图片上传之前检查大小、尺寸、格式并预览的js代码

下面的代码，需要保存到本地种执行，在线看不到实际效果。

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
<head> 
<title>validate before upload by Lance Zhang</title> 
</head> 
<body style="font-family:Calibri"> 
<form name="Myform"> 
<p> 
<font color="red" size="6">File type :gif,jpg,jpeg,png,bmp</font></p> 
<p> 
File Size limit: 
<input type="text" size="4" value="100" name="fileSizeLimit" id="fileSizeLimit" /> 
K</p> 
<p> 
Image Height Limit: <input type="text" size="4" value="100" name="heightLimit" id="heightLimit" /> 
</p> 
<p> 
Image Width Limit: <input type="text" size="4" value="100" name="widthLimit" id="widthLimit" /> 
</p> 
<p> 
<input type="file" name="photo" onchange="changeSrc(this)" /> 
Image Preview<img src="https://www.jb51.net/article/about:blank" id="fileChecker" name="fileChecker" alt="test" /> 
</p> 
<p> 
<input type="submit" value="submit"> 
</p> 
</form>

if (oFileChecker.fileSize > limit) 
{ 
alert("FileSize :"+oFileChecker.fileSize+"byte--too large!"); 
} 
else 
{ 
alert("FileSize :"+oFileChecker.fileSize+"byte--ok"); 
}

if( oFileChecker.height> height) 
{ 
alert("ImageHeight :"+oFileChecker.height+"--too height!"); 
} 
else 
{ 
alert("ImageHeight :"+oFileChecker.height+"--ok"); 
} 
if(oFileChecker.width> width) 
{ 
alert("ImageWidth :"+oFileChecker.width+"--too width!"); 
} 
else 
{ 
alert("ImageWidth :"+oFileChecker.width+"--ok"); 
} 
return false; 
}

var right_type=new Array(".gif",".jpg",".jpeg",".png",".bmp");

var oFileChecker = document.getElementById("fileChecker");

function changeSrc(filePicker) 
{ 
if(!checkImgType(filePicker.value)) 
{ 
alert("the file type is not correct"); 
return; 
} 
oFileChecker.src = filePicker.value; 
}

oFileChecker.onreadystatechange = function () 
{ 
if (oFileChecker.readyState == "complete") 
{ 
CheckFileSize(); 
} 
}

[Ctrl+A 全选注:如需引入外部Js需刷新才能执行]