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Bun in 2026 (new 1.4 version was released)

#bunjs #javascript #typescript

For whom this article is: For JavaScript/TypeScript developers who want to learn about major events and new capabilities of Bun in 2026.

2026 was a turning point for Bun. Rewriting in Rust using Claude Fable 5 in 11 days demonstrated new possibilities for AI-accelerated development, and on August 20, 2026 Bun v1.4 shipped — the first production release running fully on the Rust rewrite (until then Rust was only used internally, for example in Claude Code). Version 1.3.14 added Bun.Image, HTTP/3, and Global Virtual Store, while v1.4 brought a sharp jump in Node.js compatibility and dozens of new built-in APIs. Bun became part of Anthropic after acquisition in December 2025.

Key Features

Command Description Node.js Alternative
bun run Execute JavaScript/TypeScript files node / ts-node
bun install Fast dependency manager npm install / yarn
bun build Bundler and minifier webpack / esbuild
bun test Built-in test runner jest / vitest
bun create Initialize projects from templates npm create
Bun.serve() HTTP/HTTP2/HTTP3 server express / fastify
Bun.SQL Built-in SQL client pg / mysql2

TypeScript: Built-in Support

Bun natively supports running TypeScript and JSX without a separate compilation step. Path aliases from tsconfig.json work out of the box.

Important: Bun does NOT replace static type checking. For full type checking, use tsc --noEmit in CI.

Bun vs Node.js: TypeScript Support in 2026

Node.js v24 (LTS 2026) made type stripping the default behavior. However, important differences remain between Bun and Node.js:

Capability Bun v1.4 Node.js v24+
Run .ts files ✅ bun app.ts ✅ node app.ts
JSX support ✅ Built-in ❌ No
Path aliases (@app/*) ✅ From tsconfig.json ❌ Ignored
Enums, namespaces ✅ Work ⚠️ Needs --experimental-transform-types
Startup speed ~5.1ms on Linux ~19ms
Built-in bundler ✅ bun build ❌ No

When to choose Bun: JSX/React projects, using path aliases, maximum development speed.

When Node.js is enough: Simple .ts scripts without JSX and path aliases.

Quick Start

Install Dependencies 

bun install
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Run a Script 

bun run start
# or directly
bun index.ts
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Build Your Project 

bun build index.ts --target node --outfile dist/bundle.js
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Run Tests 

// tests/example.test.ts
import { describe, it, expect } from "bun:test";

describe("sum", () => {
  it("should correctly add numbers", () => {
    const sum = (a: number, b: number) => a + b;
    expect(sum(1, 2)).toBe(3);
  });
});
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bun test
bun test --watch
bun test --parallel  # new in 2026
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Major Events 2026

Bun v1.4 (August 20, 2026)

Bun v1.4 is the first production release running on the rewritten Rust engine — previously the Rust version lived only inside Anthropic, Claude Code included, and the last Zig build was v1.3.14. Startup got 50% faster on Linux (5.1ms vs 10.9ms) and 2.5x faster on Windows (15.5ms vs 39.0ms), idle CPU usage dropped fivefold, and HTTP server memory under load fell by 35-48%.

Node.js compatibility took its biggest leap since v1.0: the test suite gained 1,517 tests from Node.js itself, node:http, node:fs, node:sqlite and other key modules now pass 97-100% of them, and Playwright, Next.js 16, vitest, and OpenTelemetry just work. On the feature side, Bun.markdown, Bun.Archive, and Bun.JSON5/JSONC/XML/TOML replace another 15 npm dependencies with native code. Windows ARM64 is now officially supported.

The most telling number comes from Anthropic's own production: Claude Code's p99 CPU usage dropped from 24% to 10%, p50 from 5.8% to 2.5%.

Part of the community criticized the AI-driven rewrite for over 13,000 unsafe blocks in the resulting Rust code — hand-written Rust projects of similar size average around 70 — and for the fact that the largest PR in GitHub history was reviewed only by AI agents (coderabbitai and claude), with no human in the loop. A 99.8% first-pass test suite score sounds reassuring, but it's not the same thing as a person actually reading a million-line diff, and that gap is a fair thing to be uneasy about. Upgrading via bun upgrade at least requires no breaking changes.

Bun v1.3.14 (May 13, 2026)

Bun v1.3.14 — the last stable release on Zig before the Rust move — shipped Bun.Image (image processing 7x faster than sharp), HTTP/3 and QUIC support in Bun.serve() and fetch(), the Global Virtual Store (7x faster cached installs), and Bun.Terminal, terminal APIs on Windows via ConPTY.

Rewritten in Rust (May 4-14, 2026)

Bun was rewritten from Zig to Rust in 11 days using Claude Fable 5 — 535,496 lines of Zig became over a million lines of Rust across 6,502 commits, with up to 64 Claude agents working in parallel and an API bill of roughly $165,000 (5.9 billion tokens).

