What are 'Mapped Type As-Clauses' and how are they used for filtering?

The 'as' clause in a mapped type allows you to filter out or rename keys by mapping them to 'never' or template strings[cite: 1].

It provides a declarative way to create sub-types based on key or value attributes. This is used to implement logic like 'extract only string-based properties from this interface'[cite: 1].