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0.0.34 • Public • Published


A Bazel rule set for prerendering HTML pages.


NOTE: This project is currently experimental. Feel free to install it to try it out, give feedback, and suggest improvements! Just don't use it in production quite yet.


Start with an @aspect_rules_js project.

Then add a workspace dependency on @rules_prerender. See the releases page for the latest release and copy the snippet into your WORKSPACE file. Then install the rules_prerender NPM package. We'll also want @rules_prerender/preact and preact itself to use JSX as a templating system.

pnpm install rules_prerender @rules_prerender/preact preact --save-dev


Optionally, you likely want to configure TypeScript by installing it and creating a tsconfig.json file.

pnpm install typescript --save-dev
node_modules/.bin/tsc --init

Declarative Shadow DOM

Also optional, but it is recommended to include the declarative shadow DOM package as it is a key part of @rules_prerender components.

pnpm install @rules_prerender/declarative-shadow-dom --save-dev

Then update your root BUILD.bazel file to include:

load("@rules_prerender//:index.bzl", "link_prerender_component")

    name = "prerender_components/@rules_prerender/declarative_shadow_dom",
    package = ":node_modules/@rules_prerender/declarative_shadow_dom",
    visibility = ["//visibility:public"],

With that all done, you should be ready to use rules_prerender! See the next section for how to use the API, or you can check out some examples which shows most of the relevant features in action.


The exact API is not currently nailed down, but it is expected to look something like the following.

There are two significant portions of the rule set. The first defines a "component": an HTML template and the associated JavaScript, CSS, and other web resources (images, fonts, JSON) required for to it to function.

# my_component/BUILD.bazel

load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
load("@rules_prerender//:index.bzl", "prerender_component", "web_resources")

# A "library" target encapsulating the entire component.
    name = "my_component",
    # The library which will prerender the HTML at build time in a Node process.
    prerender = ":prerender",
    # Client-side JavaScript to be executed in the browser.
    scripts = ":scripts",
    # Styles for the component.
    styles = ":styles",
    # Other resources required by the component (images, fonts, static JSON, etc.).
    resources = ":resources",

# Compile the prerendering logic (can also be a `js_library`).
    name = "prerender",
    srcs = ["my_component_prerender.tsx"],
    deps = [
        # See "Component composition" to learn more about how to depend on
        # another `prerender_component`.

        # Regular dependencies.

# Client-side scripts to be executed in the browser (can also be a `js_library`).
    name = "scripts",
    srcs = ["my_component.mts"],
    declaration = True,
    deps = ["//some/other/package:ts_proj"],

# Any styles needed by this component to render correctly.
    name = "styles",
    srcs = ["my_component.css"],
    deps = ["//some/other/package:css_lib"],

# Other resources required for this component to function at the URL paths they
# are expected to be hosted at.
    name = "resources",
    entries = {
        "/images/foo.png": ":foo.png",
        "/fonts/roboto.woff": "//fonts:roboto",
// my_component/my_component_prerender.tsx

import { Templates } from '@rules_prerender/declarative_shadow_dom/preact.mjs';
import { includeScript, inlineStyle } from '@rules_prerender/preact';
import { VNode } from 'preact';
import { OtherComponent } from '../my_other_component/my_other_component_prerender.js';

/** Render partial HTML with Preact. */
export function MyComponent({ name }: { name: string }): VNode {
    return <div>
        {/* Use declarative shadow DOM to isolate styles. If you're not familiar
            with declarative shadow DOM, you don't have to use it. But if you
            don't you'll need to manually namespace your styles or else styles
            in different components could conflict with each other! */}
        <Template shadowrootmode="open">
            {/* Render some HTML. */}
            <h2 class="my-component-header">Hello, {name}</h2>!
            <button id="show">Show</button>

            {/* Use related web resources. */}
            <img src="/images/foo.png" />

            {/* Compose other components from the light DOM. */}

            {/* Inject the associated client-side JavaScript. */}
            {includeScript('./my_component.mjs', import.meta)}

            {/* Inline the associated styles, scoped to this shadow root. */}
            {inlineStyle('./my_component.css', import.meta)}

