Parameters and Return Types

Annotate the parameters; let the return type be inferred unless stating it adds something.

Stating it does add something in one important case: it makes the compiler check the function against your intention, rather than infer whatever your mistake produced. A function meant to return a number that accidentally returns a string is only caught if you said what it should return.

Two versions of the same mistake. The first reports it where the mistake is; the second reports it wherever somebody uses the result.

Optional and default parameters.

function f(a: string, b?: number) makes b optional, so its type is number | undefined and you must handle that.

function f(a: string, b = 0) gives b a default, which also gives it its type, and it is never undefined inside.

Optional parameters must come after required ones, for the obvious reason.