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    1.52.1 • Public • Published


    jsii-diff compares two jsii assemblies for compatibility.

    In the future, it will be able to do generic comparisons, but for now it will compare assemblies for API compatibility, and exit with a non-zero exit code if any stable APIs have had incompatible changes.

    API items that have no stability are treated as stable. To treat unmarked API items as experimental, pass the --default-experimental flag.


    To compare two JSII packages:

    jsii-diff <old> [new]

    Packages can be identified by either:

    • A path, in which case it should be the path to a JSII package directory, or to a .jsii file.
    • An NPM package specifier of the form npm:[<package>[@version]], in which case the indicated version is downloaded and used. If @version is left out, the latest version will be used. If package is left out, the assembly name of .jsii in the current directory will be used.

    To compare current package against latest published NPM release:

    jsii-diff npm:


    jsii-diff will assert that code written against version A of a library will still typecheck when compiled against version B of that library. It does this by verifying the following properties:

    • Any type (class/interface/enum) in A must also exist in B.
    • Enums have only added members.
    • Classes and interfaces have only added members, or modified existing members in an allowed way.
    • Property types are the same or have been strengthened (see below).
    • Methods have only added optional arguments, existing argument types have only been weakened, and the return type has only been strengthened (see below).

    Strengthening and weakening

    • Strengthening a type refers to excluding more possible values. Changing a field from optional to required, or changing a type from any to string are examples of strengthening.

    • As the opposite of strengthening, weakening refers to allowing more possible values. Changing a field from required to optional, or changing a type to a superclass or interface are examples of weakening.

    An API can change in the following way without breaking its consumer:

    • It can weaken its input (require less from the caller); and
    • It can strengthen its output (guarantee more to the caller).

    Struct types

    Structs (interfaces consisting completely of readonly properties) are treated as bags of data. Their API compatibility will be evaluated depending on whether they appear in input or output position of operations.

    • Structs are weakened if all types of all of its properties are weakened. Normally removing properties would also be considered weakening, but because that may cause references to the fields in existing code bases to become undefined (which is not allowed in most programming languages) we disallow removing properties.
    • Structs are strengthened if all types of all of its properties are strengthened, or if fields are added.

    jsii-diff will check the evolution of structs against their position in an operation, similar to other types. Input structs may be weakened, and output structs may be strengthened.

    Reference types

    Classes and non-struct interface types are considered "reference types". By default we treat them as being the result of a function call:

    • Class instances are the return values calling their constructors.
    • Interfaces are only ever implemented by objects returned from the framework, or returned by factory functions.

    This means their evolution falls under the rules of "strengthening": they may only add fields, never take any away or make them optional.


    Some classes or interfaces may be intended to be implemented by consumers. Those should be marked with the docstring tag @subclassable.

    This will effectively cause changes against those types to be checked against the rules for weakening as well (i.e., no new (abstract) fields or members added). This is necessary because otherwise any existing implementor of that interface would be broken, since they wouldn't be implementing the new abstract members yet.

    @subclassable is not the default since most interfaces are not intended for subclassing, but treating them as such would limit the evolvability of libraries too much.


    jsii-diff is distributed under the Apache License, Version 2.0.

    See LICENSE and NOTICE for more information.




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