        {/* Light DOM content goes here, local styles are *not* applied to these
            elements. */}
        <OtherComponent id="other" name={name.reverse()} />
// my_component/my_component.mts

import { showDialog } from '../some/other/package/show_dialog.mjs';

// Register an event handler to show the other component. Could just as easily
// use a framework like Angular, LitElement, React, or just define an
// implementation for a custom element that was prerendered.
document.getElementById('show').addEventListener('click', () => {
    // Show the composed `other` component.
/* my_component/my_component.css */

/* @import dependencies resolved and bundled at build time. */
@import '../some/other/package/styles.css';

/* Styles for the component. */
@font-face {
    font-family: Roboto;
    src: url(/fonts/roboto.woff); /* Use related web resources. */

.my-component-header {
    color: red;
    font-family: Roboto;

The second part of the rule set leverages such components to prerender an entire web page.

// my_page/my_page_prerender.tsx

import { PrerenderResource, renderToHtml } from '@rules_prerender/preact';
import { MyComponent } from '../my_component/my_component_prerender.js';

// Renders HTML pages for the site at build-time.
// If you aren't familiar with generators and the `yield` looks scary, you could
// also write this as simply returning an `Array<PrerenderResource>`.
export default function* render(): Generator<PrerenderResource, void, void> {
    // Generate an HTML page at `/my_page/index.html` with this content:
    yield PrerenderResource.fromHtml('/my_page/index.html', renderToHtml(
                <title>My Page</title>
                <meta charSet="utf8" />
                <MyComponent name="World" />
# my_page/BUILD.bazel

load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
load("@rules_prerender//:index.bzl", "prerender_pages", "web_resources_devserver")

# Renders the page, bundles JavaScript and CSS, injects the relevant
# `<script />` and `<style />` tags, and combines with all transitive resources
# to create a directory with the following paths:
#     /my_page/index.html - Final prerendered HTML page with CSS styles inlined.
#     /my_page/index.js - All transitive client-side JS source files bundled
#         into a single file.
#     /images/foo.png - The image used in `my_component`.
#     /fonts/roboto.woff - The Robot font used in `my_component`.
#     ... - Possibly other resources from `my_other_component` and transitive
#         dependencies.
    name = "prerendered_page",
    # Import specifier for the JavaScript output from `:prerender` which
    # generates the page.
    entry_point = "./my_page_prerender.js",
    # Depend on the library containing `my_page_prerender.js`.
    prerender = ":prerender",

    name = "prerender",
    srcs = ["my_page_prerender.tsx"],
    deps = [
        # See "Component composition" to learn more about how to depend on
        # another `prerender_component`.

        # Other dependencies.

# Small dev server to test out this page. `bazel run` / `ibazel run` this target
# to check out the page at `/my_page/index.html`.
    name = "devserver",
    resources = ":prerendered_page",

The //my_page:prerendered_page target generates a directory which contains its HTML, JavaScript, CSS, and other resources from all the transitively included components at their expected paths.

Multiple prerender_pages() directories can then be composed together into a single web_resources() target which contains a final directory of everything merged together, representing an entire prerendered web site.

This final directory can be served with a simple devserver for local builds or uploaded directly to a CDN for production deployments.

# my_site/BUILD.bazel

load("@rules_prerender//:index.bzl", "web_resources", "web_resources_devserver")

# Combines all the prerendered resources into a single directory, composing a
# site from a bunch of `prerender_pages()` and `web_resources()` rules. Just
# upload this to a CDN for production builds!
    name = "my_site",
    deps = [

# A simple devserver implementation to serve the entire site.
    name = "devserver",
    resources = ":site",

With this model, a user could do ibazel run //my_site:devserver to prerender the entire application composed from various self-contained components in a fast and incremental fashion. They could also just run bazel build //my_site to generate the application as a directory and upload it to a CDN for production deployments. They could even make a separate bazel run //my_site:deploy target which performs the upload and run it from CI for easy deployments!

Component composition

The prerender_component target generates aliases to the targets passed in as inputs. Consider the following example:

load("@rules_prerender//:index.bzl", "prerender_component")

    name = "component",
    prerender = ":my_prerender_lib",
    scripts = ":my_scripts_lib",
    styles = ":my_styles_lib",
    resources = ":my_resources_lib",

This will generate the following aliases:

  • :component_prerender -> :my_prerender_lib
  • :component_scripts -> :my_scripts_lib
  • :component_styles -> :my_styles_lib
  • :component_resources -> :my_resources_lib

If you want to use any part of a component, you can use it directly rather than depending on :component. However, you must depend on that part through one of the above aliases.

