Typing functions

When declaring a function we can type both the arguments and the return value:
function multiply(a: number, b: number): number {
return a * b;

Reusable function type

Let's say the above function implements a very common pattern in out app: do some calculation with two numbers.
function sum(a: number, b: number): number {
return a + b;
function modulo(a: number, b: number): number {
return a % b;
We can create a function type:
type BinaryOperator = (a: number, b: number) => number;
const subtract: BinaryOperator = (a, b) => a - b;

The void type

One type is particularly useful with functions is void: to signify that nothing will be returned from the function:
function log(message): void {

Optional and default arguments

Function arguments can be marked as optional using the ? sign
function done(error?: Error): void {
/* ... */
We can also provide a default value; any argument with a default value will automatically be marked as optional.
function complete(success: boolean = true): void {
/* ... */