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
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
An NPM package specifier of the form
npm:[<package>[@version]], in which case the indicated version is downloaded and used. If
@versionis left out, the latest version will be used. If
packageis left out, the assembly name of
.jsiiin the current directory will be used.
To compare current package against latest published NPM release:
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
required, or changing a type from
stringare examples of strengthening.
As the opposite of strengthening, weakening refers to allowing more possible values. Changing a field from
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).
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.
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
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.