For example, consider the following component:

// my_component/prerender.mts

import { VNode } from 'preact';

/** Render partial HTML with Preact. */
export function MyComponent({ name }: { name: string }): VNode {
    return <div>Hello, {name}!</div>

With the following BUILD.bazel file:

# my_component/BUILD.bazel

load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
load("@rules_prerender//:index.bzl", "prerender_component")

    name = "my_component",
    prerender = ":prerender_lib",
    # ...

    name = "prerender_lib",
    srcs = ["prerender.mts"],

To use this, you can import MyComponent directly like you would any other function.

// my_other_component/prerender.mts

import { MyComponent } from '../my_component/prerender.js';

// ...

However instead of depending on //my_component:prerender_lib, depend on //my_component:my_component_prerender.

# my_other_component/BUILD.bazel

load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
load("@rules_prerender//:index.bzl", "prerender_component")

# Does not reference `//my_component` at all.
    name = "my_other_component",
    prerender = ":prerender_lib",
    # ...

    name = "prerender_lib",
    srcs = ["prerender.mts"],
    # IMPORTANT: Depend on `:my_component_prerender` instead of `:prerender_lib`.
    deps = ["//my_component:my_component_prerender"],

While this looks like just an alias, it is actually load bearing and required.

The same requirement to use the aliases applies to client-side JavaScript (_scripts), CSS styles (_styles), and generated resources (_resources).

prerender_component rules

As indicated by component composition, the prerender_component macro is a bit unique compared to most Bazel macros/rules and has a few special rules for how it is used.

  1. Any direct dependency of a prerender_component target should only be used by that prerender_component.
  2. Any additional desired dependencies should go through the relevant _prerender, _scripts, _styles, _resources aliases generated by prerender_component.
    • Exception: Unit tests may directly depend on targets, provided they do not use any prerender_* rules as part of the test.
  3. Never depend on a prerender_component target directly. Always depend on the alias of the specific part of the component you actually want to use.
    • Exception: You may bazel build a prerender_component target directly or have a build_test depend on it in order to verify that the component is buildable.
  4. Any direct dependency of a prerender_component target must be defined in the same Bazel package and have private visibility.
    • This is enforced at build time.
    • Acts as a guardrail to make it less likely to run afoul of the above rules.

A general best practice is to give every prerender_component target its own directory and Bazel package. Leave everything private visibility except for the prerender_component target itself. This will set visibility for the alias targets as well. Doing so makes it impossible to accidentally forget to use the component aliases, since doing so would be a visibility error. This pattern helps you naturally follow the above rules without even thinking about it.

Generating multiple pages

We can generate multiple pages just as easily as the one. We just need to yield more files. Take this example where we render HTML files for a bunch of markdown posts in a blog.

// my_blog/posts_prerender.tsx

import * as fs from 'fs';
import * as path from 'path';
import { PrerenderResource, renderToHtml } from '@rules_prerender/preact';
import * as md from 'markdown-it';

export default async function* render():
        AsyncGenerator<PrerenderResource, void, void> {
    // List all files in the `posts/` directory.
    const postsDir = `${process.env['RUNFILES']}/wksp/my_blog/posts`;
    const posts = await fs.readdir(postsDir, { withFileTypes: true });

    for (const post of posts) {
        // Read the post markdown, convert it to HTML, and then emit the file to
        // `rules_prerender` which will write it at
        // `/post/${post_file_name_with_html_extension}`.
        const postMarkdown =
            await fs.readFile(path.join(postsDir, post), 'utf8');
        const postHtml = md.render(postMarkdown);
        const postBaseName = post.split('.').slice(0, -1).join('.');
        const htmlName = `${postBaseName}.html`;
        yield PrerenderResource.fromHtml(`/posts/${htmlName}`, renderToHtml(
                    <title>Post {postBaseName}</title>
                    <meta charSet="utf8" />
                    <article dangerouslySetInnerHTML={{ __html: postHtml }} />

