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Bun 1.4's Rust Rewrite: What Changed and What to Check Before You Upgrade

webhani·

Bun 1.4 is out, and the headline isn't a feature — it's that the runtime's internals were rewritten from Zig to Rust. Over 2,900 issues got resolved along the way, and more than 1,500 additional Node.js compatibility tests now pass. This is a foundational change to the runtime, not a routine point release, and it deserves to be treated that way.

New built-in APIs

Bun 1.4 ships 15 new built-in APIs that replace functionality teams typically pulled in as npm packages:

  • Bun.WebView — headless browser automation built in, covering a lot of what you'd normally reach for Puppeteer to do
  • Bun.Image — image resizing and format conversion
  • Bun.markdown — Markdown rendering
  • Bun.cron() — cron-style scheduled jobs
  • Native JSON5 and JSONL parsing
import { cron } from "bun";
 
cron.schedule("0 * * * *", () => {
  console.log("Runs every hour, no external dependency needed");
});

The value here isn't just convenience in prototypes — fewer runtime dependencies means a smaller supply-chain surface in production, too.

Performance gains

The Rust rewrite shows up directly in the numbers:

  • 5x lower idle CPU usage
  • Up to 35% lower memory usage
  • 50% faster startup on Linux
  • Up to 7x faster installs with the new opt-in global virtual store

If your CI runs on Bun, faster installs translate directly into shorter pipeline times. In monorepos with a lot of packages, enabling the global virtual store is worth testing specifically for that reason.

Breaking changes to check before migrating

Three changes are easy to miss and worth verifying before you flip the switch:

  1. Stricter TLS certificate verification — certificate configurations that used to pass may now be rejected. If you're relying on internal self-signed certificates, test this explicitly.
  2. Bun.YAML now follows YAML 1.2 — configs that depended on YAML 1.1's type inference (like yes/no parsing as booleans) will parse differently.
  3. No more automatic .env loading when Bun is invoked as node — you'll need to load environment variables explicitly with dotenv or equivalent.

A sane migration path

Rather than flipping production over directly, we'd recommend:

  1. Run Bun 1.4 as a parallel CI job first and confirm your existing test suite still passes
  2. Specifically re-test YAML config parsing and any TLS-dependent connections
  3. Audit scripts relying on implicit .env loading and switch them to explicit loading

For a release with a change this foundational, mapping the blast radius of the breaking changes should come before enjoying the new features.

Source: Bun Blog (bun.com/blog/bun-v1.4)

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