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HTML

              
                <div id="root"></div>
              
            
!

CSS

              
                
              
            
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JS

              
                //上記Bについて:もともとの配列の存在
let ar=[2,4,6,8];


//上記Aについて:map()メソッドによって呼び出される(使われる)関数(コールバックと呼ぶ)を定義(上記
function Kakezan(x){
  return x*2
}

//上記CとDについて:map()メソッドで新しい配列を自動的に作成
let new_ar=ar.map(Kakezan);

//新しい配列をコンソールに表示する
console.log(new_ar);


//以下はフレームワークのReactを使った場合の表示。コンソールではなく、htmlファイルに表示される
let element=<p>新しい配列は[{new_ar[0]},{new_ar[1]},{new_ar[2]},{new_ar[3]}]です。</p>;
ReactDOM.render(element, document.getElementById("root"));
              
            
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999px

Console