We can easily execute this at build time like so:

# my_blog/BUILD.bazel

load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
load("@rules_prerender//:index.bzl", "prerender_pages", "web_resources_devserver")

# Renders a page for every `posts/*.md` file. Also performs all the bundling and
# merging of required JS, CSS, and other resources.
    name = "prerendered_posts",
    # Script to invoke the default export of to generate the page.
    entry_point = "./posts_prerender.js",
    # Library which generates the entry point JavaScript.
    prerender = ":prerender",

    name = "prerender",
    srcs = ["posts_prerender.tsx"],
    # Include all the markdown files at runtime in runfiles.
    data = glob(["posts/*.md"]),
    deps = [

# Simple server to test out this page. `bazel run` / `ibazel run` this target to
# check out the posts at `/posts/*.html`.
    name = "devserver",
    resources = ":prerendered_posts",

With this, all markdown posts in the posts/ directory will get generated into HTML files. Using this strategy, we can scale static-site generation for a large number of files with common generation patterns.


Since prerender_pages() and related rules invoke user code at build time, debugging can get a little complicated. Add the following to your .bazelrc:

# `@rules_prerender` specific options.
build --flag_alias=debug_prerender=@rules_prerender//tools/flags:debug_prerender

Then, you can use the --debug_prerender flag to specify a target which you want to open a breakpoint on. Use it with:

bazel run //path/to/my:devserver --debug_prerender=//path/to/my:prerender_pages_target

You should then see a log statement indicating that a target is being debugged and a hung execution on the Prerendering action.

DEBUG: /home/doug/Source/rules_prerender/packages/rules_prerender/prerender_resources.bzl:165:14: Debugging @//examples/minimal:page_page_annotated from //examples/minimal:page
INFO: Analyzed target //examples/minimal:devserver (1 packages loaded, 5038 targets configured).
INFO: Found 1 target...
[290 / 294] Prerendering (@//examples/minimal:page_page_annotated); 8s linux-sandbox

This target is hanging because it has --inspect-brk set on the Node invocation and is waiting for the debugger. Connect your preferred Node debugger and you should be able to step through rendering.

Note that the Bazel cache can get a little tricky here, as repeated runs may skip the target altogether. If so, make any arbitrary whitespace change to a the rendering code to invalidate the cache and rerun.

The target you actually bazel run or bazel build (//path/to/my:devserver above) doesn't actually matter, as long as it includes //path/to/my:prerender_pages_target as a transitive dependency.

Custom Bundling

The previous example automatically bundled all the JavaScript and CSS for a given page. This is very simple and easy to use, but also somewhat limited. The prerender_pages_unbundled() rule provides unbundled JavaScript and CSS resources so a user can manually bundle them with whatever means they like.

There is also an extract_single_resource() rule, which pulls out a resource from a directory generated by a prerender_*() rule (assuming the directory contains only one resource). This can be useful to post-process a prerendered resource with tools that expect a single file as input, rather than a directory.


To get started, simply download / fork the repository and run:

bazel run @pnpm -- install --dir $PWD --frozen-lockfile
bazel test //...

Prefer using bazel run @pnpm -- ... and bazel run @nodejs_host//:node -- ... over using pnpm and node directly so they are strongly versioned with the repository. Alternatively, you can install nvm and run nvm use to switch the node version to the correct one for this repository.

There are bazel and ibazel scripts in package.json so you can run any Bazel command with:

npm run -s -- bazel # ...

Or, if you want to live-reload on changes:

npm run -s -- ibazel # ...

Alternatively, you can run npm install -g @bazel/bazelisk @bazel/ibazel to get a global install of bazel and ibazel on your $PATH and just use them directly instead of proxying through the NPM wrapper scripts. This repository has a .bazelversion file used by bazelisk to manage and download the correct Bazel version for you and pass through all commands to it (not totally sure if it applies to ibazel though).

You can also use npm run build and npm test to build and test everything. npm test also tests external workspaces used as test cases which would normally be skipped by bazel test //....


Most tests are run in Jasmine using jasmine_node_test(), a slightly customized implementation of @aspect_rules_jasmine. These tests run in a Node Jasmine environment with no available browser (unless they depend on WebDriverIO). The test can be executed with a simple bazel test //path/to/pkg:target.