The payoff: the binary shrank by about 20%, 128 bugs got fixed along the way, memory leaks in Bun.build() dropped from 6,745 MB to 609 MB — more than 11x — and every test passed on every platform.

The reason for the rewrite is mundane: the Zig version had 13 memory-safety vulnerabilities (use-after-free, double-free, leaks), and Rust catches that class of bug at compile time. The rewritten code spent three months running inside Bun itself, including in Claude Code, before becoming the production version in v1.4.

Acquired by Anthropic (December 2, 2025)

Bun was acquired by Anthropic and is now used as the foundation for Claude Code and Claude Agent SDK. Bun remains open source under MIT license.

New APIs and Features

Bun.Image (v1.3.14, matured in v1.4)

Built-in image processing, 7x faster than alternatives:

import { Image } from "bun";

const img = new Image(await Bun.file("photo.jpg").arrayBuffer());
const resized = await img.resize(800, 600);
const webp = await resized.webp();
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Support: JPEG, PNG, WebP, GIF, BMP, HEIC, AVIF.

Bun.WebView (v1.3.12, in v1.4 — system WebKit or Chrome/Chromium/Edge)

Headless browser without Puppeteer and Playwright:

const webview = await Bun.WebView.open("https://example.com");
const screenshot = await webview.screenshot();
await webview.close();
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Bun.cron (v1.3.12)

Built-in task scheduler:

import { cron } from "bun";

cron("0 * * * *", () => {
  console.log("Every hour");
});
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Bun.SQL (v1.3)

Built-in SQL client without external dependencies:

import { sql } from "bun";

const users = await sql`SELECT * FROM users WHERE id = ${userId}`;
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Support: PostgreSQL, MySQL, MariaDB, SQLite.

Markdown in Terminal (v1.3.12) 

bun ./README.md  # renders markdown in terminal
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Bun.markdown (v1.4)

Programmatic markdown parser — render to HTML, React, or a custom format:

import { markdown } from "bun";

const html = markdown.toHTML("# Hello, **Bun**!");
const react = markdown.toReact("- one\n- two");
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Bun.Archive (v1.4)

Create and extract archives without tar/node-tar:

import { Archive } from "bun";

const archive = new Archive();
await archive.add("dist/", { recursive: true });
await archive.write("release.tar.gz");

await Archive.extract("release.tar.gz", { to: "./out" });
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Data Formats: JSON5, JSONC, XML, TOML (v1.4)

Built-in format parsers — 15 more npm dependencies eliminated:

import { JSON5, JSONC, XML, TOML } from "bun";

JSON5.parse("{ unquoted: 'ok', trailing: 1, }");
JSONC.parse("{ /* comment */ \"a\": 1 }");
XML.parse("<root><item>1</item></root>");
TOML.parse("[server]\nport = 8080");
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HTTP/3 and QUIC (v1.3.14, now also in fetch() — v1.4) 

Bun.serve({
  port: 443,
  tls: { /* ... */ },
  // HTTP/3 enabled automatically with TLS
  fetch(req) {
    return new Response("Hello HTTP/3!");
  },
});
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In v1.4, experimental HTTP/2 and HTTP/3 support was added to the client-side fetch() as well:

await fetch("https://example.com", { protocol: "http3" });
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Parallel Testing (v1.3.13, + --timings in v1.4) 

bun test --parallel        # parallel execution
bun test --isolate         # test isolation
bun test --shard=1/4       # sharding for CI
bun test --changed         # only changed tests
bun test --timings         # shard by actual execution time (v1.4)
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Compile to HTML (v1.3.10, + --asset in v1.4) 

bun build ./app.ts --compile --target=browser
# Result: single .html file

bun build ./app.ts --compile --asset ./data.db
# Embeds a file into the compiled binary (v1.4)
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Package Management Commands (v1.4)

New commands for maintaining dependencies:

bun audit fix     # automatically patches vulnerabilities
bun dedupe        # removes duplicate versions from the lockfile
bun prune         # deletes unused packages
bun pm diff        # compares package versions
bun pm licenses    # license report for dependencies
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bun build: React Compiler and Barrel Imports (v1.4)

Built-in React Compiler — 19x faster than the Babel plugin — plus tree-shaking of barrel files, markdown bundle reports, and standard TC39 decorators support:

bun build ./app.tsx --react-compiler
bun build ./app.ts --metafile-md=report.md
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Developer Tooling (v1.4) 

bun --cpu-prof-md ./app.ts    # markdown CPU profiling report
bun --heap-prof-md ./app.ts   # markdown heap profiling report
bun run --parallel "build:*"  # run multiple scripts concurrently
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process.on("memoryPressure", (level) => {
  console.log("OS reports low memory:", level);
});
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Performance

