ボタンを押すごとに、boldが変わる

fontWeightを判定します。

<p id="list1">Catcher In The Light</p>
<p id="list2">About You</p>
<p id="list3">GAME</p>
<input type="button" value="ボタン" onclick="changeFontWeight();">

<script>
function changeFontWeight(){
	var obj1 = document.getElementById("list1");
	var obj2 = document.getElementById("list2");
	var obj3 = document.getElementById("list3");
	if(obj1.style.fontWeight == "bold"){
		obj1.style.fontWeight = "normal";
		obj2.style.fontWeight = "bold";
	} else if(obj2.style.fontWeight == "bold") {
		obj2.style.fontWeight = "normal";
		obj3.style.fontWeight = "bold";
	} else if(obj3.style.fontWeight == "bold"){
		obj3.style.fontWeight = "normal";
		obj1.style.fontWeight = "bold";
	} else {
		obj1.style.fontWeight = "bold";
	}
}
</script>

変数をHTMLに渡して表示する

<h1>MyStory <div style="display:inline;" id="title"></div></h1>

<p id="list1">Catcher In The Light</p>
<p id="list2">About You</p>
<p id="list3">GAME</p>
<input type="button" value="ボタン" onclick="changeFontWeight();">

<script>
function changeFontWeight(){
	var obj1 = document.getElementById("list1");
	var obj2 = document.getElementById("list2");
	var obj3 = document.getElementById("list3");
	if(obj1.style.fontWeight == "bold"){
		obj1.style.fontWeight = "normal";
		obj2.style.fontWeight = "bold";
		list = "list2";
		

	} else if(obj2.style.fontWeight == "bold") {
		obj2.style.fontWeight = "normal";
		obj3.style.fontWeight = "bold";
		list = "list3";
	} else if(obj3.style.fontWeight == "bold"){
		obj3.style.fontWeight = "normal";
		obj1.style.fontWeight = "bold";
		list = "list1";
	} else {
		obj1.style.fontWeight = "bold";
		list = "list1";
	}
	var obj4 = document.getElementById(list).innerHTML;
	var title = document.getElementById("title");
	title.textContent = obj4
}
</script>

ボタンを押す度に、タイトルとfontWeightがboldに切り替わる。