Yesterday, Daniel Rosenwasser, Program Manager at TypeScript, announced the release of TypeScript 3.5. This release has great new additions in compiler and language, editor tooling, some breaking changes as well. Some key features include speed improvements, ‘omit’ helper type, improved excess property checks, and more. The earlier version of TypeScript 3.4 was released two months ago.
Compiler and Language
Speed improvements
Typescripts team have been focusing heavily on optimizing certain code paths and stripping down certain functionality, since the past release. This has resulted in TypeScript 3.5 being faster than TypeScript 3.3 for many incremental checks. The compile time of TypeScript 3.5 has also fallen compared to 3.4, but users have been alerted that code completion and any other editor operations would be much ‘snappier’. This release also includes several optimizations to compiler settings such as why files were looked up, where files were found, etc. It’s also been found that in TypeScript 3.5, the amount of time rebuilding can be reduced by as much as 68% compared to TypeScript 3.4.
The ‘Omit’ helper type
Usually, users create an object that omits certain properties. In TypeScript 3.5, a new version of ‘Omit’ has been defined. It will include its own lib.d.ts which can be used everywhere. The compiler itself will use this ‘Omit’ type to express types created through object rest, destructuring declarations on generics.
Improved excess property checks in union types
TypeScript has this feature of excess property checking in object literals. In the earlier versions, certain excess properties were allowed in the object literal, even if it didn’t match between Point and Label. In this new version, the type-checker will verify that all the provided properties belong to some union member and have the appropriate type.
The –allowUmdGlobalAccess flag
In TypeScript 3.5, you can now reference UMD global declarations like export as namespace foo. This is possible from anywhere, even modules by using the new –allowUmdGlobalAccess flag.
Smarter union type checking
When checking against union types, TypeScript usually compares each constituent type in isolation. While assigning source to target, it typically involves checking whether the type of source is assignable to target. In TypeScript 3.5, when assigning to types with discriminant properties like in T, the language actually will go further and decompose types like S into a union of every possible inhabitant type. This was not possible in the previous versions.
Higher order type inference from generic constructors
TypeScript 3.4’s inference allowed newFn to be generic. In TypeScript 3.5, this behavior is generalized to work on constructor functions as well. This means that functions that operate on class components in certain UI libraries like React, can more correctly operate on generic class components.
New Editing Tools
Smart Select
This will provide an API for editors to expand text selections farther outward in a syntactical manner. This feature is cross-platform and available to any editor which can appropriately query TypeScript’s language server.
Extract to type alias
TypeScript 3.5 will now support a useful new refactoring, to extract types to local type aliases. However, for users who prefer interfaces over type aliases, an issue still exists for extracting object types to interfaces as well.
Breaking changes
Generic type parameters are implicitly constrained to unknown
In TypeScript 3.5, generic type parameters without an explicit constraint are now implicitly constrained to unknown, whereas previously the implicit constraint of type parameters was the empty object type {}.
{ [k: string]: unknown } is no longer a wildcard assignment target
TypeScript 3.5 has removed the specialized assignability rule to permit assignment to { [k: string]: unknown }. This change was made because of the change from {} to unknown, if generic inference has no candidates.
Depending on the intended behavior of { [s: string]: unknown }, several alternatives are available:
- { [s: string]: any }
- { [s: string]: {} }
- object
- unknown
- any
Improved excess property checks in union types
- Typescript 3.5 adds a type assertion onto the object (e.g. { myProp: SomeType } as ExpectedType)
- It also adds an index signature to the expected type to signal, that unspecified properties are expected (e.g. interface ExpectedType { myProp: SomeType; [prop: string]: unknown })
Fixes to unsound writes to indexed access types
TypeScript allows you to represent the operation of accessing a property of an object via the name of that property. In TypeScript 3.5, samples will correctly issue an error. Most instances of this error represent potential errors in the relevant code.
Object.keys rejects primitives in ES5
In ECMAScript 5 environments, Object.keys throws an exception if passed through any non-object argument. In TypeScript 3.5, if target (or equivalently lib) is ES5, calls to Object.keys must pass a valid object. This change interacts with the change in generic inference from {} to unknown.
The aim of this version of TypeScript is to make the coding experience faster and happier. In the announcement, Daniel has also given the 3.6 iteration plan document and the feature roadmap page, to give users an idea of what’s coming in the next version of TypeScript.
Users are quite content with the new additions and breaking changes in TypeScript 3.5.
TypeScript 3.5 fixes ‘unbearably slow’ type-checking bug https://t.co/eaYg9oWFal
— David Papp (@DavidPapp) May 21, 2019
Great to see Omit is now included in #typescript 3.5
— Sebastien Lorber (@sebastienlorber) May 29, 2019
A user on Reddit comments, “Those are some seriously impressive improvements. I know it’s minor, but having Omit built in is just awesome. I’m tired of defining it myself in every project.”
To read more details of TypeScript 3.5, head over to the official announcement.
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