Benjamin Lannon

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Bundling a Rust library to WebAssembly with Webpack & wasm-pack

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On top of using wasm-pack directly, you can also use it in JS bundlers. In this case, there has been an implementation to integrate wasm-pack into a webpack bundle with little code.

Project setup

You can copy over the project from my getting started with wasm-pack post. The file structure you want to start with is like the following.

├── Cargo.toml
└── src

So you have a src directory that includes your rust code and a Cargo.toml file that will describe the project.

name = "webpack-wasmpack"
version = "0.1.0"
authors = ["Benjamin Lannon <>"]
edition = "2018"

crate-type = ["cdylib"]

wasm-bindgen = "0.2"
extern crate wasm_bindgen;

use wasm_bindgen::prelude::*;

pub fn add(a: i32, b: i32) -> i32 {
a + b

Following, we will want to create a JS file that is the entrypoint for our webpack bundle at lib.js in the root directory.

import("./pkg/").then((lib) => {
// lib is the wasm library you can now use.
console.log(`2 + 2 = ${lib.add(2, 2)}`);

With this, when we build our project, in the pkg folder, there will be a index.js file that will include glue code to fetch the WebAssembly. By using a dynamic import, on this wrapper file, it will asynchronously load the wasm code and then expose the various functions from the rust code, like add.

Setup Webpack

With Webpack in this use case, we can compile rust code alongside our JS into a final bundle.

On top of the default code for a usual webpack bundle, there's a plugin called @wasm-tool/wasm-pack-plugin that makes the process of pulling in rust code built with wasm-pack in better.

Setup a package.json file and install the following packages:

npm init --yes
npm install --save-dev webpack webpack-cli @wasm-tool/wasm-pack-plugin

For this example, we are going to bundle towards a browser environment. Here is the webpack config that will be used for this project:

const path = require("path");
const WasmPackPlugin = require("@wasm-tool/wasm-pack-plugin");

module.exports = {
entry: "./lib.js", // input file of the JS bundle
output: {
filename: "bundle.js", // output filename
path: path.resolve(__dirname, "dist"), // directory of where the bundle will be created at
plugins: [
new WasmPackPlugin({
crateDirectory: __dirname, // Define where the root of the rust code is located (where the cargo.toml file is located)

The only thing out of the ordinary is we have a new plugin of WasmPackPlugin. The one required option is crateDirectory to tell wasm-pack where the root of the Rust code is. In this instance we have it at the root, but we could contain all of the rust code in its own folder if we wanted to.

Then add a build script as part of the package.json and then run it.

// ...
"scripts": {
"build": "webpack"

Loading the outputs in the browser

What is outputted is various build artifacts. There will be a pkg file that includes the output of wasm-pack. Then it will take the index.js and index_bg.wasm file and use them as part of the outputted bundle.

Finally, there will be a dist folder which includes the end result. Create an index.html file in this directory and import the script in:

<!DOCTYPE html>
<html lang="en">
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Wasm-pack / Webpack example</title>
<script src="bundle.js"></script>

Now if you serve this dist folder on a web server that properly serves WASM, you will be able to see it functioning. A notice about that previous statement is the WebAssembly web API which will run WASM needs the .wasm file to be loaded with the MIME type of application/wasm or it won't run. At the moment, there are some hiccups with Firefox loading this, but other browsers that support WASM will work, like Chrome, Edge, Safari, and others. These issues with browsers not being able to load WASM or HTTP Servers not being able to serve it properly will likely be resolved over time.

A http server that serves .wasm files as application/wasm is the node package http-server. You can run it with npx like so:

npx http-server dist

Now this can be used as a starting point and then can be used for bigger applications or in other environments. If you want to target NodeJS or other environments, minimal modifications are needed other than telling webpack and wasm-pack that you want to build for Node. As WebAssembly gains traction, the tooling around it will continue to evolve and be easier and easier to build.