Bun 1.4.2: Elysia build collision, AsyncLocalStorage leak, CMYK JPEGs (5 Sep 2026)

No affiliate links. This is a frontend runtime note from the Bun v1.4.2 blog, the GitHub release tag, and the npm bun registry. ShopperCove has not run bun upgrade on a production host for this article.

Why 1.4.2 now

npm bun@1.4.2 is the latest dist-tag as of 5 Oct 2026, published 5 Sep 2026 (06:01 UTC / 11:31 IST). The Bun blog post landed the same day (Dylan Conway). Among Prettier (3.9.9, 23 Sep), Rspack (2.2.8, 28 Sep — already packaged), Vite 8.3.2, and Turborepo 2.11.7, this is the open Bun stable pin for the Bun/Prettier/oxc frontend slot.

1.4.2 is a tight patch on the 1.4 line (1.4.0 announced 20 Aug 2026; 1.4.1 on 4 Sep 2026). The operator story is Elysia/bun build correctness, an AsyncLocalStorage memory leak, worker_threads online ordering for @discordjs/ws, and CMYK/YCCK JPEG decode in Bun.Image—plus a JavaScriptCore pull of ~350 WebKit commits. Not a new major.

What 1.4.2 changes

From the Bun v1.4.2 blog:

Fixed: bun build variable name collision (Elysia)

A 1.4.1 regression renamed a nested var to the same name as a let in the same block. Builds importing Elysia failed to load with SyntaxError: Cannot declare a var variable that shadows a let/const/class variable. Fixed, with a regression test. The same bug could also give a let the name of a function parameter or catch binding, producing code that evaluates but computes incorrect values.

Fixed: AsyncLocalStorage memory leak

A 1.4.1 regression: a timer, immediate, or pending promise created inside store.exit() or a nested store.run() kept the outer store value alive for as long as that timer/promise existed. getStore() still returned the correct value—only memory usage grew. Fixed in 1.4.2.

Fixed: worker_threads 'online' event order

A Worker from node:worker_threads did not emit 'online' first, causing @discordjs/ws to hang waiting on a worker's first message. Ordering now matches Node.js.

Fixed: Bun.Image decodes CMYK and YCCK JPEGs

Bun.Image now decodes 4-component CMYK and YCCK JPEGs (previously Image: decode failed). They convert to RGB on decode so transforms and output formats work.

Upgraded JavaScriptCore

About 350 more upstream WebKit commits, including:

• Intl.PluralRules bigint selectors (was TypeError)
• TypedArray constructor/slice correctness when valueOf mutates or Symbol.species overlaps
• Intl.DurationFormat style: "digital" stray : when minutes is 0 and hidden
• Proxy handler crash after Object.setPrototypeOf(handler, null) on a hot class-defined handler

Other bugfixes called out

• Rare JIT crash in long-running processes after a prototype used for cached property lookups was GC'd
• musl (Alpine): Array.prototype.splice / shift / shrinking array.length on object arrays could crash on a GC thread or hang during marking
• .json() on Response / Blob / Bun.file() / proc.stdout now surfaces the real JSON.parse SyntaxError message instead of a generic parse failure
• bun install / bun add no longer panic with range end index out of range on package-name hash mismatches in bun.lockb / cached manifests
• Linux: Bun.file().writer() FileSink no longer double-closes a descriptor when epoll registration fails (e.g. fs.epoll.max_user_watches exhausted)

Upgrade checklist

1. Upgrade exactly: bun upgrade (or install via bun.com/docs / the install script) and confirm bun --version prints 1.4.2.
2. If you ship Elysia (or any app that hit the bun build shadowing SyntaxError on 1.4.1): rebuild and smoke-load the output.
3. If you use AsyncLocalStorage with store.exit() / nested store.run() plus long-lived timers or promises: watch RSS after upgrade; the leak was silent.
4. If you use @discordjs/ws (or other code that awaits Worker 'online' before the first message): re-test connect.
5. If you decode print/CMYK JPEGs through Bun.Image: re-run those transform paths.
6. Alpine/musl hosts: prefer 1.4.2 before relying on array mutate + GC under load.
7. No urgency invented beyond the named regressions—this is a patch on an already-shipped 1.4 line, not a CVE advisory of its own.

ShopperCove did not change production Bun versions for this article. Prefer bun.com/blog and the GitHub tags over third-party roundups.

Bottom line

Bun 1.4.2 (5 Sep 2026) is the pin to take if you are on 1.4.1 and hit Elysia/bun build shadowing, an ALS leak under exit()/run(), a @discordjs/ws hang, or CMYK JPEG decode failures—plus the bundled JSC correctness pull. No affiliate.

Sources

• 01bun.com/blog/bun-v1.4.1
• 02bun.com/blog/bun-v1.4.0
• 03registry.npmjs.org/bun/1.4.2
• 04github.com/oven-sh/bun

Related

• 01shoppercove.com/blog/rspack-2-2-8-resolver-cache-memory-october-2026
• 02shoppercove.com/blog/vite-8-3-2-renderbuilturl-bundled-dev-sourcemaps-october-2026
• 03shoppercove.com/blog/turborepo-2-11-7-sdkroot-oidc-cache-october-2026
• 04shoppercove.com/blog/pnpm-12-9-1-rust-rewrite-wasm-split-october-2026
• 05shoppercove.com/blog/nodejs-26-lts-october-2026-schedule-change
• 06shoppercove.com/blog/oxlint-1-87-react-suggestions-a11y-fixes-october-2026
• 07shoppercove.com/blog/biome-2-5-15-type-inference-nursery-rules-october-2026
• 08shoppercove.com/blog/eslint-10-12-release-october-2026