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h-include.js

Declarative client-side transclusion, using Custom Elements V1. Perfect for Microfrontend architectures, in combination with server-side transclusion technologies like Edge-Side Includes.

Based on hinclude.js by @mnot.

Breaking changes in version 4.0.0:

Rename alt attribute to when-false-src to support future feature for error handling.

Breaking changes in version 3.0.0:

  • Because h-include is now using Custom Elements V1, we recommend that you update your polyfill (i.e. document-register-element) to the latest version.
  • If you have created your own custom elements that inherit from h-include, they too need to be based on Custom Elements V1. See EXTENDING.md for an example how to extend h-include.
  • The navigate attribute is broken out into the separate element <h-include-navigate>, located in lib/h-include-extensions.js.
  • Changes to @src attribute don't automatically refresh an h-include element anymore

Usage

Include an HTML resource like this:

<h-include src="/url/to/fragment.html"></h-include>

Each <h-include> element will create an AJAX request to the URL and replace the innerHTML of the element with the response of the request.

See the demo page for live examples.

Installation

Install using npm:

$ npm install h-include

Install using bower:

$ bower install h-include

Rendering Mode

By default, each include is fetched in the background and the page is updated only when they all are available.

This is bounded by a timeout, by default 2500 ms. After the timeout, h-include will show what it has and keep on listening for the remaining responses.

However, it's also possible to have responses from <h-include> elements become visible as they become available, by providing configuration:

HIncludeConfig = { mode: 'async' };

While this shows the included content quicker, it may be less visually smooth.

Custom Elements polyfill

You need to use a polyfill for enabling W3C Custom Elements for browsers not supporting Custom Elements V1.

We recommend using document-register-element (5KB minified and gzipped) as the polyfill for W3C Custom Elements.

Example:

<head>
  <script>this.customElements||document.write('<script src="//unpkg.com/document-register-element"><\x2fscript>');</script>
  <script type="text/javascript" src="/path/to/h-include.js"></script>
</head>
<body>
  ...
  <h-include src=">
  ...
</body>

Extensions

Additional extensions are located in lib/h-include-extensions.js and have lib/h-include.js as a dependency:

<script type="text/javascript" src="/lib/h-include.js"></script>
<script type="text/javascript" src="/lib/h-include-extensions.js"></script>

All extensions inherit h-include's base behavior, when applicable.

To create your own elements that inherit from <h-include>, see EXTENDING.md.

Example usage, in summary:

Resource fragments Content fragments Example
ESI ESI Short static pages
ESI h-include Dynamic web app with static content fragments (i.e. search)
ESI ESI + h‑include‑lazy Pages with homogeneous lists, lazy loaded on scroll below the fold
ESI + h‑import‑lazy ESI + h‑include‑lazy Pages with heterogeneous content, lazy loaded on scroll below the fold together with resource fragments
h‑import h‑include (etc) Sites without access to ESI

h-include-lazy

Only includes the HTML resource if the element is about to enter the viewport, by default 400 pixels margin, using the Intersection Observer API (which needs to be polyfilled).

After page load, elements in the DOM need to registered to the Intersection Observer:

<script src="https://polyfill.io/v2/polyfill.min.js?features=IntersectionObserver"></script>
<script type="text/javascript" src="/lib/h-include.js"></script>
<script type="text/javascript" src="/lib/h-include-extensions.js"></script>
<script>
window.addEventListener('load', function() {
  HInclude.initLazyLoad();
});
</script>

Example:

<h-include-lazy src="fragment.html"></h-include-lazy>

...


<h-include-lazy src="lazy-loaded-fragment.html"></h-include-lazy>

Example repo using plain h-include (without lazy) and the Intersection Observer API to pull in content on ‘in-view’ scroll interaction can be found here.

h-import

Request an HTML resource and include all found script and stylesheet references.

Example:

<h-import src="resource-fragment.html"></h-import>

If possible, use Edge-Side Includes (or similar) to import statically loaded resource fragments, due to performance reasons.

