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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

            
              <p>状態をblockNumというグローバル変数で管理する。この変数には、ブロックの数が保持されている。blockNumで管理されたブロックの数を参照して、render関数で実際にDOMに反映させる。</p>
<p>ただしこのコードだと、一旦DOMをすべて削除してから、再度すべてを再レンダリングしているので、無駄が多い。</p>
<p>⇨Reactは仮想DOMという仕組みで、最小限のDOMを更新するするので、効率がいい。</p>

<button class="add">Add</button>
<button class="delete">Delete</button>
<div class="wrapper">
</div>

            
          
!
            
              p
	margin : 10px
	width: 500px

.block
	float: left
	width: 100px
	height: 100px
	box-sizing: border-box
	border: 1px solid black
	background-color: blue

.add
	display: block
	width: 100px
	height: 50px

.delete
	display: block
	width: 100px
	height: 50px
	
            
          
!
            
              const initialBlock = 1;
//ブロックの状態を管理する変数
//(グローバル変数で管理するのはよくないですが)
let blockNum = initialBlock;

//blockNumを参照して、レンダリングする関数
function render(blockNum) {
	//初期化。blockをすべて消す。
	$('.block').remove();
	
	//(効率の悪い書き方ですが、わかりやすく)
	//blockNumの数だけ、四角を追加する
	for (let i=0; i<blockNum; i++){
		$('.wrapper').append('<div class="block"></div>');	
	}
};

//レンダリングを一回実行し、初期画面を生成する
render(blockNum);

//blockNumを一個増やして、再レンダリング
$('.add').on('click', () => {
	blockNum ++;
	render(blockNum);
});

//blockNumを一個減らして、再レンダリング
$('.delete').on('click', () => {
	blockNum --;
	render(blockNum);
});


            
          
!
999px
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