TypeScript type system is just a formalisation of JavaScript types. You can represent runtime types of JavaScript statically by annotating them in the source code. But at the same time it is quite unique compared to other programming language type systems as it is optional and not “provably type safe”.

• Optional

Because TypeScript is a strict superset of JavaScript, it must accept all legal existing JavaScript code as valid TypeScript code. This means that TypeScript can’t enforce programmers to annotate every type with type annotations.

With optional typing, you can incrementally change your existing JavaScript code into TypeScript code. As you add more type annotations, you get more help from the toolset.

• Not “provably type safe”

TypeScript type system is not for correctness safe or performance safe because it is impossible to guarantee the correctness of JavaScript with only optional typing. Traditionally, a type system that is unsound is useless as it proves nothing.

However, TypeScript type system is still meaningful because it provides information needed to implement static analysis, code completion and refactoring capabilities in tools. So we can say that TypeScript type system is for tooling safe.

TypeScript also supports type inference and structural typing to help programmers write type annotations more succinctly. It is still evolving by introducing Generics, Tuple Type, Union Type and Type Alias. I will talk more about these in later articles.