现场演示：我的 HTML 文档

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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>Setting Default Values for Function Parameters in Older Versions of JavaScript</title>
</head>
<body>
    <script>
    function sayHello(name){
        var name = name || 'World'; 
        return 'Hello ' +  name + '!';
    }
    
    document.write(sayHello()); // Hello World!
    document.write("<br>");
    document.write(sayHello('John')); // Hello John!
    </script>
</body>
</html>