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TypeScript Restrictions

Since the API exported by jsii modules will be represented in a variety of other languages, certain restrictions are enforced by the jsii compiler.


Since those restrictions are solely intended to ensure the exported API can be represented in other languages, they do not apply to any internal declarations such as private members and declarations annotated with the /** @internal */ tag.


A jsii module can declare dependencies on any other jsii module by adding entries in the dependencies key in the package.json file. Since most other platforms do not support multiple different versions of the same library to coexist in the same closure, it is recommended to also declare all such dependencies as peerDependencies.

non-jsii dependencies

Occasionally, a dependency on a non-jsii module is useful. Since such dependencies do not have generated bindings in all the supported languages, they must be bundled with the jsii module that depends on them, by adding the library into the bundleDependencies array in package.json.

The API of the jsii module can not expose any type from bundled dependencies, since those types would not be available in other languages. TypeScript files that include a non-jsii dependency (e.g. a lambda handler for an AWS CDK Construct) cannot be exported from the main/types entry point.


For more information on package.json file contents, refer to the npm documentation.


Class Members

Members of classes cannot share the same PascalCased representation as the declaring class itself, as this results in invalid C# code:

export class Foo {
  // 💥 Illegal property name
  public foo: string;

export class Bar {
  // 💥 Illegal method name
  public bar(): void { /* ... */ }


Due to existing usage, this restriction is only enforced when jsii is invoked with the --strict option. The generated C# class name will receive a _ suffix if any of it's members conflict, which is a breaking change to existing .NET consumers when a conflicting member is introduced after the initial release.


The jsii type model distinguishes two kinds of interfaces:

  • Behavioral interfaces, which can declare methods and properties
  • Structs, which are immutable pure data entities, and can consequently only declare readonly properties

A name convention is used to distinguish between these two: behavioral interfaces must have a name that starts with a I prefix, while structs must not have such a prefix.


The /** @struct */ type system hint can be used to force an interface with a name starting with the I prefix to be handled as a struct instead of a behavioral interface.

 * Since there is no `I` prefix, Foo is considered to be a struct.
export interface Foo {
  // 💥 Structs are not allowed to declare methods
  foo(): void;
  // 💥 Structs are not allowed to declare mutable properties
  mutableProperty: number;
  // ✅ Structs can declare immutable properties
  readonly immutableProperty: string;

 * Since there is an `I` prefix, IFoo is considered to be a behavioral interface.
export interface IFoo {
  // ✅ Behavioral interfaces can declare methods
  foo(): void;
  // ✅ Behavioral interfaces can declare mutable properties
  mutableProperty: number;
  // ✅ Behavioral interfaces can declare immutable properties
  readonly immutableProperty: string;



As structs are pure data entities, they can only be extended by other structs:

export interface Struct { /* ...readonly properties... */ }

// 💥 Structs cannot be extended by behavioral interfaces
export interface IBehavioralInterface extends Struct { /* ... */ }

// 💥 Structs cannot be implemented by classes
export class ConcreteType implements Struct { /* ... */ }

// ✅ Structs can extend other structs
export interface SuperStruct extends Struct { /* ...readonly properties */ }

Member Visibility

The C# language does not allow overriding members to change the visibility of the overridden declaration, even if the updated visibility is more permissive. As a consequence, overridden members are required to retain the same visibility as their parents.