Comparison with Node.js and Deno (August 2026, Bun v1.4 figures)

Metric Bun v1.4 Node.js v26 Deno v2.5
Cold start (Linux) ~5.1ms ~19ms ~18ms
HTTP throughput 290k req/s 71k req/s ~75k req/s
Package installation 4.2s 38s (npm) 35s
Build 50k lines 1.1s - -

Bun v1.4 vs v1.3: what the Rust move delivered

According to Bun, moving the Rust rewrite to production brought a noticeable boost in real-world scenarios:

Metric v1.3 (Zig) v1.4 (Rust) Change
Startup on Linux 10.9ms 5.1ms −50%
Startup on Windows 39.0ms 15.5ms 2.5x faster
Idle CPU baseline −5x
Binary size (Linux/Windows) baseline −17%
new URL() baseline up to 4.6x faster
Regex operations baseline 138-200x faster
FFI calls (bun:ffi) baseline 3x faster

Peak HTTP server memory under 1M requests:

Framework v1.3 v1.4 Reduction
Fastify 233 MB 120 MB −48%
Express 169 MB 92 MB −46%
node:http 135 MB 81 MB −40%

In Claude Code's production, which had already been running on the Rust version for several months, p99 CPU usage dropped from 24% to 10%, and p50 from 5.8% to 2.5%.

Package Installation Speed

Scenario npm v12 pnpm v11 bun v1.3
Clean install 40s 15s 7s
With cache 18s 5.5s 2.5s
Monorepo 110s 22s 12s

On a test Next.js app (T3 stack), Bun v1.4 installs dependencies even faster: first install — 1.41s (15x faster than npm), fresh checkout with cache — 251ms (30x faster), CI without cache — 951ms (19x faster), CI with cache — 210ms (21x faster).

When to Use Bun

Go for it without hesitation: local development and CLI tools, new TypeScript/JSX projects, CI/CD (5-8x faster installs, sometimes 20-30x in v1.4 scenarios), and full-stack apps with built-in SQL. Windows development belongs here too — ARM64 included, now officially supported and starting up 2.5x faster in v1.4.

Think twice about a few things: HTTP/3 in Bun.serve() and fetch() is still experimental, too early for production. Native N-API module compatibility jumped a lot in v1.4, but a test-pass percentage isn't a guarantee for the specific module in your specific project — you'll still need to test it by hand. Cold start dropped sharply, good news for serverless, but how much it actually helps depends on the platform, not just the runtime. For applications with tight memory budgets, Bun is still the less predictable choice compared to Node.

Bottom Line: npm Packages You No Longer Need

Bun's idea isn't to add one more tool to node_modules — it's to remove as much from it as possible. By v1.4, Bun had replaced 15+ popular npm dependencies with native implementations — instead of dozens of packages in package.json, you can get by with a single runtime:

Task Typical npm packages Native Bun replacement Since
Image processing sharp, jimp Bun.Image v1.3.14
Headless browser puppeteer, playwright Bun.WebView v1.3.12
SQL client pg, mysql2, better-sqlite3 Bun.SQL v1.3
Cron jobs node-cron, cron Bun.cron() v1.3.12
Pseudo-terminal node-pty Bun.Terminal v1.3.14
Markdown rendering marked, markdown-it, remark Bun.markdown v1.4
Archives tar, node-tar, archiver Bun.Archive v1.4
JSON5 json5 Bun.JSON5 v1.4
JSON with comments jsonc-parser, comment-json Bun.JSONC v1.4
XML xml2js, fast-xml-parser Bun.XML v1.4
TOML toml, @iarna/toml Bun.TOML v1.4
Test runner jest, vitest, mocha bun test v1.3–1.4
Bundler webpack, esbuild, rollup, parcel bun build built-in
React Compiler babel-plugin-react-compiler bun build --react-compiler v1.4
Package manager npm, yarn, pnpm bun install built-in
License checking license-checker bun pm licenses v1.4
Dependency dedup manual npm dedupe bun dedupe v1.4
Unused package detection depcheck bun prune v1.4
Vulnerability patching npm audit + manual patches bun audit fix v1.4
Auto-restart nodemon bun --watch built-in
Parallel scripts concurrently, npm-run-all bun run --parallel v1.4
Environment variables dotenv built-in .env support built-in
Running TypeScript ts-node, tsx bun app.ts built-in

Not every replacement is a full drop-in — specific webpack loaders or advanced Puppeteer flows aren't going anywhere, and a third-party package will stay necessary there. But for a typical full-stack TypeScript project in 2026, package.json can genuinely shrink by a large factor, with bun ending up as pretty much the only dependency left.

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