Functions and Type Annotations

Functions also get to participate in the static typing fun. The parameters and return values can have type annotations, and the function as a whole has its own static type.

Function parameters

TypeScript function parameters can have type annotations, with a similar syntax to what we’ve seen for variable declarations. The annotations work whether the function is a named function, an anonymous function, or a method of a class. All three of these examples have the same typing:

function priceOfTicket(row: number, accessType: string) : number { }

let priceOfTicket = function(row: number, accessType: string) : number { }

class Ticket {
  priceOfTicket(row: number, accessType: string) : number { }
}

In all three cases, the function priceOfTicket expects two arguments: the first a number, the second a string, and returns a number.

Let’s talk about the return type first. As currently written, all three of these functions would fail compilation because they claim to return a number, but at the moment they don’t return anything. The TypeScript compiler will not compile a function that sets a return type but does not return a value.

If you want to claim explicitly that the function will not return a value, then you can use the special type void, which means “no value”:

function sendAnAlert(message: string) : void { }

Now you get the opposite behavior from the compiler—if you try to return a value from a void function then the compiler ...