NodeJS Interop

A Juvix module can be compiled to a Wasm module. When a Wasm module is instantiated by a host, functions from the host can be injected into a Wasm module and functions from the Wasm module can be called by the host.

In this tutorial you will see how to call host functions in Juvix and call Juvix functions from the host using the Wasm mechanism.

The Juvix module

The following Juvix module has two functions.

The function hostDisplayString is an axiom with no corresponding compile block that implements it. We will inject an implementation for this function when we instantiate the module from NodeJS.

The function juvixRender is a normal Juvix function. We will call this from NodeJS.

-- NodeJsInterop.juvix
module NodeJsInterop;

open import Stdlib.Prelude;

axiom hostDisplayString : String → IO;

juvixRender : IO;
juvixRender := hostDisplayString "Hello World from Juvix!";


Compiling the Juvix module

The Juvix module can be compiled using the following command:

juvix compile -t wasm -r standalone NodeJsInterop.juvix

This will create a file containing a Wasm module called NodeJsInterop.wasm.

The NodeJS module

The following NodeJS module demonstrates both calling a Juvix function from NodeJS and injecting a NodeJS function into a Juvix module.

The NodeJS function hostDisplayString is passed to the Wasm module NodeJSInterop.wasm when it is instantiated. After instantiation the Juvix function juvixRender is called.

The functions ptrToCstr and cstrlen are necessary to convert the char pointer passed from Juvix to a JS String.

// NodeJSInterop.js
const fs = require('fs');
let wasmModule = null;

function cstrlen(mem, ptr) {
    let len = 0;
    while (mem[ptr] != 0) {
    return len;

function ptrToCstr(ptr) {
    const wasmMemory = wasmModule.instance.exports.memory.buffer;
    const mem = new Uint8Array(wasmMemory);
    const len = cstrlen(mem, ptr);
    const bytes = new Uint8Array(wasmMemory, ptr, len);
    return new TextDecoder().decode(bytes);

function hostDisplayString(strPtr) {
    const text = ptrToCstr(strPtr);

const wasmBuffer = fs.readFileSync("NodeJsInterop.wasm");
WebAssembly.instantiate(wasmBuffer, {
    env: {
}).then((w) => {
    wasmModule = w;

Running the Wasm module

Now you should have the files NodeJsInterop.wasm and NodeJsInterop.js in the same directory. Run the following command to execute the module:

node NodeJsInterop.js

You should see the following output:

Hello World from Juvix!