Exporting JavaScript functions through WebAssembly

To export exported JavaScript functions, you can pass a .wit file and wit world when running javy build.

Only ESM exports are supported (that is, Node.js/CommonJS exports are not supported). For each exported JavaScript function, Javy will add an additional function export to the WebAssembly module. Exported functions with arguments and generators are not supported. Return values will also be dropped and not returned. The Wasm module generated is a core Wasm module, not a Wasm component.

An example looks like:

index.js:

export function foo() {
  console.log("Hello from foo!");
}

console.log("Hello world!");

index.wit:

package local:main;

world index-world {
  export foo: func();
}

Run:

$ javy build index.js -C wit=index.wit -C wit-world=index-world -o index.wasm
$ wasmtime run --invoke foo index.wasm
Hello world!
Hello from foo!

The wit package name and wit world name do not matter as long as they are present and syntactically correct (that is, it needs to be two names separated by a :). The name of the wit world (that is, the value after world and before {) must be passed via -C wit-world argument. The -C wit-world argument identifies the wit world in the wit file for the Wasm module generated by javy build.

Exports with multiple words

Exported function names with multiple words have to be written in kebab-case in the .wit file (a restriction imposed by wit), they are exported from the Wasm module as kebab-case to match the WIT, and Javy will match the WIT export to a JavaScript export with the same name but in camel-case.

index.js:

export function fooBar() {
  console.log("In foo-bar");
}

index.wit:

package local:main;

world index {
  export foo-bar: func();
}

Run:

$ javy build index.js -C wit=index.wit -C wit-world=index -o index.wasm
$ wasmtime run --invoke foo-bar index.wasm
In foo-bar

Exporting a default function

Exporting a function named default in the WIT world exports a function named default on the Wasm module and corresponds to either an exported default function or exported default arrow function in JS.

index.js:

export default function () {
  console.log("In default");
}

index.wit:

package local:main;

world index {
  export default: func();
}

In the terminal:

$ javy build index.js -C wit=index.wit -C wit-world=index -o index.wasm
$ wasmtime run --invoke default index.wasm
In default

You can also export a default function by writing:

export default () => {
  console.log("default");
}