Migration Guide: From itk.js to ITK-Wasm

Note: ITK-Wasm is currently in the beta development stage. The interface is relatively stable while functionality is currently being fleshed out. Updates for the rest of the documentation for changes from itk.js to ITK-Wasm is in progress. Last updated December 14th, 2023.

ITK-Wasm is a major upgrade with a focus on universal, 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 is now standard.

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

itkConfig.js removed

The itkConfig.js module in itk-js and initial versions of ITK-Wasm specified where to look for the io webassembly modules and an optional default base URL for pipeline modules.

This build-time approach has been replaced by a run-time configuration approach, which is much easier to specify and works transparently in a nested dependency situation.

There are now only two configuration settings.

  1. The location of the web worker module.

  2. The location of wasm module assets.

Specification of the web worker module location

By default, the web worker module will be picked up by modern bundlers like Vite or WebPack without additional configuration, and the bundler will manage the location of and deployment of the bundle in a application build.

ITK-Wasm packages also contain module bundle that embed the web workers.

Finally, on an individual ITK-Wasm package scope or bundle scope via the itk-wasm package, the web worker can be vendored and its location at specified at runtime via setPipelineWorkerUrl.

Specification of the wasm asset location

Previously, separate wasm asset locations were configured for io modules and general pipelines. Now, there is a single wasm asset configuration. It can be set on a per-package level or a common configuration for a bundle can be set.

By default, the wasm asset location is configured to use the JsDelivr CDN for a package.

On an individual ITK-Wasm package scope or bundle scope via the itk-wasm package, wasm assets can be vendored and their location specified at runtime via setPipelinesBaseUrl.

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 io packages are:

  1. @itk-wasm/image-io

  2. @itk-wasm/mesh-io

  3. @itk-wasm/dicom

An example that vendors these package’s webassembly assets into an application for deployment can be found in the Vite-based test applcation configuration.

DICOM image IO functions

The following DICOM image IO functions have been migrated to the @itk-wasm/dicom package. Their interface changed in some cases.

  1. readImageLocalDICOMFileSeries -> readImageDicomFileSeriesNode

  2. readImageDICOMFileSeries -> readImageDicomFileSeries

  3. readImageDICOMArrayBufferSeries -> readImageDicomFileSeries

  4. readDICOMTags -> readDicomTags

  5. readDICOMTagsArrayBuffer -> readDicomTags

  6. readDICOMTagsLocalFile -> readDicomTagsNode

Migrated image-io and mesh-io functions

The following image and mesh IO functions have been migrated to the @itk-wasm/image-io and @itk-wasm/mesh-io package. Their interface changed in some cases.

  1. readImageArrayBuffer -> readImageFile

  2. readImageBlob -> readImageFile

  3. readImageFileSeries -> readImageFileSeries

  4. readImageHTTP -> readImageHttp

  5. readImageLocalFile -> readImageNode

  6. readMeshArrayBuffer -> readMeshFile

  7. readMeshBlob -> readMeshFile

  8. readMeshLocalFile -> readMeshNode

  9. writeImageArrayBuffer -> writeImageFile

  10. writeImageLocalFile -> writeImageFileNode

  11. writeMeshArrayBuffer -> writeMeshFile

  12. writeMeshLocalFile -> writeMeshNode


The following mesh to polydata conversion functions have been migrated to the @itk-wasm/mesh-to-poly-data package.

  1. meshToPolyData

  2. meshToPolyDataNode

  3. polyDataToMesh

  4. polyDataToMeshNode

Removed high level IO functions

The high level IO functions, that dealt with both images and meshes have been removed.

These are:

  1. readArrayBuffer

  2. readBlob

  3. readFile

  4. readLocalFile

  5. writeArrayBuffer

  6. writeLocalFile

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.

For compatibility with vtk.js vtkImageData, use vtk.js vtkITKHelper 23.4.1 or later (vtkITKHelper.convertItkToVtkImage example).

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.

GE image file formats

Support for the legacy GE image file formats, i.e. GE4, GE5, GEAdw, is no longer provided.


The use of IOTypes in pipelines is deprecated has been removed. 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.


runPipelineBrowser was renamed to runPipeline.