name: writing-dev-server-tests description: Guides writing HMR/Dev Server tests in test/bake/. Use when creating or modifying dev server, hot reloading, or bundling tests.
Writing HMR/Dev Server Tests
Dev server tests validate hot-reloading robustness and reliability.
File Structure
-
test/bake/bake-harness.ts- shared utilities:devTest,prodTest,devAndProductionTest,Devclass,Clientclass -
test/bake/client-fixture.mjs- subprocess forClient(page loading, IPC queries) -
test/bake/dev/*.test.ts- dev server and hot reload tests -
test/bake/dev-and-prod.ts- tests running on both dev and production mode
Test Categories
-
bundle.test.ts- DevServer-specific bundling bugs -
css.test.ts- CSS bundling issues -
plugins.test.ts- development mode plugins -
ecosystem.test.ts- library compatibility (prefer concrete bugs over full package tests) -
esm.test.ts- ESM features in development -
html.test.ts- HTML file handling -
react-spa.test.ts- React, react-refresh transform, server components -
sourcemap.test.ts- source map correctness
devTest Basics
import { devTest, emptyHtmlFile } from "../bake-harness";
devTest("html file is watched", {
files: {
"index.html": emptyHtmlFile({
scripts: ["/script.ts"],
body: "<h1>Hello</h1>",
}),
"script.ts": `console.log("hello");`,
},
async test(dev) {
await dev.fetch("/").expect.toInclude("<h1>Hello</h1>");
await dev.patch("index.html", { find: "Hello", replace: "World" });
await dev.fetch("/").expect.toInclude("<h1>World</h1>");
await using c = await dev.client("/");
await c.expectMessage("hello");
await c.expectReload(async () => {
await dev.patch("index.html", { find: "World", replace: "Bar" });
});
await c.expectMessage("hello");
},
});
Key APIs
-
files: Initial filesystem state -
dev.fetch(): HTTP requests -
dev.client(): Opens browser instance -
dev.write/patch/delete: Filesystem mutations (wait for hot-reload automatically) -
c.expectMessage(): Assert console.log output -
c.expectReload(): Wrap code that causes hard reload
Important: Use dev.write/patch/delete instead of node:fs - they wait for hot-reload.
Testing Errors
devTest("import then create", {
files: {
"index.html": `<!DOCTYPE html><html><head></head><body><script type="module" src="/script.ts"></script></body></html>`,
"script.ts": `import data from "./data"; console.log(data);`,
},
async test(dev) {
const c = await dev.client("/", {
errors: ['script.ts:1:18: error: Could not resolve: "./data"'],
});
await c.expectReload(async () => {
await dev.write("data.ts", "export default 'data';");
});
await c.expectMessage("data");
},
});
Specify expected errors with the errors option:
await dev.delete("other.ts", {
errors: ['index.ts:1:16: error: Could not resolve: "./other"'],
});
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Skill Details
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Created
Jan 2026
Last Updated
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