Mastering JavaScript Functional Programming - Second Edition

Signatures for functions 

The specification of a function's arguments and the result is given by a signature. Type signatures are based on a type system called Hindley-Milner, which influenced several (mostly functional) languages, including Haskell, though the notation has changed from that of the original paper. This system can even deduce types that are not directly given; tools such as TypeScript or Flow also do this, so developers don't need to specify all types. Instead of going for a dry, formal explanation about the rules for writing correct signatures, let's work by examples. We only need to know the following:

• We will be writing the type declaration as a comment.
• The function name is written first, and then ::, which can be read ...