Breaking Changes List: TypeScript 3.9 -> TypeScript 4.7

typescript47-breaking-changes.md

The following breaking changes occurred between the release of TypeScript 3.9 and the release of TypeScript 4.7:

TypeScript 4.0

• Attempts to override a parent class's accessor with a property declaration now yield an error (4.0).
• If you enable the --strictNullChecks compiler flag, you have to provide optional arguments for delete operators (4.0)

TypeScript 4.1

• Users cannot mark abstract members as async (4.1).
• The resolve() function in a Promise must return a value (4.1).
• When you enable the --strictNullChecks compiler flag, && expressions that contain any or unknown type variables become falsy (4.1).
• Conditional object spreads create optional properties (4.1).

TypeScript 4.2

• TypeScript outputs a noImplicitAny error when it encounters a yield expression that isn't contextually typed (4.2).
• New parsing approach for illegal Type arguments in JavaScript code (4.2).
• Placing a primitive type on the right side of an in expression yields an error (4.2).

TypeScript 4.3

• TypeScript does not allow the comparison of union enum objects to Numbers (4.3).

TypeScript 4.4

• The TypeScript compiler discards the this value of functions on import (4.4).
• unknown type variables in catch clauses yield errors with the --strict compilation flag (4.4).
• The detection of missing await calls has been improved (4.4).
• TypeScript no longer allows you to initialize abstract properties (4.4).

TypeScript 4.5

• Minor changes to lib.d.ts declarations may affect the compilation process (4.4 and 4.5).
• The introduction of the Awaited type changed the handling of explicit type arguments for asynchronous functions (4.5).
• TypeScript concatenates Template Strings regardless of the build target (4.5).
• The compiler yields an error when it discovers a valid compiler option at the root of tsconfig.json (4.5).
• New assignability restrictions for conditional types (4.5).

TypeScript 4.6

• TypeScript omits unspreadable properties from generic rest types (4.6).
• The TypeScript compiler now yields syntax and binding errors for JavaScript code (4.6).

TypeScript 4.7

• JSX files are subject to stricter ...spread checks (4.7).
• Strings in template expressions are subject to stricter checks (4.7).
• The readFile method is now mandatory for LanguageServiceHost instances (4.7).
• The length property of readonly tuples is now also readonly (4.7).