関数型プログラミングとは

なぜ機能JavaScript？

さあ、やりましょう！

...どうやって？

すべてを関数で行う

input -> output

機能しません:

var name = “Lakshya”;
var greeting = “Hi, I’m ”;
console.log(greeting + name);
=> “Hi, I’m Lakshya”

機能性

function greet(name) {
return “Hi, I’m ” + name;
}
greet(“Lakshya”);
=> “Hi, I’m Lakshya”

避けて副作用

use “pure” functions

純粋ではない

var name = “Lakshya”;
function greet() {
console.log(“Hi, I’m ” + name);
}

純粋

function greet(name) {
return “Hi, I’m ” + name;
}

高次関数の使用

functions can be inputs/outputs

function makeAdjectifier(adjective) {
return function (string) {
     return adjective + “ ” + string;
   };
}
var coolifier = makeAdjectifier(“dev”);
coolifier(“to”);
  => “dev to”

反復しない

use map, reduce, filter

Source: https://www.datasciencecentral.com/forum/topics/what-is-map-reduce

変異を避ける

use immutable data

突然変異

var rooms = [“H1”, “H2”, “H3”];
rooms[2] = “H4”;
rooms;
=> ["H1", "H2", "H4"]

突然変異はない

var rooms = [“H1”, “H2”, “H3”];
Var newRooms = rooms.map(function (rm) {
  if (rm == “H3”) { return “H4”; }
  else { return rm; }
});
newRooms; 
=> ["H1", "H2", "H4"]
rooms; 
=> ["H1", "H2", "H3"]