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+# Tests
+
+## Finding tests
+
+Tests are located in the [`test/`](test/) directory and are organized using the following structure:
+
+* `test/`
+  * `js/` - tests for JavaScript APIs.
+  * `cli/` - tests for commands, configs, and stdout.
+  * `bundler/` - tests for the transpiler/bundler.
+  * `regression/` - tests that reproduce a specific issue.
+  * `harness.ts` - utility functions that can be imported from any test.
+
+The tests in [`test/js/`](js/) directory are further categorized by the type of API.
+
+* `test/js/`
+  * `bun/` - tests for `Bun`-specific APIs.
+  * `node/` - tests for Node.js APIs.
+  * `web/` - tests for Web APIs, like `fetch()`.
+  * `first_party/` - tests for npm packages that are built-in, like `undici`.
+  * `third_party/` - tests for npm packages that are not built-in, but are popular, like `esbuild`.
+
+## Running tests
+
+To run a test, use Bun's built-in test command: `bun test`.
+
+```sh
+bun test # Run all tests
+bun test js/bun # Only run tests in a directory
+bun test sqlite.test.ts # Only run a specific test
+```
+
+If you encounter lots of errors, try running `bun install`, then trying again.
+
+## Writing tests
+
+Tests are written in TypeScript (preferred) or JavaScript using Jest's `describe()`, `test()`, and `expect()` APIs.
+
+```ts
+import { describe, test, expect } from "bun:test";
+import { gcTick } from "harness";
+
+describe("TextEncoder", () => {
+  test("can encode a string", async () => {
+    const encoder = new TextEncoder();
+    const actual = encoder.encode("bun");
+    await gcTick();
+    expect(actual).toBe(new Uint8Array([0x62, 0x75, 0x6E]));
+  });
+});
+```
+
+When fixing a bug, add the test to the existing test file for that code (e.g. a fetch bug → `test/js/web/fetch/fetch.test.ts`). Only use `test/regression/` when the bug has a GitHub issue number **and** is a true regression — it worked in a previous release and then broke.
+
+```ts
+// test/regression/issue/02005.test.ts
+
+import { it, expect } from "bun:test";
+
+it("regex literal should work with non-latin1", () => {
+  const text = "这是一段要替换的文字";
+  expect(text.replace(new RegExp("要替换"), "")).toBe("这是一段的文字");
+  expect(text.replace(/要替换/, "")).toBe("这是一段的文字");
+});
+```
+
+In the future, a bot will automatically close or re-open issues when a regression is detected or resolved.
+
+## TypeScript
+
+Test files should be written in TypeScript. The types in `packages/bun-types` should be updated to support all new APIs. Changes to the `.d.ts` files in `packages/bun-types` will be immediately reflected in test files; no build step is necessary.
+
+Writing a test will often require using invalid syntax, e.g. when checking for errors when an invalid input is passed to a function. TypeScript provides a number of escape hatches here.
+
+- `// @ts-expect-error` - This should be your first choice. It tells TypeScript that the next line *should* fail typechecking.
+- `// @ts-ignore` - Ignore the next line entirely.
+- `// @ts-nocheck` - Put this at the top of the file to disable typechecking on the entire file. Useful for autogenerated test files, or when ignoring/disabling type checks an a per-line basis is too onerous.