A function is an object inside itself. They have properties, methods, and can include inner functions. It's a way to encapsulate a functionality you want to reuse in more than one place in your application; you just need to write the function name instead of all the code inside that, just like the following example:

function sum(numberA, numberB){
    return numberA + numberB
}
sum(4,5) //9
sum(5,2) //7
sum(sum(5,1),9) //15