本文制作一个用户自定义的AlertBox,效果如图:

js文件中插入如下代码：
复制代码 代码如下:
// JScript 文件
// constants to define the title of the alert and button text.
var ALERT_TITLE = "Oops!";
var ALERT_BUTTON_TEXT = "Close";
// over-ride the alert method only if this a newer browser.
// Older browser will see standard alerts
if(document.getElementById) {
window.alert = function(txt) {
createCustomAlert(txt);
}
}
function createCustomAlert(txt) {
// shortcut reference to the document object
d = document;
// if the modalContainer object already exists in the DOM, bail out.
if(d.getElementById("modalContainer")) return;
// create the modalContainer div as a child of the BODY element
mObj = d.getElementsByTagName("body")[0].appendChild(d.createElement("div"));
mObj.id = "modalContainer";
// make sure its as tall as it needs to be to overlay all the content on the page
mObj.style.height = document.documentElement.scrollHeight + "px";
// create the DIV that will be the alert
alertObj = mObj.appendChild(d.createElement("div"));
alertObj.id = "alertBox";
// MSIE doesnt treat position:fixed correctly, so this compensates for positioning the alert
if(d.all && !window.opera) alertObj.style.top = document.documentElement.scrollTop + "px";
// center the alert box
alertObj.style.left = (d.documentElement.scrollWidth - alertObj.offsetWidth)/2 + "px";
// create an H1 element as the title bar
h1 = alertObj.appendChild(d.createElement("h1"));
h1.appendChild(d.createTextNode(ALERT_TITLE));
// create a paragraph element to contain the txt argument
msg = alertObj.appendChild(d.createElement("p"));
msg.innerHTML = txt;
// create an anchor element to use as the confirmation button.
btn = alertObj.appendChild(d.createElement("a"));
btn.id = "closeBtn";
btn.appendChild(d.createTextNode(ALERT_BUTTON_TEXT));
btn.href = "#";
// set up the onclick event to remove the alert when the anchor is clicked
btn.onclick = function() { removeCustomAlert();return false; }
// removes the custom alert from the DOM function removeCustomAlert() {
// document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
}
// removes the custom alert from the DOM
function removeCustomAlert()
{
document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
}

将如下代码粘贴到你的HTML你的HTML的HEAD部分。 
复制代码 代码如下:
<script type="text/javascript" src="/UploadFiles/2021-04-02/customAlertBox.js"><!-- Paste this code into your external style sheet or the
CSS section of your HTML document -->
<style type="text/css">
#modalContainer {
background-color:transparent;
position:absolute;
width:100%;
height:100%;
top:0px;
left:0px;
z-index:10000;
}
#alertBox {
position:relative;
width:300px;
min-height:100px;
margin-top:50px;
border:2px solid #000;
background-color:#F2F5F6;
background-image:url(alert.png);
background-repeat:no-repeat;
background-position:20px 30px;
}
#modalContainer > #alertBox {
position:fixed;
}
#alertBox h1 {
margin:0;
font:bold 0.9em verdana,arial;
background-color:#78919B;
color:#FFF;
border-bottom:1px solid #000;
padding:2px 0 2px 5px;
}
#alertBox p {
font:0.7em verdana,arial;
height:50px;
padding-left:5px;
margin-left:55px;
}
#alertBox #closeBtn {
display:block;
position:relative;
margin:5px auto;
padding:3px;
border:1px solid #000;
width:70px;
font:0.7em verdana,arial;
text-transform:uppercase;
text-align:center;
color:#FFF;
background-color:#78919B;
text-decoration:none;
}
</style>

在你的HTML文档的Body部分插入如下代码:
复制代码 代码如下:
<input type="button" value = "Test the alert" onclick="alert('This is a custom alert dialog that was created by overriding the window.alert method.');">