What is the 'Double Assertion' pattern and when is it necessary?

Double assertion (expr as unknown as T) is used to force a type conversion between two types that have no overlap.

TypeScript normally prevents assertions between types that it deems completely unrelated to prevent obvious bugs. However, in low-level library code, unit testing, or when dealing with legacy JS, you might know more than the compiler. By first asserting to unknown or any, you effectively 'reset' the type checking, allowing the final assertion to the target type.