writing-dev-server-tests

Original：🇺🇸 English

Translated

Guides writing HMR/Dev Server tests in test/bake/. Use when creating or modifying dev server, hot reloading, or bundling tests.

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Sourceoven-sh/bun

Added on2026-02-17

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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

ts

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

ts

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:

ts

await dev.delete("other.ts", {
  errors: ['index.ts:1:16: error: Could not resolve: "./other"'],
});