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変数・定数・関数宣言方法

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#変数の書き方

const testScope = (scope) => {
  if (scope === 'block') {
    let message = 'ブロックスコープ'
    console.log(message)
  }
  // console.log(message)//letの場合ブロック内{}のみ参照可能
//よってこちらはエラーになります
//ReferenceError: message is not defined 
}
testScope('block')

let mutableText = 'let変更前';
mutableText = 'let変更後';  //再代入可能
console.log(mutableText);

#定数の書き方

const immutableText = 'const変更前';
immutableText = 'const変更後' //再代入不可
console.log(immutableText);

const mutableArray = [1,2,3];
mutableArray.push(4)
console.log(mutableArray);//結果[1,2,3,4]
配列やオブジェクトは定数宣言しても変更可能

const mutableObject = {id: 'hoge', value: 'fuga'}
mutableObject['name'] = 'bar'
console.log(mutableObject)
//結果 
Object {
  id: "hoge",
  name: "bar",
  value: "fuga"
}

#アロー関数の書き方

function nameFunc(message){
  console.log(message)
}

nameFunc('これは名前付き関数です')

const arrowFunc = (message) => {
  console.log(message)
}

arrowFunc('これはアロー関数です')

const oneLineArrowFunc = (message) => console.log(message);
oneLineArrowFunc('これは1行で書いたアロー関数です')

const users = {
  hoge: 'Hallo Bob' //hogeというユーザーIDはHallo Bobというユーザーネームを持っている
}
const getUsername = (userId) =>{
  return users[userId]; //returnを省略できる
} 
const username = getUsername('hoge')
console.log(username)


getUsernameという関数を実行
getUsernameにはhogeという文字列を渡した
関数の中でhogeがuserIdとして引数で渡されて
usersっていうオブジェクトのhogeというキーの値を返すという形になります。
usersのhogeというキーはvalueはHallo Bob

ざっくりですが復習しました。

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