What is functional programming?

What is functional programming?

a programming paradigm (worldview, mindset)
it's kind of a way of looking at a world in terms of what the world is composed of.

Imperative
follow my commands
do this, then that

Object-Oriented
keep state to yourself
send/receive messages

Declarative
this is what I want
I don't care how you do it e.g. SQL

Functional
sort of a sub-paradigm of Declarative

What is functional programming?
one simple idea. pure functions (only input in, only output out)

Not pure:
var name = "Saint Petersburg";
function greet() {
    console.log("Hello, " + name + "!");
}

greet();
// Hello, Saint Petersburg!


Pure:
function greet(name) {
    return "Hello, " + name + "!";
}

greet("Saint Petersburg");
// Hello, Saint Petersburg!

Why functional programming?
- more predictable, safer
- easier to test/debug
  all you need to do to test pure functions is to pass in the input and check if it gives the right thing

makes you look/feel smart
is THE BEST paradigm

Why functional JavaScript?
- object-oriented JS gets tricky
  (prototypes? this?!?)
- established community/tools
- reuse functions like Lego blocks between different projects

https://codewords.recurse.com/issues/one/an-introduction-to-functional-programming