js画像アップロードとプレビュー関数例分析_javascriptスキル

<img id="imgTag" style="height: 100px;" alt="" />
<input type="file" />

function DisplayImage(fileTag,imgTagId){
var allowExtention=".jpg.png.gif";
var extentionArr=fileTag.value.split('.');
var extention = extentionArr[extentionArr.length-1];
if(!(allowExtention.indexOf(extention)>-1)){
alert("Please upload image!");
}else{
//for adveced broswer(the newest ie,chrome,ff)
if(typeof(FileReader)!=="undefined"){
var reader = new FileReader();
reader.readAsDataURL(fileTag.files[0]);
reader.onload = function(e){
document.getElementById(imgTagId).setAttribute("src", e.target.result);
}
}else{
//for(ie6)
document.getElementById(imgTagId).setAttribute("src",fileTag.value);
}
}
}