Functions are just objects in Javascript, which means that they can have properties just like any other object.
Having properties on a function can be useful if you want to make your function configurable, or if you want a static propery for your function. This can be a nice alternative to creating a class or a wrapper object to maintain properties.
Example
sayHello.firstName = 'Chris';
console.log(sayHello()); // "Hello Chris"
function sayHello() {
return `Hello ${sayHello.firstName}`;
}