WebAssembly and nodejs

Call WebAssembly functions from a nodejs web application.

In previous tutorials, we discussed how to access WebAssembly functions from JavaScript applications hosted inside web browsers. However, as we also noted, there are great use cases for WebAssembly on the server-side, especially for AI, blockchain, and big data applications. In this example, I will show you how to incorporate WebAssembly functions, written in Rust, into nodejs applications on the server. We can provide WebAssembly functions as a microservice.

The demo application is structured as follows.

  • The host application is a nodejs web application written in JavaScript. It makes WebAssembly function calls.

  • The WebAssembly bytecode program is written in Rust. It is called from the nodejs web application.

The source code of the tutorial is here.


As in the previous tutorial, we use the wasm-pack tool to compile the Rust source code and generate the corresponding JavaScript module. The generated module makes it easy to pass complex and dynamic data between JavaScript and Rust functions. You can learn more about this issue here. Follow the steps below to install Rust and the wasm-pack tool.

# Install Rust
$ sudo apt-get update
$ sudo apt-get -y upgrade
$ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
$ source $HOME/.cargo/env
# Install wasm-pack tools
$ curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh

WebAssembly program in Rust

In this example, our Rust program appends the input string after “hello”. Let’s create a new cargo project. Since this program is intended to be called from a host application, not to run as a stand-alone executable, we will create a hello project.

$ cargo new --lib hello
$ cd hello

Edit the Cargo.toml file to add a [lib] section. It tells the compiler where to find the source code for the library and how to generate the bytecode output. We also need to add a dependency of wasm-bindgen here. It is the utility wasm-pack uses to generate the JavaScript binding for the Rust WebAssembly program.

name = "hello_lib"
path = "src/lib.rs"
crate-type =["cdylib"]
wasm-bindgen = "0.2.50"

Below is the content of the Rust program src/lib.rs. You can actually define multiple external functions in this library file, and all of them will be available to the host JaveScript app via WebAssembly.

use wasm_bindgen::prelude::*;
pub fn say(s: String) -> String {
let r = String::from("hello ");
return r + &s;

Next, you can compile the Rust source code into WebAssembly bytecode and generate the accompanying JavaScript module for the nodejs host environment.

$ wasm-pack build --target nodejs

The result are the following three files. the .wasm file is the WebAssembly bytecode program, and the .js files are for the JavaScript module.


The nodejs host application

Next, let’s create a node folder for the nodejs web application. Copy over the generated JavaScript module files.

$ mkdir node
$ cp pkg/hello_lib_bg.wasm node/
$ cp pkg/hello_lib_bg.js node/
$ cp pkg/hello_lib.js node/

With the generated hello_lib.js module, it is very easy to write JavaScript to call WebAssembly functions. Below is the node application app.js. It simply imports the say() function from the generated module. The node application takes the name parameter from incoming an HTTP GET request, and responds with “hello name”.

const { say } = require('./hello_lib.js');
const http = require('http');
const url = require('url');
const hostname = '';
const port = 8080;
const server = http.createServer((req, res) => {
const queryObject = url.parse(req.url,true).query;
res.statusCode = 200;
res.setHeader('Content-Type', 'text/plain');
server.listen(port, hostname, () => {
console.log(`Server running at http://${hostname}:${port}/`);

Start the nodejs application server as follows.

$ node app.js
Server running at

Then, you can test it.

$ curl
hello Wasm

What’s next?

Rust developers rejoice! Web services can now offload computationally intensive, unsafe, and novel hardware access tasks to WebAssembly. More examples and use cases to come. Stay tuned!