あなたの本日の運勢は…？
<div id="omikuji">
<h1>おみくじ</h1>
<p id="output">あなたの本日の運勢は…？</p>
<input type="button" id="btn" value="占う">
</div>
window.onload = function(){
var luck = ["大凶","凶","末吉","小吉","中吉", "吉", "大吉"];
var colors = ["赤","青","黄","緑","白","黒","金","銀","茶","紫"];
var message, random_luck, random_color;
var btn = document.getElementById("btn"); // id="btn"を取得
var output = document.getElementById("output"); // id="output"を取得
btn.onclick = function(){
random_luck = luck[ Math.floor( Math.random() * luck.length ) ]; // luckの[i]番目の要素
random_color = colors[ Math.floor( Math.random() * colors.length ) ]; // colorsの[i]番目の要素
message = "<h2>" + random_luck + "</h2>";
message += "<p>ラッキーカラーは " + random_color + "</p>";
output.innerHTML = message;
} // End btn.onclick
} // End JavaScript