[cknitt]

Follow-up to #9043: bumps js_of_ocaml and wasm_of_ocaml-compiler from 6.4.1 to 6.5.0 (pins in dune-project and rescript.opam).

Motivation

js_of_ocaml only builds the playground compiler, so this is about the playground and the CI job that builds it. 6.5.0 (changelog) brings:

• Faster linking. Upstream reports about 25% faster compilation of large programs. For the playground bundle, js_of_ocaml uses about 28% less CPU and wasm_of_ocaml about 10% less (table below).
• Smaller output. Identical literals share one variable, and the generated JavaScript is simpler (!x conditions, fewer temporaries, return c ? a : b). compiler.js shrinks by 10% raw and 5.5% gzipped. The Wasm build shrinks by 19%.
• Soundness fixes. These include JavaScript simplification passes that were unsound in corner cases, reference unboxing, an invariant violation with recursive inlining, and the JS printer.
• Keeping current. 6.5.0 supports OCaml 5.6, which matters for later toolchain bumps.

The two breaking changes (Typed_array.float*Array_fromArray and the removed js_of_ocaml-lwt.logger) don't affect us. compiler/jsoo uses neither.

cmij files are now written without build-fs

In 6.5.0, js_of_ocaml build-fs encodes file contents as base64 literals, decoded by the runtime primitive caml_jsbytes_of_base64. Two problems follow for the playground's cmij files:

1. The playground loads them as standalone scripts next to compiler.js, where that primitive isn't in scope, so make playground-test fails with ReferenceError: caml_jsbytes_of_base64 is not defined.
2. Base64 hides the byte structure from compression, so the cmij files got about 40% larger gzipped (runtime cmij: 303 KB → 424 KB). Together the playground's download grew by 13%.

A cmij file is just a list of jsoo_create_file(name, bytes) calls. generate_cmijs.mjs now writes them itself, as escaped byte-string literals like the ones 6.4.1 produced. Loaded into a pseudo filesystem, the new files hold the same 306 files with byte-identical contents as 6.4.1's build-fs output. Generating them no longer needs js_of_ocaml on the PATH.

Measurements

These were taken in a Docker container on macOS with OCaml 5.5.2 and the browser profile, from cold builds. Sizes are deterministic. Times are two runs each, read from dune's trace (process CPU time), and should be treated as indicative only.

Files the playground downloads:

	6.4.1 raw	6.4.1 gzip / brotli	6.5.0 (this PR) raw	6.5.0 gzip / brotli
compiler.js	2,454,408	625,014 / 493,403	2,210,871	590,680 / 473,235
runtime cmij	2,217,670	302,555 / 215,181	2,277,129	309,680 / 217,946
belt cmij	956,802	127,000 / 84,689	978,346	128,796 / 85,333
react cmij	267,784	40,743 / 31,338	270,786	40,501 / 30,853
total	5,896,664	1,095,312 / 824,611	5,737,132	1,069,657 / 807,367 (−2.3% / −2.1%)

With 6.5.0's own build-fs the total would have been 1,239,890 gzip / 935,592 brotli (+13%).

Not shipped: the Wasm build (.bc.wasm) goes from 3,377,349 to 2,734,673 bytes.

Link steps in make playground (CPU time, two runs):

	6.4.1	6.5.0
js_of_ocaml	7.4 s, 7.4 s	5.2 s, 5.4 s
wasm_of_ocaml	27.1 s, 28.1 s	23.8 s, 26.0 s

Notes

• An internal changelog fragment, as in Upgrade OCaml toolchain to 5.5.2 #9043: this changes the toolchain and how the playground is packaged, not compiler behavior.
• CI's opam cache key hashes *.opam, so CI builds a fresh switch with the new pins.
• Locally: opam install js_of_ocaml.6.5.0 wasm_of_ocaml-compiler.6.5.0.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RXfJoyYrXm1fPyDMWV8xnx

[github-actions]

Changelog

🏠 Internal

• Upgrade js_of_ocaml and wasm_of_ocaml to 6.5.0 for a smaller, faster-building playground compiler.

Semantic Diff

2 commits · Full page

Upgrade js_of_ocaml and wasm_of_ocaml to 6.5.0

_{0841977 · cknitt + Claude · Oct 26, 2026, 09:18 UTC}

6.5.0 links the playground compiler faster and emits a smaller bundle. Its build-fs now encodes file contents in base64, which compresses worse and needs a runtime primitive the standalone cmij files cannot reach, so the playground writes its cmij files itself, with the same contents.

✨ Add changelog fragment

_{88c1ef9 · cknitt + Claude · Oct 26, 2026, 09:44 UTC · new}

The changelog check requires a fragment; this one notes the toolchain bump.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.37%. Comparing base (eb08738) to head (88c1ef9).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9051      +/-   ##
==========================================
+ Coverage   81.33%   81.37%   +0.04%     
==========================================
  Files         456      458       +2     
  Lines       64273    64503     +230     
==========================================
+ Hits        52275    52490     +215     
- Misses      11998    12013      +15     

see 10 files with indirect coverage changes

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

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commit: 88c1ef9

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-9051

[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-26T10:25:53.252350Z	88c1ef9	Manual request

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Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: 88c1ef9950

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