Debugging Tests

To debug these tests, simply add --config debug, which will opt in to additional flags specifically for testing. Most notably, this includes --inspect-brk so Node will not begin executing until a debugger has connected. You can use chrome://inspect or the "Attach" run configuration in VSCode to attach a debugger and start test execution.

Debugging WebDriver tests

End-to-end tests using a real browser are done with WebDriver using jasmine_web_test_suite().

When executing WebDriver tests and using --config debug, the browser will open non-headless, giving you the opportunity to visually inspect the page under test and debug it directly. This is done via an X server, so make sure the $DISPLAY variable is set. For example, if debugging over SSH, you'll need to enable X11 forwarding.

When using WSL 2 make sure you have also installed WSLg. That will give you the X server implementation necessary to debug.

Then running a bazel test //path/to/pkg:target --config debug for a WebDriver test should open Chrome visually and give you an opportunity to debug and inspect the page.


Most model types are stored under //common/models/... and generally consist of interfaces rather than classes. This provides immutable, pure-data structured types which work well with functional design patterns. They are also easy to assert in Jasmine with expect().toEqual().

These models typically include a _mock.ts file which exposes mock*() functions. These provide simple helpers to generate a mock for a model using default values with override values as inputs. Using these mocks, a test can explicitly specify only the properties of an object that it actually cares about and trust that the mock function will provide reasonable and semantically accurate defaults for all other values. For example:

// Some model interface.
interface MyModel {
    name: string;
    path: string;

// Some real function.
function getName(model: MyModel): string {

// A mock for the model.
function mockModel(overrides: Partial<MyModel> = {}): MyModel {
    return {
        name: 'MockName',
        // Default is semantically accurate, even if it is an arbitrary value.
        path: 'some/mocked/path.txt',
        // Allow caller to specify any given value.

// Test of a real function.
it('`getName()` returns the name', () => {
    const model = mockModel({
        name: 'Ollie',
        // path uses the default value.


    // There are several benefits with this approach:
    // 1.  `path` isn't used, so no need to specify it for the test, making the
    //     test and its intent much clearer.
    // 2.  If `path` is accidentally used for an important operation as part of
    //     `getName()`, the test would almost certainly fail and the `path`
    //     value can be explicitly specified as part of the test.
    // 3.  Even if `path` is used as part of unimportant operations in
    //     `getName()` (such as simply validating the type), it will not break
    //     the test because the default value is semantically accurate.
    // 4.  This isolates the test from unrelated changes to `MyModel`.
    //     Introducing another property is not likely to break `getName()` and
    //     would not require changes to the test to support.

This is a semi-experimental mocking strategy, so whether or not it is actually a good idea is still to be determined.

VSCode Snippets

The repository includes a few custom snippets available if you use VSCode. Type the given name and hit Tab to insert the snippet. Then type out the desired value for various parameters using Tab and Shift+Tab to navigate between them. The snippet will take care of making sure certain values match as expected.

  • Typing ts_proj in a BUILD.bazel file with a filename will generate a ts_project() rule for that file, a rule for its test file, and a jasmine_node_test() rule. Useful when creating a new file to auto-generate its default BUILD rules.
  • Typing jas in a TypeScript file will generate a base Jasmine setup with imports and an initial test with a TODO.
  • Typing desc in a TypeScript file will generate a Jasmine test suite, moving the cursor exactly where you want it to go.
  • Typing it in a TypeScript file will generate a Jasmine test, moving the cursor exactly where you want it to go. It will generate an async test by default, which you can either skip over with Tab to accept, or delete with Backspace (and then move on with Tab) to make synchronous.


To actually publish a release to NPM, follow these steps:

  1. Go to the Publish workflow and click Run workflow.
    • Make sure to fill out all the requested information.
    • This will install the package, execute all tests, and then publish as the given semver to NPM.
    • It will also tag the commit with releases/${semver} and push it back to the repository.
    • Finally, it will create a draft GitHub release for that tag with a link to NPM for this particular version.
  2. Once the workflow is complete, go to releases to update the draft and add a changelog or other relevant information before publishing.


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