TypeScript 5.1, featuring easier implicit returns for
undefined-returning functions and allowing unrelated types for getters and setters, has moved to a release candidate stage as of May 18. The release candidate (RC) changes the behavior for
init hooks in decorators and emit behavior under
The changes to the emit behavior under
isolatedModules ensure that script files are not rewritten to modules. They also ensure that using the
transpileModule API will not result in script files being interpreted as modules, as
transpileModule assumes the usage of
To correct the behavior of
init hooks in decorators, the evaluation order of decorator-injected initializers was changed for accessor and non-accessor fields. Also new in the RC is the addition of refactoring support for moving declarations to existing files. This feature will be previewed in TypeScript 5.1, and can be tried out in a recent version of the Visual Studio Code editor.
TypeScript 5.1 features a whole host of coding enhancements. With easier implicit returns for
undefined-returning functions, TypeScript 5.1 allows
undefined-returning functions to have no return statement, fixing a situation in which only functions that could have absolutely no return statements were
any-returning functions. This meant that even if a developer explicity indicated that a function returned
undefined, at least one return statement was required.
This was a problem if an API expected a function to return
undefined, in which case at least one explicit return of
undefined or a
return statement and an explicit annotation was needed. This behavior was confusing and frustrating, Microsoft said. Also with TypeScript 5.1, if a function has no return expression and is being passed to something that expects a function to return
undefined, TypeScript infers
undefined for that function’s return type.
TypeScript 5.1 also now allows completely unrelated types for
set accessor properties, provided they have explicit type annotations. This follows TypeScript 4.3 allowing a
set accessor pair to specify two different types.
TypeScript 5.1 is due in a final, production release on May 30. Predecessor TypeScript 5.0 was released last month. The TypeScript 5.1 RC is available through NuGet or via NPM:
npm install -D typescript@rc
Other new capabilities and improvements in TypeScript 5.1:
- TypeScript now provides snippet completions when typing out a
- TypeScript 5.1 avoids performing type instantiation within object types known not to contain references to outer type parameters. This could cut down on many unnecessary computations and reduce type-checking time of Material. UI’s docs directory by more than 50%.
- When checking if a source type is part of a union type, TypeScript 5.1 first does a fast lookup using an internal type identifier for that source. If the lookup fails, TypeScript checks for compatibility against every type within the union.
- TypeScript now only runs on Node.js 14.17 and later.
- Namespaced attribute names now are supported when using the JSX syntax. Also for JSX, TypeScript 5.1 now looks up a type called
JSX.ElementType, which specifies precisely what is valid to use as a tag in a JSX element.
- When TypeScript’s specified module lookup strategy is unable to resolve a path, it now will resolve packages relative to the specified
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