# Bun v1.3.8

> Source: <https://bun.com/blog/bun-v1.3.8>
> Published: 2026-01-29 10:04:52+00:00

To install Bun
curl -fsSL https://bun.sh/install | bash
npm install -g bun
powershell -c "irm bun.sh/install.ps1|iex"
scoop install bun
brew tap oven-sh/bun
brew install bun
docker pull oven/bun
docker run --rm --init --ulimit memlock=-1:-1 oven/bun
To upgrade Bun
bun upgrade
Bun.markdown
— Built-in Markdown Parser
Bun now includes a fast, CommonMark-compliant Markdown parser written in Zig. This is a Zig port of the popular md4c
library.
The new Bun.markdown
API provides three ways to render Markdown:
Bun.markdown.html()
— Render to HTML
const html = Bun.markdown.html("# Hello **world**");
// "<h1>Hello <strong>world</strong></h1>\n"
// With options
Bun.markdown.html("## Hello", { headingIds: true });
// '<h2 id="hello">Hello</h2>\n'
Bun.markdown.render()
— Custom Callbacks
Render with JavaScript callbacks for each element, perfect for custom HTML, ANSI terminal output, or stripping formatting:
// Custom HTML with classes
const html = Bun.markdown.render("# Title\n\nHello **world**", {
heading: (children, { level }) =>
`<h${level} class="title">${children}</h${level}>`,
paragraph: (children) => `<p>${children}</p>`,
strong: (children) => `<b>${children}</b>`,
});
// ANSI terminal output
const ansi = Bun.markdown.render("# Hello\n\n**bold**", {
heading: (children) => `\x1b[1;4m${children}\x1b[0m\n`,
paragraph: (children) => children + "\n",
strong: (children) => `\x1b[1m${children}\x1b[22m`,
});
// Return null to omit elements
const result = Bun.markdown.render("# Title\n\n![logo](img.png)", {
image: () => null,
heading: (children) => children,
});
Bun.markdown.react()
— React Elements
Returns a React Fragment directly usable as a component return value:
function Markdown({ text }: { text: string }) {
return Bun.markdown.react(text);
}
// With custom components
const element = Bun.markdown.react("# Hello", {
h1: ({ children }) => <h1 className="title">{children}</h1>,
});
// Server-side rendering
import { renderToString } from "react-dom/server";
const html = renderToString(Bun.markdown.react("# Hello **world**"));
For React 18 and older, pass reactVersion: 18
since the default targets React 19's element format.
GFM Extensions
GitHub Flavored Markdown extensions are enabled by default:
- Tables
- Strikethrough (
~~deleted~~
) - Task lists (
- [x] done
) - Permissive autolinks
Additional options include wikiLinks
, latexMath
, headingIds
, and autolinkHeadings
.
--metafile-md
CLI option for LLM-friendly bundle analysis
bun build
now supports a --metafile-md
option that generates a Markdown visualization of your bundle's module graph. This is particularly useful for analyzing bundle composition with LLMs like Claude—paste the output into a chat to identify bloat, understand dependency chains, and optimize your builds.
# Default filename (meta.md)
bun build entry.js --metafile-md --outdir=dist
# Custom filename
bun build entry.js --metafile-md=analysis.md --outdir=dist
# Both JSON and markdown
bun build entry.js --metafile=meta.json --metafile-md=meta.md --outdir=dist
The generated Markdown includes:
- Quick Summary — Module counts, sizes, ESM/CJS breakdown, output/input ratio
- Largest Input Files — Sorted by size to identify potential bloat
- Entry Point Analysis — Bundle size, exports, CSS bundles, and bundled modules
- Dependency Chains — Most commonly imported modules and reverse dependencies
- Full Module Graph — Complete import/export info for each module
- Raw Data for Searching — Grep-friendly markers like
[MODULE:]
,[SIZE:]
,[IMPORT:]
The Bun.build()
API also supports this via an expanded metafile
option:
const result = await Bun.build({
entrypoints: ["./index.ts"],
outdir: "./dist",
// Can now be a string path or object with json/markdown paths
metafile: {
json: "meta.json",
markdown: "meta.md",
},
});
Bugfixes
- Updated mimalloc
- Fixed:
napi_typeof
incorrectly returningnapi_object
instead ofnapi_function
for callbacks wrapped inAsyncContextFrame
, which caused native addons like encore.dev to panic with "expect Function, got: Object" when usingAsyncLocalStorage.run()
- Fixed: Crash during heap snapshot generation in certain cases
- Fixed: Crash in
node:vm
when usingSyntheticModule
withnode:async_hooks
enabled, such as when running the React Email preview server - Fixed: HTTP/2 stream state handling that caused gRPC streaming calls to fail with
DEADLINE_EXCEEDED
errors in certain cases when using libraries like@google-cloud/firestore
and@grpc/grpc-js
- Fixed:
npm i -g bun
failing on Windows due to npm'scmd-shim
generating broken wrappers that referenced/bin/sh
from placeholder scripts' shebang lines