Typing Functions

Introduction

Functions are where types pay off most. A function is a promise: "give me these inputs and I will give you this output". Types write that promise down, so every caller and the function itself can be checked against it.

Parameter and return types

• width: number types each parameter. You almost always annotate parameters, because TypeScript cannot guess them.
• : number after the brackets is the return type. It is often optional, since TypeScript infers it from the return, but writing it documents your intent and catches mistakes inside the function.

Return the wrong thing and TypeScript tells you:

Calling a function with the wrong number of arguments is an error too:

Optional and default parameters

Add ? to make a parameter optional. Inside the function its type includes undefined, so check before using it.

A default value also makes a parameter optional and gives it a type automatically:

void: no return value

A function that returns nothing has the return type void.

Rest parameters

Rest parameters collect extra arguments into an array. Type it as an array:

Function types

Sometimes a function is a value you pass to another function, like a callback. Describe its shape with an arrow-style type:

(n: number) => number reads as "a function that takes a number and returns a number". Notice you didn't have to annotate n in the arrow functions: TypeScript knows the callback's parameter type from fn's type. This is called contextual typing.

Try it yourself

Write a function average that takes an array of numbers and returns their average as a number. Give it parameter and return types. For an empty array, return 0. The stub below has a deliberate mistake in the parameter type: the type checker complains when you call it with numbers.

Summary

• Annotate parameters, and consider annotating the return type.
• ? and default values make parameters optional. void means nothing is returned.
• Rest parameters are typed as arrays, like ...numbers: number[].
• Describe callbacks with a function type such as (n: number) => number.