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    0.0.13 • 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 a rules_nodejs project, if you already have one, great! If not, the easiest way to make one is:

    npx @bazel/create ${NAME}
    cd ${NAME}
    npm install
    npm run build # Confirm project is buildable.

    Then install rules_prerender as a dev dependency. You must also satisfy a peer dep on @bazel/typescript, which itself has a peer dep on typescript.

    npm install rules_prerender @bazel/typescript typescript --save-dev

    You will also need a tsconfig.json. Easiest way to generate one is with:

    npx typescript --init

    See rules_typescript suggestions to set up absolute imports.

    Last step is to update to your WORKSPACE file. Add:

    # Load other `rules_prerender` dependencies.
    load("@npm//rules_prerender:package.bzl", "rules_prerender_dependencies")

    And make sure your npm_install() rule has strict_visibility = False.

    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 (note that where they depend on @npm//rules_prerender/packages/rules_prerender, you should depend on @npm//rules_prerender).


    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("@npm//@bazel/concatjs:index.bzl", "ts_library")
    load("@npm//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.
        srcs = ["my_component_prerender.ts"],
        # Other `ts_library()` rules used by `my_component_prerender.ts`.
        lib_deps = ["@npm//rules_prerender"],
        # Other `prerender_component()` rules used by `my_component_prerender.ts`.
        deps = ["//my_other_component"],
        # Client-side JavaScript to be executed in the browser.
        scripts = [":scripts"],
        # Styles for the component.
        styles = [":styles"],
        # Other resources required by the component.
        resources = [":resources"],
    # Client-side scripts to be executed in the browser.
        name = "scripts",
        srcs = ["my_component.ts"],
        deps = ["//my_other_component:scripts"],
        name = "styles",
        srcs = ["my_component.css"],
        deps = ["//some_common_package:styles"],
    # 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.ts
    import { includeScript, inlineStyle } from 'rules_prerender';
    import { renderOtherComponent } from '__main__/my_other_component/my_other_component_prerender';
     * Render partial HTML. In this case we're just using a string literal, but you
     * could reasonably use lit-html, React, or any other templating library.
    export function renderMyComponent(name: string): string {
        return `
        <!-- 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 shadowroot="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 via light DOM. -->
            <!-- Inject the associated client-side JavaScript. -->
            <!-- Inline the associated CSS styles, scoped to this shadow root. -->
        <!-- Other components are placed in light DOM and visible at the \`<slot />\`. -->
            id: 'other',
            name: name.reverse(),
    // my_component/my_component.ts
    import { show } from '__main__/my_other_component/my_other_component';
    // 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.addEventListener('DOMContentLoaded', () => {
        // When the "Show" button is clicked.
        document.getElementById('show').addEventListener('click', () => {
            // Show the composed `other` component.
    /* my_component/my_component.css */
    /* @import dependencies resolved and bundled at build time. */
    @import '__main__/some_common_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.ts
    import { PrerenderResource } from 'rules_prerender';
    import { renderMyComponent } from '__main__/my_component/my_component_prerender';
    // 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.of('/my_page/index.html', `
    <!DOCTYPE html>
            <title>My Page</title>
    # my_page/BUILD.bazel
    # 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",
        # Script to invoke the default export of to generate the page.
        src = "my_page_prerender.ts",
        # Components used during prerendering.
        deps = ["//my_component"],
    # Simple 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 page is built into a web_resources() rule which is a directory that 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() rule 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
    # 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!

    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.ts
    import * as fs from 'fs';
    import { PrerenderResource } from 'rules_prerender';
    import * as md from 'markdown-it';
    export default async function* render():
            AsyncGenerator<PrerenderResource, void, void> {
        // List all files in the `posts/` directory.
        const posts = await fs.readdir(
            { 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/${post_file_name_with_html_extension}`.
            const postMarkdown = await fs.readFile(post, { encoding: 'utf8' });
            const postHtml = md.render(postMarkdown);
            const htmlName = post.split('.').slice(0, -1).join('.') + '.html';
            yield PrerenderResource.of(`/posts/${htmlName}`, postHtml);

    We can easily execute this at build time like so:

    # my_blog/BUILD.bazel
    # 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.
        src = "posts_prerender.ts",
        # Other files needed to generate all the HTML.
        data = glob(["posts/*.md"]),
        # Plain TypeScript dependencies used by `posts_prerender.ts`.
        lib_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.