This makes changing the visibility of a protected member to public is a breaking change in jsii libraries!

export class Base {
  protected method(): void { /* ... */ }

export class Subclass extends Base {
  // 💥 Illegal change of visibility from protected to public
  public method(): void { /* ... */ }

Covariant Overrides & Parameter List Changes

In TypeScript, overriding members can have a signature that differs from the overridden member as long as the new signature is compatible with the parent. This is however not supported as:

  • Java and C# do not support omitting parameters when overriding or implementing a member
  • C# does not support overriding or implementing a member using covariant parameter or return types


C# 9 introduces support for covariant return types, which would allow relaxing this restriction, however jsii must build code targetting the .NET Core 3.1 runtime, which only supports C# 8. Once .NET Core 3.1 becimes end-of-life, this may change.

export class Base {
  public method(param: any): any { /* ... */ }

export class Child extends Base {
  // 💥 Parameter signatures do not match
  public method(): any { /* ... */ }

  // 💥 Parameter types do not match, even though they are covariant
  public method(param: string): any { /* ... */ }

  // 💥 Return type does not match, even though it is covariant
  public method(param: any): string { /* ... */ }

Index Signatures

TypeScript allows declaring additional properties through the use of index signatures. These are however not supported by the jsii type system and are rejected by the compiler.


Version 1.x of the compiler silently ignores index signatures instead of reporting a compilation error. Users with offending APIs migrating from jsii@1.x to jsii@5.0 or newer need to either remove those declarations, or explicitly ignore them using the @jsii ignore tag.

export interface WithIndexSignature {
  public readonly name: string;

  // 💥 Index signatures are not supported
  public [key: string]: unknown;

TypeScript Mapped Types


These are also referred to as "Generics", "Parameterized Types", "Utility Types", ...

Parameterized types are not consistently supported in all supported target languages. Go support for generics is relatively limited (for good reasons) compared to TypeScript and most Object-Oriented languages, and the differences in generic semantics between TypeScript, C# and Java make it difficult to correctly represent such types in all those languages. As a consequence, jsii does not support declaring parameterized types.

Certain built-in TypeScript types can however be used in jsii modules:

  • Array<T>, which is equivalent to T[]
  • Record<string, T>, which is equivalent to { [key: string]: T }
  • Promise<T>, which is the return type of any asynchronous method
// 💥 Parameterized types cannot be introduced
export interface Parameterized<T> {
  readonly value: T;

export interface IAsyncFooMaker {
  // ✅ Asynchronous methods must return promises
  makeFoo(): Promise<Foo>;
  // ✅ Arrays are supported
  makeFoos(count: number): Array<Promise<Foo>>;

Pick<T, Keys> and Omit<T, Keys>

Users are often tempted to use Pick<T, Keys> and Omit<T, Keys> when creating higher level abstractions of types exposed by their dependencies, and they want to expose all configuration options from the upstream implementation except for some specific properties which are determined fully by the new abstraction.

Pick<T, Keys> and Omit<T, Keys> are not supported as they would result in open-ended implementation requirements to exist in languages such as Java and C# where such things are not possible.

Users with this particular use-case should investigate generating code in order to reproduce the upstream type without the filtered out fields. For example, this can be done with projen using jsii-struct-builder.

Type Aliases

TypeScript supports type aliasing using the export type Name = ... syntax. While this is not considered a compilation error by the jsii compiler, those types are implicitly de-sugared by the compiler for all language targets except for TypeScript.

// 👻 Only visible in TypeScript
export type FooOrBar = Foo | Bar;

export interface Props {
  // ⚠️ Effectively `readonly fooOrBar: Foo | Bar;` in non-TypeScript
  readonly fooOrBar: FooOrBar;

Soft-Reserved Words

In order to guarantee a consistent developer experience across all supported languages, jsii emits warnings whenever a declaration is named after any target language's reserved words, as those will need renaming in target languages:

C# Java Python Go
abstract abstract False break
base assert None case
bool boolean True chan
byte byte and const
char char assert continue
checked double def default
decimal final del defer
delegate float elif else
double goto except fallthrough
event int from for
explicit long global func
extern native is go
fixed short lambda goto
float strictfp nonlocal if
foreach synchronized not import
goto throws or interface
implicit transient pass map
int volatile raise package
internal range
is return
lock select
long struct
namespace switch
object type
operator var


The list of reserved words is derived from jsii/lib/reserved-words.ts.