

``

`

We'd like {{x}} and {{y}} to be template parts, exposed as JavaScript objects which can be manipulated.

With this library, and that given template, one could implement the following:

`js
import {TemplateInstance} from '@github/template-parts'

const tpl = new TemplateInstance(document.getElementById('some-template'), { x: 'Hello', y: 'bar'})

document.appendChild(tpl)
`

A TemplateInstance instance is a subclass of DocumentFragment - containing the cloned contents of the template. It also has an update(params: unknown): void method - which when called will run the given "processor", with the new params.

This library has a default "processor": propertyIdentity which implements basic functionality of applying the params object values to the Template Parts (it is effectively part.value = params[part.expression])

In addition, there is a propertyIdentityOrBooleanAttribute export which adds the capability of toggling boolean style attributes like hidden or input.required.

To use the propertyIdentityOrBooleanAttribute, import it and pass it as a third argument to the TemplateInstance constructor:


`js
import {TemplateInstance, propertyIdentityOrBooleanAttribute} from '@github/template-parts'

// This will simply replace {{x}} with "Hello" and {{y}} with "bar"
const tpl = new TemplateInstance(document.getElementById('foo'), { x: 'Hello', y: 'bar'})

// The propertyIdentityOrBooleanAttribute processor will check for false/true values which map to Template Part values that are assigned to attributes, and add/remove the attribute.
const tpl = new TemplateInstance(document.getElementById('foo'), { x: 'Hello', hidden: false}, propertyIdentityOrBooleanAttribute)
``