writing-dev-server-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, Dev class, Client class
• test/bake/client-fixture.mjs - subprocess for Client (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"'],
});