TypeScript 6.0 Strict Function Types: Why Contravariance Breaks Your Existing Callbacks

TypeScript 6.0 now enforces strictFunctionTypes by default, changing how function parameter types are checked. This affects callbacks that accept base types where consumers expect derived types—a common source of migration failures. The key issue is contravariance, which determines whether a function that accepts a more general type can safely substitute for one that requires a more specific type.

Before TypeScript 6.0, the compiler allowed bivariant checking, where a function expecting a more specific type could be assigned to a variable typed with a more general type, and vice versa. Now, with strictFunctionTypes enabled, TypeScript rejects such assignments, breaking callbacks that relied on this behavior. The problem arises because callbacks registered with array methods expect specific types, but implementations pass derived types.

This mismatch causes compile-time errors, and fixing them requires understanding the underlying variance rules. The solution isn't to weaken types with any casts, which could introduce runtime bugs, but to restructure callbacks using proper variance-aware patterns or switch to method syntax where bivariance is intentional. The migration challenge lies in the hundreds of callbacks that violate contravariance in codebases that never configured strict checking.

The recommended approach is to widen parameter types to unions, use method syntax for intentional bivariance, or introduce generic constraints that preserve assignability without any. Disabling strictFunctionTypes to silence errors creates a type system that diverges from reality, undermining the compiler's safety guarantees.