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                <input type="text" id="birthday" placeholder="MM/DD/YYYY"><button id="btn-submit">計算</button>
<div id="age"></div>





                // ボタンのイベントリスナー
document.getElementById("btn-submit").addEventListener( "click", function(){

	// 誕生日を保持したDateインスタンスを生成
	var birthday = new Date( document.getElementById("birthday").value );
	// 今日の日付を保持したDateインスタンスを生成
	var today = new Date();

	// 現在の年から誕生年を減算
	var age = today.getFullYear() - birthday.getFullYear();

	// 正しい値が取れているかチェック
	if ( isNaN( age ) ){
		alert( "正しいフォーマットで誕生日を入力してください。" );
		return false;

	// 現在の年で誕生日が来ていなければ減算
	if ( today < birthday.setFullYear( today.getFullYear() ) ){

	// 計算した結果の年齢を出力
	document.getElementById("age").textContent = age + "歳";

	return false;
} );