Javascript: preview before uploading an image (TX interview question) _ form special effect

Javascript allows you to preview images before uploading (TX interview questions). If you do not know the file control before, you can also use onchange, which leads to a loss of opportunity during the interview.

Script
Function yulan ()
{
Var fileext = document. form1.UpFile. value. substring (document. form1.UpFile. value. lastIndexOf ("."), document. form1.UpFile. value. length)
Fileext = fileext. toLowerCase ()

If (fileext! Your '.jpg ') & (fileext! Using '.gif ') & (fileext! Using '.jpeg ') & (fileext! Using '.png ') & (fileext! Using '.bmp '))
{
Alert ("sorry, the system only supports photos in the standard format. Please adjust the format and upload it again. Thank you! ");
Document. form1.UpFile. focus ();
}
Else
{
// Alert (''+ document. form1.UpFile. value) // The statement used to preview the image.
Document. getElementById ("preview"). innerHTML = ""
}

}
Script