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I wanted to know if real Clojure can run on Cloudflare Workers. Not ClojureScript. Not Squint. Actual JVM Clojure, compiled to WebAssembly.

The short answer is yes.

What is the path

Cloudflare Workers runs V8 isolates. Only JavaScript and WASM are allowed. No JVM.

GraalVM 25 has an experimental feature called Web Image. The flag is --tool:svm-wasm. It compiles a JVM application to WebAssembly using the same AOT pipeline as native-image, but targeting WASM instead of a native binary.

This is different from:

  • ClojureScript / Squint - these compile Clojure syntax to JavaScript
  • Babashka - native binary, no WASM target
  • Chicory - a JVM running inside WASM, not the other way

Did anyone do this before

I searched and did not find a working example of JVM Clojure on CF Workers. The closest thing I found:

  • graal-clojure-wasm by roman01la - shows Clojure to WASM compilation working in Node.js. No CF Workers.
  • ClojureScript on CF Workers has examples, but that compiles to JavaScript, not WASM.
  • JVM languages on CF Workers in general seem unexplored. Kotlin and Scala users have asked about it but no working examples.

If you have seen this done somewhere, I would like to know.

Toolchain

  • Oracle GraalVM 25.0.3 (via Homebrew: brew install graalvm-jdk@25)
  • Binaryen v130 (brew install binaryen)
  • Clojure CLI tools

The program

A minimal Clojure namespace:

(ns clj-wasm-worker.core
  (:gen-class))


(defn greet [name]
  (str "Hello from Clojure WASM, " name "!"))


(defn -main [& _args]
  (println (greet "world")))

Build it into an uberjar with AOT compilation:

;; deps.edn
{:paths ["src"]
 :deps {org.clojure/clojure {:mvn/version "1.12.0"}}
 :aliases
 {:uberjar
  {:deps {com.github.seancorfield/depstar {:mvn/version "2.1.303"}}
   :exec-fn hf.depstar/uberjar
   :exec-args {:jar "target/app.jar"
               :main-class clj-wasm-worker.core
               :aot true}}}}
Terminal window
clojure -X:uberjar

Compile to WASM

Terminal window
export GRAALVM_HOME=/Library/Java/JavaVirtualMachines/graalvm-25.jdk/Contents/Home
export PATH=$GRAALVM_HOME/bin:$PATH


native-image --tool:svm-wasm \
  -jar target/app.jar \
  -H:Name=target/wasm/app \
  --no-fallback \
  --initialize-at-build-time

The --initialize-at-build-time flag is important. Without it, Clojure tries to load clojure.core from the classpath at runtime. There is no classpath in a WASM sandbox. The flag bakes the Clojure runtime into the image at compile time.

Output:

Build artifacts:
  target/wasm/app.js       (92KB  - JS glue)
  target/wasm/app.js.wasm  (5.3MB - the WASM binary)
  target/wasm/app.js.wat   (116MB - text format, not deployed)

Gzipped WASM is 2.44MB. CF Workers free tier limit is 3MB. We are inside it, for now.

Run locally

Terminal window
node target/wasm/app.js
# Hello from Clojure WASM, world!

Real Clojure running in WASM. Now we need to get it into CF Workers. That is Part 2.