77ca318336
### Problem The bundler's `__toESM` helper creates a new getter-wrapped proxy object every time a CJS module is imported. In a large app, a popular dependency like React can be imported 600+ times — each creating a fresh object with ~44 getter properties. This produces ~27K unnecessary `GetterSetter` objects, ~25K closures, and ~25K `JSLexicalEnvironment` scope objects at startup. Additionally, `__export` and `__exportValue` use `var`-scoped loop variables captured by setter closures, meaning all setters incorrectly reference the last iterated key (a latent bug). ### Changes 1. **`__toESM`: add WeakMap cache** — deduplicate repeated wrappings of the same CJS module. Two caches (one per `isNodeMode` value) to handle both import modes correctly. 2. **Replace closures with `.bind()`** — `() => obj[key]` becomes `__accessProp.bind(obj, key)`. BoundFunction is cheaper than Function + JSLexicalEnvironment, and frees the for-in `JSPropertyNameEnumerator` from the closure scope. 3. **Fix var-scoping bug in `__export`/`__exportValue`** — setter closures captured a shared `var name` and would all modify the last iterated key. `.bind()` eagerly captures the correct key per iteration. 4. **`__toCommonJS`: `.map()` → `for..of`** — eliminates throwaway array allocation. 5. **`__reExport`: single `getOwnPropertyNames` call** — was calling it twice when `secondTarget` was provided. ### Impact (measured on a ~23MB single-bundle app with 600+ React imports) | Metric | Before | After | Delta | |--------|--------|-------|-------| | **Total objects** | 745,985 | 664,001 | **-81,984 (-11%)** | | **Heap size** | 115 MB | 111 MB | **-4 MB** | | GetterSetter | 34,625 | 13,428 | -21,197 (-61%) | | Function | 221,302 | 197,024 | -24,278 (-11%) | | JSLexicalEnvironment | 70,101 | 44,633 | -25,468 (-36%) | | Structure | 40,254 | 39,762 | -492 |
Tests
Finding tests
Tests are located in the 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/ directory are further categorized by the type of API.
test/js/bun/- tests forBun-specific APIs.node/- tests for Node.js APIs.web/- tests for Web APIs, likefetch().first_party/- tests for npm packages that are built-in, likeundici.third_party/- tests for npm packages that are not built-in, but are popular, likeesbuild.
Running tests
To run a test, use Bun's built-in test command: bun test.
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.
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]));
});
});
If you are fixing a bug that was reported from a GitHub issue, remember to add a test in the test/regression/ directory.
// 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.
Zig tests
These tests live in various .zig files throughout Bun's codebase, leveraging Zig's builtin test keyword.
Currently, they're not run automatically nor is there a simple way to run all of them. We will make this better soon.
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.