Using itk.js in a web browser application via Webpack

This example demonstrates how to use itk.js in a web browser application built with Webpack. Find the full example in the itk-js/examples/Webpack directory of the GitHub repository.

Since we asynchronously download the itk.js JavaScript and WebAssembly Emscripten modules, a few extra configuration steps are required.

This example assumes you are creating a Node.js package. If you do not already have a package.json file, create one, first.

Add itk to your project’s dependencies:

npm install --save itk

Then, install Webpack-related development dependencies:

npm install --save-dev webpack webpack-cli webpack-dev-server worker-loader babel-loader '@babel/preset-env' '@babel/core' copy-webpack-plugin

Next, create a webpack.config.js file like the following:

const path = require('path')

const webpack = require('webpack')
const CopyPlugin = require('copy-webpack-plugin')

const entry = path.join(__dirname, './src/index.js')
const outputPath = path.join(__dirname, './dist')

module.exports = {
node: {
fs: 'empty',
output: {
path: outputPath,
filename: 'index.js',
module: {
rules: [
{ test: entry, loader: 'expose-loader?index' },
{ test: /\.js$/, loader: 'babel-loader' },
plugins: [
new CopyPlugin([
from: path.join(__dirname, 'node_modules', 'itk', 'WebWorkers'),
to: path.join(__dirname, 'dist', 'itk', 'WebWorkers'),
from: path.join(__dirname, 'node_modules', 'itk', 'ImageIOs'),
to: path.join(__dirname, 'dist', 'itk', 'ImageIOs'),
from: path.join(__dirname, 'node_modules', 'itk', 'MeshIOs'),
to: path.join(__dirname, 'dist', 'itk', 'MeshIOs'),
performance: {
maxAssetSize: 10000000

Replace src/index.js by your Webpack entry point. Replace ./dist/ and the output filename with where you want Webpack to place the generated JavaScript bundle.

The babel-loader rule will transpile JavaScript from the latest language syntax to a syntax supported by existing browser clients. Configure the target browsers to support with a .babelrc file like the following:

presets: [
['@babel/preset-env', {
targets: {
browsers: ['last 2 versions'],

The itk.js Emscripten modules are loaded and executed asynchronously and on demand. This means the client only download the content it needs and the user does not experience interruption of the main user interface thread during computation. However, a few extra configuration steps are required since the modules are not bundled by Webpack.

The CopyPlugin copies itk.js Emscripten modules to distribute along with your Webpack bundle. In this example, we copy all ImageIOs and MeshIOs. In your project, you may want to copy only the ImageIOs or a subset of the ImageIOs, based on your needs. We also copy the WebWorkers, which asynchronously perform IO or run processing pipelines in a background thread.

To change the location of the itk.js web worker and Emscripten modules, set the Webpack resolve.alias setting as described in the Karma configuration below.

Next, define commands to build the project or build the project and start a local development web server in the scripts section of the package.json file,

"scripts": {
"build": "webpack --progress --colors -p",
"start": "webpack-dev-server --content-base ./dist/ --watch-content-base"

Build the project with

npm run build

To start the development web server, run

npm run start

Testing with Karma

This section described how to configure browser-based testing with the Karma test runner.

First, install Karma and a test harness library like tape.

npm install --save-dev karma karma-chrome-launcher karma-tap karma-tap-pretty-reporter karma-webpack tape tap-spec

Next write a karma.config.js file. The itk.js specific sections of this
file are:

files: [
{ pattern: './dist/itk/ImageIOs/**', watched: true, served: true, included: false },
{ pattern: './dist/itk/MeshIOs/**', watched: true, served: true, included: false },
{ pattern: './dist/itk/WebWorkers/**', watched: true, served: true, included: false },

Here, ./test/index.js can be replaced by the path to your testing module. We also serve the itk.js Emscripten and web worker files with Karma’s web server.

Since Karma’s web server serves its files in /base by default, and our files are also in the ./dist/itk directory, we can tell Webpack to use a different path for the itk.js modules when building for Karma tests with:

webpack: {
resolve: {
alias: {
'./itkConfig$': path.resolve(__dirname, 'test', 'config', 'itkConfigTest.js'),
plugins: [
new webpack.DefinePlugin({
__BASE_PATH__: "'/base'"

Where itkConfigTest.js contains:

const itkConfig = {
itkModulesPath: __BASE_PATH__ + '/dist/itk'

export default itkConfig

Create entries in the package.json file to start Karma, and run the tests!

"scripts": {
"build": "webpack --progress --colors -p",
"start": "webpack-dev-server --mode development --content-base ./dist/ --watch-content-base",
"test": "karma start ./karma.conf.js",
"test:debug": "karma start ./karma.conf.js --no-single-run"


npm run test