writing-bundler-tests

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;