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This document provides a high level overview of jsii, starting with its design tenets. It introduces the concepts and components that compose jsii.

Updating the Specification


The jsii specification follows the guiding principles of an RFC. It is a living document that describes the current understanding of how the various components of jsii are operating, as well as the approaches used to ensure consistent behavior across the various supported languages.

The document is hosted with the jsii codebase, making it easy to determine what specification was in place at the time of a give jsii release (by ways of referring to the vX.Y.Z git tag). A single version of the specification is considered active at any given time: the version of the specification that is represented on the HEAD commit of the main branch of the aws/jsii repository. The active specification must be the base version for any update proposal.

The process to update the specification is intended to be as lightweight as possible, while ensuring sufficient conversation takes place before implementing significant (and breaking) changes. Since the process to update the specification is part of the specification itself, it is amenable to be changed following the process described in the currently active specification.


While the general process for updating the specification is to create a GitHub pull request against the aws/jsii repository, the exact requirements for what should be included in the pull request vary depending on the type of update that is proposed:

  • ⚠ Changing Behavior describes the process to be followed when introducing changes to the behavior of any component of jsii: new features, breaking changes to existing features, ...
  • 🔍 Addressing Gaps is the process used for adding specification around existing but unspecified behavior.
  • 👍 Trivial Changes explains how to propose changes that improve the specification without changing its meaning.

⚠ Changing Behavior

If the change is not backwards compatible (it is a breaking change to an existing feature, or it is a new feature that requires all runtime libraries implement support immediately), a new RFC should be created in the awslabs/aws-cdk-rfcs repository, following the RFC Process. This ensures enough time is spent considering alternatives to breaking changes, and to create consensus that the change is desirable before time is spent implementing it.


While going through the RFC process upfront is strongly recommended, contributors may choose not to file an RFC for a behavior change. In this case however, any core maintainer may decide that an RFC is required and block the contribution until the RFC process has been followed.

It is worth noting that a draft pull request with proposed modifications to the specification (and possibly a proof-of-concept implementation), can be extremely useful.

When the RFC is ready, a GitHub pull request is created that must contain:

  • Relevant additions or modifications to the specification documents
  • Relevant additions or modifications to the compliance suite
  • Implementation of the new behavior, including new or updated tests in all the language bindings

The pull request's body must reference the RFC if there has been one, and otherwise must include all discussion necessary to explain the reasoning behind the proposal (including alternatives considered, risks, ...).

🔍 Addressing Gaps

Proposals that increase the specification's coverage (desribing behavior that already exists) are handled as GitHub pull requests that must contain the following elements:

  • Relevant additions to the specification documents
  • New compliance test(s) that enshrine the described behavior
  • Implementation of the new compliance test(s) for all Generally Available language bindings

The pull request body should provide pointers to any and all elements that can be used to verify that the behavior that is described is indeed what is currently implemented.

👍 Trivial Changes

Proposal of trivial changes, such as correcting typos in the document, or re-phrasing elements of the specification without altering the meaning (typically to improve clarity) are handled in a simple GitHub pull request.

Design Tenets (unless you know better ones)

  • jsii APIs strive to feel idiomatic in all supported languages.
  • jsii applications behave identically regardless of the language they are written in. It favors correctness over performance.
  • jsii does not attempt to support all TypeScript idioms (many features of TypeScript cannot be expressed in some target languages).
  • Unsupported idioms will cause a compile-time error to be emitted.
  • When prohibiting an idiom, jsii strives to provide an error message that gives the user insight into why the pattern cannot be supported.
  • jsii does not force API design opinions on the developer:
  • Reserved names are limited to a minimum.
  • TypeScript API design patterns that are known to result in poor developer experience when represented in other languages will cause warnings to be issued, but the developer is ultimately entitled to decide whether they want to take or leave the advice.
  • jsii produces artifacts compatible with idiomatic tools whenever possible:
  • Generated libraries can be easily published to the "standard" package repository for the language.
  • Standard tools can be used to work with the generated libraries, and do not require any special configuration.


Annotations are present in the jsii specification to provide additional information to the reader that is non-normative. Those take the form of block-quotes that use the following chart:


Is used to annotate parts of the specification that are known to be partially or incorrectly implemented in the current releases. Those are known issues in the current implementation that will be addressed in the future.


Is used to annotate open questions. They are typically in parts of the specification that is likely to change in future releases, and that may be good candidates for introducing RFCs.


Is used to draw the reader's attention on specific points. They are used primarily to help the reader identify areas of the specification that, when incorrectly implemented, may result in hard-to-troubleshoot bugs; or to identify behavior that is intentionally undefined.


Is used to provide additional context which may not be obvious to the reader. They typically contain trivia that can help the reader understand the motivation for certain behaviors that are not always intuitive.


jsii allows developers to author code once in TypeScript, while allowing use in a variety of other programming languages (including C#, Java and Python).


The jsii Assembly document contains a specific representation of the API exported by the TypeScript module. Similar to a header file in the C++ world, it contains only information about the signatures of APIs (type names with method and property signatures, associated documentation elements, ...) and no implementation.

The npm package produced as a result of compiling the TypeScript source remains the source of truth with respects to implementation of the API.

Host & Kernel

The jsii runtime architecture defines two processes:

  1. The host process runs the users' code native environment (a Java virtual machine, the .NET Runtime, ...).
  2. The kernel process hosts the JavaScript code from the standard npm package defined by the user (and their dependencies), which is loaded and managed by a standardized @jsii/kernel package.

The host process is responsible for starting the kernel process as needed. A designated host runtime library provides helper functions that will perform the necessary initialization when needed, so the host app does not need to include any special boilerplate code.

The two processes exchange messages over a designated communication channel (for example, using pipes), using a kernel API that is standardized in the jsii specification.


Several tools are involved in making this possible:

  • jsii is a modified TypeScript compiler. In addition to generating JavaScript code from the source, it produces a jsii Assembly document.
  • jsii-pacmak generates language bindings from a package compiled using jsii. It generates code in host languages that expose the API declared in the jsii Assembly document.
  • Host runtime libraries centralize features used by code generated by jsii-pacmak, such as primitives to interact with the kernel process, so that this code does not need to be duplicated in every generated module.
  • @jsii/kernel (and @jsii/runtime) provide the functionality exposed by the kernel process, and allow the host code to seamlessly interact with the JavaScript implementation.