js determines the type and size of uploaded files sample code_javascript skills

//Detect file size and type
function fileChange(target) {
//Detect the type of uploaded file
if(!(/(?:jpg|gif|png|jpeg)$/i.test(target.value))) {
alert("only Allow uploading images in jpg|gif|png|jpeg format");
if(window.ActiveXObject) {//for IE
target.select();//select the file, and clear selection
document.selection.clear();
} else if(window.opera) {//for opera
target.type="text";target.type="file";
} else target. value="";//for FF,Chrome,Safari
return;
} else {
return; //alert("ok");//or you can do nothing here.
}

//Detect the size of the uploaded file
var isIE = /msie/i.test(navigator.userAgent) && !window.opera;
var fileSize = 0;
if ( isIE && !target.files){
var filePath = target.value;
var fileSystem = new ActiveXObject("Scripting.FileSystemObject");
var file = fileSystem.GetFile (filePath);
fileSize = file.Size;
} else {
fileSize = target.files[0].size;
}
var size = fileSize / 1024;
if(size>(500) ){
alert("File size cannot exceed 500KB");
if(window.ActiveXObject) {//for IE
target.select();//select the file, and clear selection
document.selection.clear();
} else if(window.opera) {//for opera
target.type="text";target.type="file";
} else {
target.value="";//for FF, Chrome, Safari
}
return;
}else{
return;
}
}