sideEffects field breaks bundler tree-shaking in Bun 1.3.10 #27709
Description
What version of Bun is running?
Bun 1.3.10 (regression from 1.3.9)
What platform is your computer?
Darwin 25.2.0 arm64 arm
What steps can reproduce the bug?
mkdir repro && cd repro// a.ts
import { createContext, createElement, useContext } from "react"
const Ctx = createContext(null)
function Root({ children }) {
return createElement(Ctx.Provider, { value: {} }, createElement("div", null, children))
}
function Trigger({ children }) {
const ctx = useContext(Ctx)
return createElement("button", null, children)
}
export const A = { Root, Trigger }// b.ts
import { createContext, createElement, useContext } from "react"
const Ctx = createContext(null)
function Root({ children }) {
return createElement(Ctx.Provider, { value: {} }, createElement("div", null, children))
}
function Trigger({ children }) {
const ctx = useContext(Ctx)
return createElement("button", null, children)
}
export const B = { Root, Trigger }// index.ts
export { A } from "./a.js"
export { B } from "./b.js"// package.json
{ "name": "test", "type": "module", "sideEffects": ["./dist/index.js"] }bun build index.ts --outdir out --format esm --external react --minify
cat out/index.jsWhat is the expected behavior?
Bundled 3 modules in 2ms
index.js 0.76 KB
Full output with all function bodies and correct exports.
What do you see instead?
Actual (Bun 1.3.10):
Bundled 1 module in 2ms
index.js 61 bytes
export{l as B,t as A};Variables l and t are never declared, the bundler tree-shook away all module contents.
Additional information
Summary: Bun.build() / bun build incorrectly applies the sideEffects field from the project's own package.json to source files during bundling. This causes the bundler to tree-shake away all re-exported modules, producing an empty output with dangling export references.
Root cause: The bundler reads the sideEffects field from the project's own package.json and applies it to source files during the build. Since ./dist/index.js doesn't match any source file path, the bundler treats all source modules as side-effect-free and drops them entirely. The sideEffects field is intended for downstream consumers, not for the package's own build process.