TypeScript's specification is very out-of-date. The best "specification" for the language is "whatever the compiler does." (It's a double-edged sword because i

TypeScript's specification is very out-of-date. The best "specification" for the language is "whatever the compiler does." (It's a double-edged sword because it lets the TypeScript compiler move much faster than any other I've seen, but the lack of standardization regularly causes unintended breaking changes, and developing a competing-but-compatible type checker is basically impossible.)