在FireFox、Chrome浏览器中可以根据document.getElementById(“id_file”).files[0].size 获取上传文件的大小（字节数）使用的 api是FileReader，不多说了，看例子吧。
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<html>
<head>
<title>test</title>
<script language="
JavaScript">
function importPrompt()
{
var tipMsg = "
browser version too low, suggest you to upgrade or try other browsers!";
var errMsg = "
import file size is greater than 250kb!";
if (!
window.FileReader)
{
alert(tipMsg);
return false;
}
var oFile =
document.getElementById("
fileupload").files[0];
// input box with type file;
var img =
document.getElementById("
imgtag");
var fileSize = 0;
var maxsize = 250*1024;
//250kb
var reader =
new FileReader();
reader.onload =
function (e)
{
console.log(e.total);
// file size
img.src = e.target.result;
// putting file in dom without server upload.
};
reader.readAsDataURL(oFile );
fileSize = oFile.size;
if(fileSize>maxsize)
{
alert(errMsg);
return false;
}
else
{
return confirm("
select yes to upload");
}
}
</script>
</head>
<body>
<img />
<form action='#' onsubmit="
return importPrompt();">
<input type="
file"
name="
ImportFile" >
<input type="
submit" >
</form>
</body>
</html>
请一定要去看下面的参考资料，你会受益更多
reference:
http://caniuse.com/filereader
http://blog.teamtreehouse.com/reading-files-using-the-html5-filereader-api
http://www.micmiu.com/lang/javascript/js-check-filesize/
http://stackoverflow.com/questions/1606842/how-can-i-get-a-files-upload-size-using-simple-javascript