TypeScript 3.5 fixes ‘unbearably slow’ type-checking bug

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TypeScript 3.5, the latest version of Microsoft’s popular typed superset of JavaScript, is now available in a release candidate. The update is set to add optimizations for type-checking, addressing a serious speed regression introduced with the previous release.

With TypeScript 3.5, Microsoft’s TypeScript development team focused on optimizing code paths and stripping down certain functionality to make TypeScript faster. Compile times have fallen compared to ; code completion and other editor operations should be “snappier” as well, the .

In making these optimizations, the TypeScript team sought to mend a regression introduced in TypeScript 3.4 that could lead to a lot more work for the type-checker and increase type-checking times. The regression was serious because, in addition to leading to longer build times, editor operations for TypeScript and JavaScript users became “unbearably slow.” Those most impacted were developers using the styled-components library.

The general release of TypeScript 3.5 is due at the end of May. Other improvements in TypeScript 3.5 include:

  • Caching optimizations impacting the incremental compiler option that reduce the time for rebuilds.
  • A lib.d.ts declaration file to provide an Omit helper type. The compiler will use this type to express types created through object rest destructuring on generics.
  • Improved excess property checks in union types. Excess property checking is intended to find typos when a type is not expecting a specific property. In TypeScript 3.5, the type checker verifies that all provided properties belong to some union member and have the appropriate type.
  • When assigning to types with discriminant properties, the language will decompose types into a union of every possible inhabitant type. This provides smarter type checking.
  • Referencing of UMD (Universal Module Definition) global declarations using an allowUmdGlobalAccess flag.
  • Higher-order type inferencing from generic constructors.
  • TypeScript 3.5 contains some breaking changes, such as generic type parameters constrained to unknown.

You can download the TypeScript 3.5 release candidate from or by using the following NPM command:

npm install -g typescript@rc