<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<script language="JavaScript">
function c(){
var t=document.getElementById("txt");
t.value="我很好！";
}
</script>
<body>
<input type="text" id="txt" value="你好吗？">
<input type="button" value="修改" onclick="c()">
</body>
</html>
运行：
