Generating HTML Snippets

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When using optimized builds with steal-tools, separate bundles are created which are able to be loaded in parallel via async script tags like so:

<script src="/dist/bundles/app/app.js" async></script>
<script src="/dist/bundles/app/page-one.js" async></script>

Given that steal-tools intelligently splits bundles to optimize for page load times, you don't necessary know which bundles are going to be created. You might have a new bundle be created, for example, when you add a new dependency to your project.

This is where the bundle manifest file comes into play. It creates a file that can be used to know, for a given entry module, what bundles (JavaScript and CSS) need to be loaded.

In this guide we'll take an existing build script and update it to generate snippets of HTML, that you can use from your server-side templating engine.

Creating the bundle manifest

If you've checked out the progressive loading guide then you have probably seen (or built) the myhub app. We're going to use this app as the example, but you can use your existing StealJS app as well.

If you'd like to follow along, start by cloning the myhub app from GitHub:

git clone

Then change the build.js file (if using your own app you likely have a similar file, and if not you can create one). It currently looks like:

var stealTools = require("steal-tools");{}, {
  bundleSteal: true

Change it to look like this instead:

var stealTools = require("steal-tools");

stealTools.optimize({}, {
  bundleManifest: true

Note here:

  1. This uses the stealTools.optimize function rather than This build is needed in order to use the async script tags described before.
  2. The bundleManifest: true option is used to generate the bundle manifest file that is used by steal-bundle-manifest.

Generating snippets

The goal here is to create snippets of HTML that we can save to a folder and then use in our backend application server. To do this we will use steal-bundle-manifest.

We'll create these snippets in a subfolder of the generate dist/ folder, dist/snippets. First let's install a couple of utilities we need:

npm install steal-bundle-manifest make-dir --save

In addition to steal-bundle-manifest, we are also installing make-dir, a library to let us recursively create directories.

Create a new file generate-snippets.js and paste this code:

var BundleManifest = require("steal-bundle-manifest");
var bundles = require("./package.json").steal.bundle;
var fs = require("fs");

module.exports = function(){
  var manifest = new BundleManifest();

  bundles.forEach(bundleName => {
    var bundle = manifest.for(bundleName);
    var prefix = bundleName.split('/').pop();
    var fnprefix = `dist/snippets/${prefix}`;

    var styles = bundle.assets.filter(a => a.type === 'style');
    var stylesHTML = bundle.toHTML(styles);
    fs.writeFile(`${fnprefix}-styles.html`, stylesHTML, () => {});

    var scripts = bundle.assets.filter(a => a.type === 'script');
    var scriptsHTML = bundle.toHTML(scripts);
    fs.writeFile(`${fnprefix}-scripts.html`, scriptsHTML, () => {});

And update your build.js script to use it:

var stealTools = require("steal-tools");
var makeDir = require("make-dir");
var generateSnippets = require("./generate-snippets");

stealTools.optimize({}, {
  bundleManifest: true
.then(() => makeDir("dist/snippets"))

What this does is:

  1. Reads the steal.bundle property from your package.json to determine which bundles should have HTML snippets created for.
  2. Creates a build that includes the bundle manifest.
  3. Uses steal-bundle-manifest to create HTML snippets for styles and scripts, and saves them as dist/snippets/page-scripts.html and dist/snippets/page-styles.html.

You can run this script with:

node build.js

Which should give you a dist/snippets folder that looks like:


Add snippets to your route views

This part differs depending on which application framework you are using. Whether you are using Ruby on Rails or Java or something else, it's best to refer to your framework's documentation on how to use HTML snippets in your templates.

This example shows use in PHP, using the include statement.

<!doctype html>
<html lang="en">
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">

    <?php include('dist/snippets/puppies-styles.html'); ?>
    <div class="container">Hello World.</div>

    <?php include('dist/snippets/puppies-scripts.html'); ?>

And that's it! Now every time you run your build you will also generate these snippets of HTML, so that every JavaScript bundle can load in parallel.

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