本文介绍了一个简单的JavaScript脚本，用于实现用户在上传图片前的实时预览功能。该脚本仅支持jpg、gif和bmp格式的图片，并通过判断文件扩展名来确保正确显示预览图。
<HTML>
<wbr><HEAD></wbr>
<wbr> <TITLE> 上传图片预览 </TITLE></wbr>
<wbr><wbr><wbr></HEAD></wbr></wbr></wbr>
<wbr><BODY></wbr>
<wbr> <script type="text/javascript"></wbr>
<wbr><wbr><wbr> function lookImg()</wbr></wbr></wbr>
<wbr>{</wbr>
<wbr><wbr><wbr> var fileURL=document.myForm.myFile.value;</wbr></wbr></wbr>
<wbr><wbr><wbr> fileURLSplit = fileURL.split(".");</wbr></wbr></wbr>
<wbr><wbr><wbr> fileExt=fileURLSplit[fileURLSplit.length-1].toLowerCase();</wbr></wbr></wbr>
<wbr><wbr><wbr><wbr><wbr><wbr> //toLowerCase()返回一个字符串，该字符串中的字母被转换为小写字母。</wbr></wbr></wbr></wbr></wbr></wbr>
<wbr><wbr><wbr><wbr><wbr><wbr> if(fileExt=="jpg" || fileExt=="gif" ||fileExt=="bmp")</wbr></wbr></wbr></wbr></wbr></wbr>
<wbr><wbr><wbr> {</wbr></wbr></wbr>
<wbr><wbr><wbr><wbr><wbr><wbr> document.imgA.src=fileURL;</wbr></wbr></wbr></wbr></wbr></wbr>
<wbr><wbr><wbr> }</wbr></wbr></wbr>
<wbr><wbr><wbr> else</wbr></wbr></wbr>
<wbr><wbr><wbr> {</wbr></wbr></wbr>
<wbr><wbr><wbr><wbr><wbr><wbr><wbr> alert("请选择jpg、gif或bmp图片文件");</wbr></wbr></wbr></wbr></wbr></wbr></wbr>
<wbr><wbr><wbr> }</wbr></wbr></wbr>
<wbr><wbr><wbr> }</wbr></wbr></wbr>
<wbr> </script></wbr>
<wbr> <form name="myForm" enctype="mulitipart/form-data" method="post"></wbr>
<wbr><wbr><wbr><wbr><wbr><wbr> 请选择文件：<input type="file" name="myFile" onchange="lookImg()"><br></wbr></wbr></wbr></wbr></wbr></wbr>
<wbr><wbr><wbr></wbr></wbr></wbr>
<wbr> </form></wbr>
<wbr> <img src="hyopec2.jpg" name="imgA"></wbr>
<wbr></BODY></wbr>
</HTML>
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