Writing Bundler Tests

Bundler tests use itBundled() from test/bundler/expectBundled.ts to test Bun's bundler.

Basic Usage

import { describe } from "bun:test"; import { itBundled, dedent } from "./expectBundled";

describe("bundler", () => { itBundled("category/TestName", { files: { "index.js": console.log("hello");, }, run: { stdout: "hello", }, }); });

Test ID format: category/TestName (e.g., banner/CommentBanner , minify/Empty )

File Setup

{ files: { "index.js": console.log("test");, "lib.ts": export const foo = 123;, "nested/file.js": export default {};, }, entryPoints: ["index.js"], // defaults to first file runtimeFiles: { // written AFTER bundling "extra.js": console.log("added later");, }, }

Bundler Options

{ outfile: "/out.js", outdir: "/out", format: "esm" | "cjs" | "iife", target: "bun" | "browser" | "node",

// Minification minifyWhitespace: true, minifyIdentifiers: true, minifySyntax: true,

// Code manipulation banner: "// copyright", footer: "// end", define: { "PROD": "true" }, external: ["lodash"],

// Advanced sourceMap: "inline" | "external", splitting: true, treeShaking: true, drop: ["console"], }

Runtime Verification

{ run: { stdout: "expected output", // exact match stdout: /regex/, // pattern match partialStdout: "contains this", // substring stderr: "error output", exitCode: 1, env: { NODE_ENV: "production" }, runtime: "bun" | "node",

// Runtime errors
error: "ReferenceError: x is not defined",

}, }

Bundle Errors/Warnings

{ bundleErrors: { "/file.js": ["error message 1", "error message 2"], }, bundleWarnings: { "/file.js": ["warning message"], }, }

Dead Code Elimination (DCE)

Add markers in source code:

// KEEP - this should survive const used = 1;

// REMOVE - this should be eliminated const unused = 2;

{ dce: true, dceKeepMarkerCount: 5, // expected KEEP markers }

Capture Pattern

Verify exact transpilation with capture() :

itBundled("string/Folding", { files: { "index.ts": capture(\${1 + 1}`);`, }, capture: ['"2"'], // expected captured value minifySyntax: true, });

Post-Bundle Assertions

{ onAfterBundle(api) { api.expectFile("out.js").toContain("console.log"); api.assertFileExists("out.js");

const content = api.readFile("out.js");
expect(content).toMatchSnapshot();

const values = api.captureFile("out.js");
expect(values).toEqual(["2"]);

}, }

Common Patterns

Simple output verification:

itBundled("banner/Comment", { banner: "// copyright", files: { "a.js": console.log("Hello") }, onAfterBundle(api) { api.expectFile("out.js").toContain("// copyright"); }, });

Multi-file CJS/ESM interop:

itBundled("cjs/ImportSyntax", { files: { "entry.js": import lib from './lib.cjs'; console.log(lib);, "lib.cjs": exports.foo = 'bar';, }, run: { stdout: '{"foo":"bar"}' }, });

Error handling:

itBundled("edgecase/InvalidLoader", { files: { "index.js": ... }, bundleErrors: { "index.js": ["Unsupported loader type"], }, });

Test Organization

test/bundler/ ├── bundler_banner.test.ts ├── bundler_string.test.ts ├── bundler_minify.test.ts ├── bundler_cjs.test.ts ├── bundler_edgecase.test.ts ├── bundler_splitting.test.ts ├── css/ ├── transpiler/ └── expectBundled.ts

Running Tests

bun bd test test/bundler/bundler_banner.test.ts BUN_BUNDLER_TEST_FILTER="banner/Comment" bun bd test bundler_banner.test.ts BUN_BUNDLER_TEST_DEBUG=1 bun bd test bundler_minify.test.ts

Key Points

• Use dedent for readable multi-line code

• File paths are relative (e.g., /index.js )

• Use capture() to verify exact transpilation results

• Use .toMatchSnapshot() for complex outputs

• Pass array to run for multiple test scenarios