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Bun Playground

Toolchain

Transpiler

Section titled “Transpiler”

Bun’s own parser and printer, compiled to WebAssembly. Output matches bun build --no-bundle --target=browser.

  • ✅ .ts, .tsx, .jsx, .mts, .cts, .js, .mjs
  • ✅ JSX with Bun’s defaults (React automatic runtime)
  • ✅ Legacy decorators with experimentalDecorators and emitDecoratorMetadata in tsconfig.json
  • ✅ Top-level await
  • ❌ tsconfig.json paths, baseUrl, jsx, jsxImportSource and extends are not read
  • ❌ No type checking

Module resolution

Section titled “Module resolution”
  • ✅ Relative imports, with .ts .tsx .js .jsx .mjs .json and index.* tried in that order
  • ✅ Packages from node_modules, walking up folders: exports (nested conditions, arrays, ./* patterns), then module, then main
  • ✅ Conditions: bun, node, import, development, default for server code; browser, module, import, development, default for code in the preview page
  • ✅ #imports from the nearest package.json
  • ✅ JSON imports
  • ✅ CSS imported from code that goes through an HTML import (bundled)
  • ✅ http:/https: imports are left to the browser
  • rolldown is replaced by the playground’s own @rolldown/browser, since the native binding can’t load

An import that can’t be resolved shows cannot resolve "x" in the console.

CommonJS and require

Section titled “CommonJS and require”
  • ✅ .cjs files, and .js files without ESM syntax in packages that aren’t "type": "module"
  • ✅ require("x") with a string literal. Modules run on first require, as in Node; cycles see partial exports.
  • ✅ Named imports from CommonJS, found by a static scan (exports.x =, module.exports = { … }, re-exports). Exports computed at runtime aren’t visible as named imports.
  • ✅ createRequire(import.meta.url) and import.meta.require with string literals
  • ❌ require(variable) throws. So does a require that can’t be resolved, when it runs.
  • ❌ require.cache, require.main, native .node addons

bun install

Section titled “bun install”

Runs in the console (bun install, bun add, bun remove) and automatically when a project opens with missing dependencies.

  • ✅ Packages from the npm registry, with Bun’s semver
  • ✅ dependencies, devDependencies, optionalDependencies and required peer dependencies
  • ✅ bun.lock (text format): exact versions, nested node_modules, integrity checks. A stale lockfile logs a warning.
  • ✅ Optional packages limited to other platforms (native bindings) are skipped
  • 🟡 Without a lockfile, the install is flat: the first version seen wins
  • ❌ No bun.lock is written. bun add refuses to run when a bun.lock exists.
  • ❌ No lifecycle scripts (postinstall, …)
  • ❌ No git, tarball, file:, workspace: or npm: dependencies
  • ❌ Only registry.npmjs.org; .npmrc isn’t read
  • ❌ Binary bun.lockb

Bundler

Section titled “Bundler”

Bun.build and HTML imports use Bun’s real bundler, compiled to WebAssembly (~10 MB, loaded on first use). See Bun.build for the supported options.

  • Single-threaded; each build starts fresh
  • HTML imports in Bun.serve are built for the browser, without minification or source maps
  • If a build fails, errors go to the console and the page is served unbundled