itk.js to itk-wasm Migration Guide

Note: this migration guide is a work in progress. itk-wasm is currently in
the beta development stage. The interface is relatively stable while functionality is currently being fleshed out. While nearly all tests in the test suite pass, GE and JPEG format currently have issues. Last updated March 5th, 2022.

itk-wasm is a major upgrade with a focus on univeral, performant computing in
WebAssembly. The itk.js to itk-wasm transition also brings improvements
including modern, elegant programming interfaces, accelerated performance,
and execution beyond JavaScript thanks to WASI.

Most itk.js client code will simply need to update its import statements to
leverage itk-wasm. However, there are also other more advanced changes. A
description of the changes, their motivation, and example migration code can
be found below.

Module import statements

Browser module import statements have changed from:

import IntTypes from 'itk/IntTypes'


import { IntTypes } from 'itk-wasm'

Node module import migration:


const IntTypes = require('itk/IntTypes.js')


import { IntTypes } from 'itk-wasm'

TypeScript support

Typescript support was added, but, of course, you can still just use JavaScript if you wish. In itk-wasm, adoption of modern JavaScript constructs was extended, e.g. classes.


ECMAScript Module (ESM) support was improved.

There is now an improved approach for web workers for bundlers, CORS constraints.

IO modules are available in separate packages.

IO modules are now installed in separate npm packages to limit the itk-wasm package size. You can install only the packages that you need.

The version of these packages follow the itk-wasm package version and should be kept in sync. The io packages are:

  1. itk-image-io
  2. itk-mesh-io

An example that vendors these package’s webassembly assets into an application for deployment can be found in the Webpack Example.

itkConfig.js content and usage

The itkConfig.js specifies where to look for the io webassembly modules and an optional default base URL for pipeline modules.

The default itkConfig.js is configured to use the JsDelivr CDN, but you may want to override this

In the Webpack Example the io webassembly module assets are vendored into /itk directories, this module looks like:

const itkConfig = {
webWorkersUrl: '/itk/web-workers',
imageIOUrl: '/itk/image-io',
meshIOUrl: '/itk/mesh-io',
pipelinesUrl: '/itk/pipelines',

export default itkConfig

And it can be injected into an application bundle by setting defining alias‘s to the configuration module for ../itkConfig.js and ../../itkConfig.js. For other override configuration options, see the Webpack and Rollup (todo) examples.

CLI options

The --image, -i command line option for the build subcommand is now a global option for all subcommands. For examples

itk-wasm build --image itkwasm/emscripten src/dir

is now:

itk-wasm --image itkwasm/emscripten --source-dir src/dir  build

Image data structure

The itk.Image.direction is now directly a Float64Arrayitk.Matrix has been removed.

Addresses an important issue in image orientation support.

Pixel type, component type identifiers

PixelTypes, used in imageType.pixelType, is now a string enumeration to make the type immediately evident in JSON serializations.

IntTypes and FloatTypes string identifiers have new string identifiers with explicit sizes.

Image file format

The JSON file format is replaced by a WASM file format with a different layout. A directory ending in .iwi or a .iwi.cbor file is supported for images and a directory ending in .iwm and .iwm.cbor is supported for meshes and geometry data structures.

Node Sync functions

Node *Sync functions have been removed – use the equivalent async versions instead.

VTK support

The VTK Docker images, VTK PolyData IO, readPolyDataFile, readPolyDataBlob, readPolyDataArrayBuffer, based on VTK were removed.


The use of IOTypes in pipelines is deprecated and not expected to work in the future. These have been replaced by InterfaceTypes.

Argument order when writing image, mesh binaries

The useCompression argument is now last in writeArrayBuffer, writeImageArrayBuffer, writeImageLocalFile, writeMeshArrayBuffer, writeMeshLocalFile.