    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 @nodejs_host//:npm -- ci
    bazel test //...

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

    NOTE: If you encounter "Missing inputs" errors from fsevents or other optional dependencies, make sure you are using npm ci instead of npm install. See:

    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.


    Most tests are run in Jasmine using jasmine_node_test(). 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.

    Source maps should be set up and usable, however rules_nodejs currently compiles everything to ES5, so async/await gets transpiled to generators, meaning stepping over an await can be quite fiddly sometimes. When using chrome://inspect, consider using the debugger; keyword at a particular file in order to stop execution programmatically and then set interactive breakpoints via the DevTools debugger itself. Otherwise most files are not loaded at the time --inspect-brk stops 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 there is also some additional configuration required. WSL 2 does not currently support graphical applications out of the box and Windows does not ship with an X server implementation (this may be unnecessary with WSLg, but that hasn't been tested). To debug end-to-end tests in WSL 2, you need to:

    1. Install an X server for Windows (such as VcXsrv)
    2. Launch the X server and set it up enable public access (for VcXsrv, this is the "Disable access control" box).
    3. When Windows Defender pops up about network permissions, allow access for private and public networks.
    4. In the WSL Ubuntu terminal, run:
      export DISPLAY=$(awk '/nameserver / {print $2; exit}' /etc/resolv.conf 2>/dev/null):0
      Consider adding it to your ~/.bashrc so you don't have to remember to do this.

    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 @npm//rules_prerender/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_lib in a BUILD.bazel file with a filename will generate a ts_library() 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.


    The building and publishing process of this repository is a bit unique. Because this project provides a bunch of build tools, things can get somewhat confusing. A couple definitions for the purpose of publishing:

    • Build time - Execution of bazel build by a contributor to rules_prerender directly in this repository.
    • Run time - Execution of bazel build by a user of rules_prerender depending on it via rules_nodejs in the @npm workspace, or via a direct install in their WORKSPACE file.

    Since the //examples/... directory has direct references to the same build tools as are exported to users, many of those tools must support execution at both build time and run time. Some tools only need to support build time execution (such as test targets).

    The run time workspace is generated via a pkg_npm() target at //:pkg. This includes pre-built binaries, config files, *.bzl files, and even BUILD files. This is effectively a mini Bazel workspace that is generated by another Bazel workspace.

    Most of the implementation comes down to simply copying the right files to the right place, however there are a couple things to keep in mind:

    Many packages have a BUILD.publish file in additional to a BUILD.bazel file. BUILD.bazel handles the build time bazel build command, responsible for building code in //examples/... and the run time NPM package. The BUILD.publish file handles the run time bazel build command, loaded at @npm//rules_prerender/.... These two workspaces share a lot of the same code, but BUILD.bazel generates its tools at HEAD, while BUILD.publish leverages pre-built tools for most of the same work.

    publish_files() aggregates tools and their BUILD.publish files for use in the run time package. If new files or tools are needed at run time, then a publish_files() macro is needed to copy them into the NPM package, and a BUILD.publish file may be needed to configure their runtime usage and provide consistency with build time usage.

    Testing publishable builds

    Currently there are no automated tests of the published package or run time builds (aside from a simple build test of the NPM package directory). To test this manually, you need to:

    1. Build the NPM package in rules_prerender.

      • This generates dist/bin/pkg/ which contains the contents of the NPM package.
      bazel build //:pkg.pack
    2. Set up a separate Bazel workspace and cd into it.

      • Easiest way to do this from scratch is:

        npx @bazel/create ${NAME}
        cd ${NAME}
        npm install
    3. Install the local rules_prerender build.

      npm install --save-dev path/to/rules_prerender/workspace/dist/bin/pkg
    4. Use @npm//rules_prerender/... and build some code.

    Check out the ref/external branch which includes an in-tree user workspace which can be used to more easily verify and debug run time execution.


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

    1. Consider testing the release.
      • No need for bazel test //..., the release process will do it for you.
    2. 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.
    3. 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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