Programming TypeScript

Chapter 4. Functions

In the last chapter we covered the basics of TypeScript’s type system: primitive types, objects, arrays, tuples, and enums, as well as the basics of TypeScript’s type inference and how type assignability works. You are now ready for TypeScript’s pièce de résistance (or raison d’être, if you’re a functional programmer): functions. A few of the topics we’ll cover in this chapter are:

• The different ways to declare and invoke functions in TypeScript

• Signature overloading

• Polymorphic functions

• Polymorphic type aliases

Declaring and Invoking Functions

In JavaScript, functions are first-class objects. That means you can use them exactly like you would any other object: assign them to variables, pass them to other functions, return them from functions, assign them to objects and prototypes, write properties to them, read those properties back, and so on. There is a lot you can do with functions in JavaScript, and TypeScript models all of those things with its rich type system.

Here’s what a function looks like in TypeScript (this should look familiar from the last chapter):

function add(a: number, b: number) {
  return a + b
}

You will usually explicitly annotate function parameters (a and b in this example)—TypeScript will always infer types throughout the body of your function, but in most cases it won’t infer types for your parameters, except for a few special cases where it can infer types from context (more on that in “Contextual Typing”). The return ...