HTML5 AJAX Multiple Files Upload

I came across this plain js ajax upload code (it seems that jquery $.post does not work with HTML5 for some reason),

    /* If you want to upload only a file along with arbitary data that
       is not in the form, use this */
    var fd = new FormData();
    fd.append("file", document.getElementById('file').files[0]);

    /* If you want to simply post the entire form, use this */
    //var fd = document.getElementById('form1').getFormData();

    var xhr = new XMLHttpRequest();        
    xhr.upload.addEventListener("progress", uploadProgress, false);
    xhr.addEventListener("load", uploadComplete, false);
    xhr.addEventListener("error", uploadFailed, false);
    xhr.开发者_如何学运维addEventListener("abort", uploadCanceled, false);
    xhr.open("POST", "Upload.php");
    xhr.send(fd);

But I have two issues here,

1. what does this line mean after the object FormData?

fd.append("file", document.getElementById('file').files[0]);

Why do I need an ID there? Can I do something use jquery append() with $('input[type=file]')?

1. This ajax is only for single file upload, how can I change it for multiple files upload?

what does this line mean after the object FormData?

fd.append("file", document.getElementById('file').files[0]);

The document.getElementById('file') gets the <input type="file" id="file"> element by its ID. The element.files[0] grabs the first selected file from the element. The fd.append("file", file) appends it to an FormData instance with the field name of file. The fd is later to be sent as multipart/form-data XHR request body.

Why do I need an ID there? Can I do something use jquery append() with $('input[type=file]')?

The ID is not necessary. It's after all just an example in order to be able to get the <input type="file"> from the document by its ID. Note that the append() function in this example is part of the FormData API and is not to be confused with jQuery's append() function. Also note that the 1st argument of append() represents the field name, not the ID. That they are the same is just a coincidence.

This ajax is only for single file upload, how can I change it for multiple files upload?

Just append multiple fields to FormData. Assuming that you have a File[], here's an example:

for (var i = 0; i < files.length; i++) { 
    fd.append("file" + i, files[i]);
}

It'll be available in the server end by the field names file0, file1, etc.