Deletes empty output client-side JavaScript bundles.

This updates the Rollup config to avoid generating empty bundles. This used to be a warning, but now the bundles are not emitted at all. The build process will properly skip injecting a pointless `<script>` tag, so everything works as expected.
dgp1130 · Jul 24, 2023 · 62f2eab · 62f2eab
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commit 62f2eab
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Showing 5 changed files with 107 additions and 4 deletions.
diff --git a/examples/empty_script/BUILD.bazel b/examples/empty_script/BUILD.bazel
@@ -0,0 +1,47 @@
+load("//:index.bzl", "prerender_pages", "web_resources_devserver")
+load("//tools/jasmine:defs.bzl", "jasmine_web_test_suite")
+load("//tools/typescript:defs.bzl", "ts_project")
+
+prerender_pages(
+    name = "site",
+    entry_point = "./site.js",
+    prerender = ":prerender",
+    scripts = ":scripts",
+)
+
+ts_project(
+    name = "prerender",
+    srcs = ["site.tsx"],
+    deps = [
+        "//:node_modules/@rules_prerender/preact",
+        "//:node_modules/preact",
+    ],
+)
+
+ts_project(
+    name = "scripts",
+    srcs = ["empty_script.mts"],
+)
+
+web_resources_devserver(
+    name = "devserver",
+    resources = ":site",
+)
+
+ts_project(
+    name = "test_lib",
+    srcs = ["test.mts"],
+    testonly = True,
+    deps = [
+        "//common/testing:devserver",
+        "//common/testing:webdriver",
+        "//:node_modules/@types/jasmine",
+    ],
+)
+
+jasmine_web_test_suite(
+    name = "test",
+    browsers = ["//tools/browsers:chromium-local"],
+    data = [":devserver"],
+    deps = [":test_lib"],
+)
diff --git a/examples/empty_script/empty_script.mts b/examples/empty_script/empty_script.mts
@@ -0,0 +1 @@
+// Empty script, should not be injected into the page.
diff --git a/examples/empty_script/site.tsx b/examples/empty_script/site.tsx
@@ -0,0 +1,17 @@
+import { PrerenderResource, renderToHtml, includeScript } from '@rules_prerender/preact';
+
+export default function*(): Generator<PrerenderResource, void, void> {
+    yield PrerenderResource.fromHtml('/index.html', renderToHtml(
+        <html>
+            <head>
+                <meta charSet="utf8" />
+                <title>Empty script</title>
+            </head>
+            <body>
+                <h2>Empty script</h2>
+
+                {includeScript('./empty_script.mjs', import.meta)}
+            </body>
+        </html>
+    ));
+}
diff --git a/examples/empty_script/test.mts b/examples/empty_script/test.mts
@@ -0,0 +1,20 @@
+import { useDevserver } from '../../common/testing/devserver.mjs';
+import { useWebDriver } from '../../common/testing/webdriver.mjs';
+
+describe('empty_scripts', () => {
+    const devserver = useDevserver('examples/empty_script/devserver.sh');
+    const wd = useWebDriver(devserver);
+
+    describe('index page', () => {
+        it('renders without a script', async () => {
+            const browser = wd.get();
+            await browser.url('/');
+
+            expect(await browser.$('h2').getText()).toBe('Empty script');
+
+            // Expect only the live reload script.
+            const scripts = await browser.$$('script');
+            expect(scripts.length).toBe(1);
+        });
+    });
+});
diff --git a/packages/rules_prerender/rollup.config.mts b/packages/rules_prerender/rollup.config.mts
@@ -1,18 +1,36 @@
 import { nodeResolve } from '@rollup/plugin-node-resolve';
-import { RollupOptions } from 'rollup';
+import { Plugin, RollupOptions } from 'rollup';
+
+/**
+ * Deletes any empty output bundles before they are written to disk. This
+ * prevents the build process from injecting an empty script onto the page.
+ */
+const deleteEmptyBundles: Plugin = {
+    name: 'delete-empty-bundles',
+
+    // Called right before writing output bundles to disk.
+    generateBundle(_options, bundle): void {
+        for (const [ key, value ] of Object.entries(bundle)) {
+            if (value.type === 'chunk' && value.isEntry && value.code.trim() === '') {
+                delete bundle[key];
+            }
+        }
+    },
+};
 
 export default {
     plugins: [
         // Needed to support absolute imports.
         nodeResolve({ browser: true }),
+        deleteEmptyBundles,
     ],
 
     // Fail the build on any warning.
     onwarn(warning) {
         if (warning.code === 'EMPTY_BUNDLE') {
-            // Give a better suggestion when no JavaScript is generated.
-            console.warn('Generated an empty JavaScript bundle, do you have'
-                + ' any JavaScript?\n\n' + warning.message);
+            // Ignore empty output errors because the `deleteEmptyBundles`
+            // plugin will delete them after the fact. This is not considered a
+            // problem and does not need to warn or error.
             return;
         }
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		// Empty script, should not be injected into the page.