<!DOCTYPE html>
<html>
<body>
<h2>JavaScript $</h2>
<p>下划线被视为 JavaScript 名称中的字母。</p>
<p id="demo"></p>
<script>
var _x = 2;
var _100 = 5;
document.getElementById("demo").innerHTML = _x + _100;
</script>
</body>
</html>