<html>
<body>
名: <input type="text" id="myText" value="Mickey">
<p>点击按钮修改文本域的默认值。</p>
<button type="button" onclick="myFunction()">点我</button>
<script>
function myFunction() {
document.getElementById("myText").defaultValue = "Goofy";
}
</script>
</body>
</html>