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checkValidity( )：input元素中的数据合法返回true，反之返false
setCustomValidity( )：设置validationMessage属性值，用于自定义提示
validationMessage：输出提示
<body>
<p>输入数字并点击验证按钮：</p>
<input type="number" id="nu" max="300" min="100" required>
<button type="button" onclick="myFunction()">click</button>
<p>如果输入数字100-300以外，会提示错误信息。</p>
<p id="demo"></p>
</body>
<script>
function myFunction() {
var x = document.getElementById("nu");
x.setCustomValidity("");//使用前先取消自定义，否则下次点击checkValidity总返false
if (x.checkValidity() == false) {
x.setCustomValidity("错误");
document.getElementById("demo").innerHTML = x.validationMessage;
}
else {
x.setCustomValidity("正确");
document.getElementById("demo").innerHTML = x.validationMessage;
}
}
</script>