Bun v1.3.8

Bun version 1.3.8 introduces a built-in CommonMark-compliant Markdown parser written in Zig, offering three rendering APIs: `Bun.markdown.html()` for HTML output, `Bun.markdown.render()` for custom callbacks, and `Bun.markdown.react()` for React elements. The update also adds a `--metafile-md` CLI option for `bun build` that generates Markdown-based bundle analysis, designed to help developers identify bloat and understand dependency chains when shared with LLMs.

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