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Use-Cases & Limitations

The jsii toolchain allows developers to author class libraries once in TypeScript, while allowing their customers to consume them in various other languages. This is used for libraries such as the AWS CDK ( aws-cdk-lib), and is intended for use in similar contexts: client-side, design-time SDKs that do not require long-running processes and is not in the path to serve time-sensitive requests.

Due to limitations of its runtime architecture, jsii libraries for languages other than Javascript/TypeScript are best suited for inclusion in short-running processes, outside of performance-sensitive or memory-constrained contexts.

While async APIs are supported by the jsii type system, support for these should be treated as highly experimental. In particular, support in the various target languages varies greatly, and users may encounter a variety of bugs or surprising behaviors. The current JavaScript-based cross-language interoperability kernel is also known to block execution of background micro-tasks when it's waiting for commands from the host process.

The runtime libraries used by jsii libraries are not currently designed for thread safety, and using jsii libraries in multi-thread contexts is unsupported at the moment.

TypeScript Support

The jsii compiler leverages the original TypeScript compiler API to compile TypeScript source files and produce JavaScript output and TypeScript declaration files, while also extracting the compiled module's API signatures in a jsii assembly file.

To determine the version of TypeScript that is in use by the installed release of jsii, simply look at the jsii --version output:

# jsii --version
5.1.6 (build d8f400c), typescript 5.1.6

You can then refer to the TypeScript documentation to determine which language features are available in that specific TypeScript release.


The jsii compiler imposes some restrictions on what TypeScript features can be used to declare the APIs exported by a jsii module, in order to ensure those APIs can be consistently represented in other languages. For more information, refer to the TypeScript Restrictions page.

Target Languages

The following target languages are currently offered by jsii-pacmak, or are currently being developed:

Language Status
C# Generally Available
Go Generally Available
Java Generally Available
JavaScript Generally Available
Kotlin Development
Python Generally Available
TypeScript Generally Available
Status Definitions
  • Generally Available languages are fully supported. The generated APIs will not change in breaking ways across minor and patch releases.
  • Developer Preview languages are experimental. The generated APIs may change in breaking ways across minor releases.
  • Development languages are not yet ready for consumption. They are actively developed and may still lack certain essential features.