To load resources, h-import and h-import-lazy call HInclude.loadResources with an array of urls to resources. By default, this method delegates the resource loadin to loadjs, which needs to be on the window object. However, HInclude.loadResources can be replaced with a loader of your choice.

h-import-lazy

When lazy loading fragments, it might be the case that additional style and script resources need to be loaded as well. For example, think of a lazy loaded footer with rather specific styling. For these scenarios, use h-import-lazy.

After page load, elements in the DOM need to registered to the Intersection Observer:

<script src="https://polyfill.io/v2/polyfill.min.js?features=IntersectionObserver"></script>
<script type="text/javascript" src="/lib/h-include.js"></script>
<script type="text/javascript" src="/lib/h-include-extensions.js"></script>
<script>
window.addEventListener('load', function() {
  HInclude.initLazyLoad();
});
</script>

Example:

<h-include-lazy src="fragment.html"></h-include-lazy>

...


<h-import-lazy src="lazy-loaded-resource-fragment.html"></h-import-lazy>

h-include-navigate

Use <h-include-navigate> to let link navigation events be captured by the element itself, which changes the src attribute and triggers a refresh.

Use target="_top" to let link inside h-include behave as a normal link.

Helper function: HInclude.initLazyLoad

By default, the selector for HInclude.initLazyLoad is 'h-include-lazy, h-import-lazy' and the Intersection Observer rootMargin and threshold default values are 400px 0px and 0.01 respectively. These can be overridden:

HInclude.initLazyLoad('css style selector', {rootMargin: '200px 0', threshold: 0.2});

Helper function: HInclude.loadResources

Load an array of script and stylesheet resources (to be overridden).

Advanced usage

Conditional inclusion using when

When offers a way of using a predicate for inclusion. It also features an optional when-false-src attribute that functions as the source of inclusion given that the predicate fails.

<h-include src="logged-in.html" when="org.project.predicateFunction" when-false-src="log-in.html"></h-include>

The method specified in the when attribute may be namespaced and needs to return true for the inclusion of the url specified in the src attribute to occur.

Request errors and alternative inclusion using alt

The alt attribute functions as an alternative source of inclusion should the url result in a request error.

<h-include src="unavailable.html" alt="alternative.html"></h-include>

Refresh method

Refresh an element by using the refresh() method:

const element = document.getElementsByTagName('h-include')[0];
element.refresh();

Media queries

Use media queries to have different fragments for different devices:

<h-include media="screen and (max-width: 600px)" src="small.html"></h-include>
<h-include media="screen and (min-width: 601px)" src="large.html"></h-include>

<h-include> will not listen to changes to screen orientation or size.

Fragment extraction

Include an HTML resource and extract a fragment of the response by using a selector:

<h-include src="..." fragment=".container"></h-include>

XMLHttpRequest.withCredentials

Enable XMLHttpRequest.withCredentials:

<h-include src="..." with-credentials></h-include>

Configuration

Set buffered include timeout (default is 2500 ms):

HIncludeConfig = { timeout: 10000 };

Set include mode to async (default is buffered):

HIncludeConfig = { mode: 'async' };

Throw if caught in an infinite include loop (default is false):

HIncludeConfig = { checkRecursion: true };

If checkRecursion is true, h-include will traverse the DOM upwards to find another h-include element with the same src attribute, until the root node. This operation takes a few CPU cycles per h-include, which is why it's not enable by default.

Error Handling

If fetching the included URL results in a 404 Not Found status code, the class of the include element will be changed to include_404. Likewise, a 500 Server Error status code will result in the include element’s class being changed to include_500.

Browser support

All modern browsers and IE down to IE10 are supported. If you find something quirky, please file an issue.

HTTP/2 improves XHR performance

Browsers with HTTP/2 are using HTTP/2 for xhr requests as well. So, if both the server and the current browser supports HTTP/2, all requests made with h-include will go through the same TCP connection, given that they have the same origin.

Bundler

Parcel

With this plugin, Parcel searches and optimizes all h-include tags https://github.com/joserochadocarmo/parcel-plugin-h-include

FAQ

Please see the FAQ for some frequently asked questions.