<!doctype html>
<html>
<head>
<title>Toggle (Show/Hide) Div Element by GopherJS</title>
<style>
.invisible {display: none;}
.heading {background: yellow; width: 200px; cursor: pointer;}
.content {background: aqua; width: 200px; height: 200px;}
</style>
</head>
<body>
<div id="foo" class="heading">
<span id="foo1" class="invisible">▼</span><span id="foo2">►</span>
Heading
</div>
<div id="foo3" class="content invisible"><br><br>Content</div>
<script src="demo.js"></script>
</body>
</html>