In Functional Programing, we want express our whole program in terms of functions.
Functional Programing is Declarative. Which means we focus more on ** what to do ** instead of How to do

First lets understand why Functional Programming is so important.
Functional Programming enables us to

• Write re-useable code.
• Debug easily.
• Read better.

Functions are 1st Class Citizens
because functions can be :

• Assigned to variables.
• Can be added to objects and arrays as well .
• Sent to other functions as a argument.
• Can be Returned from other functions.

Let's get right into it.

Non Functional Way

let name = "Sayuri" ;
let message = "Hey, fellow devs, I am " ;
console.log(message + name)

---> Hey, fellow devs, I am Sayuri

Functional Way

function message(name) {
    return "Hey, fellow devs, I am " + name ;
}

message("Sayuri")

---> Hey, fellow devs, I am Sayuri

--> Pure Functions

A Pure Function is a function which,
Given the same input, will always return the same output.

A Pure Function :

• Takes in at least 1 parameter.
• Return Something (A value or a function).
• Does not mutates any arguments.

Not Pure

let name = "Sayuri" ;

function message(){
    console.log("Hey, fellow devs, I am " + name )
} 

The above code is not pure because

• --> It is not taking name as an parameter.
• --> It's dealing with something in the global scope.
• --> Also it not having a return value.

Pure Functions have no side effects which means it cannot alter anything outside the function.

Pure Function

function message(name) {
   return "Hey, fellow devs, I am " + name
}

Higher Order Function

A higher order function is a function that takes a function as an argument, or returns a function or does both.

const greet = function takeName (name){
    return function message(msg){
        return msg  + name 
    }
}

greet("Sayuri ")("Hey, fellow devs, I am ")

--> Hey, fellow devs, I am Sayuri

Immutable Code

Immutability means can't be changed.

Mutation --> (Bad)

const code= [ "Javascript", "Python", "React" ]
code[ 2 ] = "Node"
console.log(code)

--> [ "Javascript", "Node", "React" ]

Immutation

const code = [ "Javascript", "Python", "React" ]
const code2 = code.map(lang=> {
  if(lang=== 'Python') {
    lang= 'Node';
  }
  return lang;
});

console.log(code2)

--> [ "Javascript", "Node", "React" ]

Last but not the Least
Do not Iterate using for or while/loops --> Use Map, Reduce, Filter etc.

Let me your thoughts.