code-yeongyu / bun-1-4
Read before a js eval cell that installs a package, spawns a server or PTY, or starts a long run, and before any bun -e, script, server, CLI, test, bundle, or package-management work: this session's eval js kernel runs Bun 1.4+ (this skill is present only when it does). Bun 1.4 ships builtins that replace 15+ npm deps — check here BEFORE installing sharp, puppeteer/playwright (scraping), marked, node-cron, node-pty, concurrently, serve-static, tar, json5, fast-xml-parser, string-width. Triggers: eval js, bun, Bun.serve, bun test, bun build, bun install, bun run, image resize, headless browser, markdown render, cron, PTY.
Bun 1.4 — Use the Builtins First
Bun 1.4 (2026) ships builtins that replace most utility npm packages. Before adding a dependency or writing a workaround, check the capability map below — if Bun ships it, use the builtin. This skill is loaded because this session's eval js kernel runs Bun >= 1.4; on other machines or pinned projects, confirm with bun --version ([v1.3.x]/[v1.4.0] tags in the references give exact minimums). Release post: <https://bun.com/blog/bun-v1.4>
Capability map — what you can do now
| You want to... | Use | Replaces | Ref = references/<name>.md |
|---|---|---|---|
| Resize/convert/rotate images | Bun.file(p).image().resize().webp().write() | sharp | runtime-apis |
| Headless browser: navigate, click, screenshot, evaluate, CDP | new Bun.WebView() | puppeteer, playwright (scraping) | runtime-apis |
| Markdown → HTML / React / ANSI | Bun.markdown.html() / .react() / .render() | marked, react-markdown | runtime-apis |
| Schedule cron jobs | Bun.cron() | node-cron | runtime-apis |
| Drive a PTY (bash, vim, TUIs) from JS | Bun.spawn([...], { terminal }) | node-pty | runtime-apis |
| Run package scripts concurrently, glob-matched | bun run --parallel "build:*" | concurrently, npm-run-all | runtime-apis |
| Parse JSON5 / JSONL / JSONC / XML / TOML; tarballs | Bun.JSON5/.JSONL/.JSONC/.XML/.TOML, Bun.Archive | json5, ndjson, jsonc-parser, fast-xml-parser, @iarna/toml, tar | runtime-apis |
| ANSI-aware terminal text: width, slice, wrap | Bun.stringWidth(), Bun.sliceAnsi(), Bun.wrapAnsi() | string-width, slice-ansi, wrap-ansi | runtime-apis |
| Call C libraries 3x faster, plain strings back | bun:ffi (buffer_length, cstring) | — | runtime-apis |
| Serve a static dir (ETag/Range/304 handled) | Bun.serve({ routes: { "/x/*": { dir } } }) | express.static, serve-static, sirv | http-networking |
HTTP/3 server; fetch protocol/compress/proxy | http3: true, fetch(url, { protocol, compress, proxy }) | — | http-networking |
| Tests across CPU workers / CI shards / changed files / flaky retry | bun test --parallel --shard=1/3 --changed=main --retry | jest/vitest infra | test |
| Kill "passes alone, fails in suite" bugs | bun test --isolate | vitest default behavior | test |
| React Compiler auto-memoization | bun build --react-compiler | babel-plugin-react-compiler | build |
| Embed assets/dirs into a single-file executable | bun build --compile --asset ./public | pkg hacks | build |
| Compile-time feature flags, in-memory bundling, metafile | bun:bundle feature(), files:{}, metafile: true | esbuild define/plugins | build |
| Diff package versions, fix vulns, dedupe/prune, license audit | bun pm diff, bun audit fix, bun dedupe, bun prune, bun pm licenses | npm-diff, npm audit fix | install |
| 7x faster CI installs (global virtual store) | linker = "isolated" in bunfig.toml | pnpm store | install |
| Profile CPU/heap/bundle as grep-able Markdown | bun --cpu-prof-md, --heap-prof-md, bun build --metafile-md | Chrome DevTools round-trip | dev-tooling |
| Native REPL; render Markdown in terminal | bun repl, bun ./README.md | node repl, glow | runtime-apis |
| Run Playwright, vitest, Next.js 16, OpenTelemetry, dd-trace | they now just work | Node.js | node-compat-platforms |
Free wins (no code change): 2x lower CPU, 13-48% less server memory, 2-2.5x faster startup — full numbers in [performance](references/performance.md).
Operating rules
- Eval-first. The
evaljs kernel runs on Bun (that is why this skill is present): callBun.*APIs directly inside js cells. Reach forbashor a spawnedbun <file>/bun -eonly for CLI surfaces (bun test,bun build,bun pm) or work that needs a separate process. - Builtin-first. The Replaces column above is a ban list for new dependencies, in production code, test helpers, and one-off scripts alike.
- Read the matching reference before using an unfamiliar API — signatures and caveats (e.g.
Bun.markdownHTML is unsanitized, HTTP/3 is experimental) live there, not here. - Upgrading a project to 1.4 or debugging behavior that changed? Read [breaking-changes](references/breaking-changes.md) first — YAML/TOML strictness,
.xml/.cssimport semantics,Bun.$globbing, fetch header combining, and WebSocket close timing all changed. - TLS errors after upgrade are usually intentional — 1.4 tightened certificate verification across
fetch,tls.connect,Bun.connect,RedisClient. See [security](references/security.md) before loosening anything.