index.html
<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF8">
<title>JavaScriptの勉強</title>
</head>
<body>
<h1>JavaScriptの勉強</h1>
<script>
// javaScriptオブジェクト(String)
var s = "this is a pen.";
console.log(s.substr(2, 2));
console.log(s.replace("pen", "pencil"));
</script>
</body>
</html>