How to upload file using pure vanilla javascript and php?

Question

I have an existing html form which uploads a file to the server as soon as the user selects an image file.

I have done something like this.

//html code
<input type="file" id="photo" name="photo" accept="image/*" />
// the usual html stuff

document.getElementById('photo').addEventListener("change",uploadImage);
function uploadImage()
{
    var xhr = new XMLHttpRequest();

    xhr.open("POST","/upload.php",true);    
    xhr.setRequestHeader("Content-type","image");

    var file = document.getElementById('photo').files[0];
    if(file)
    {
        var formdata = new FormData();
        formdata.append("pic",file);
        xhr.send(formdata);
    }
    xhr.onreadystatechange = function(){
    if(xhr.readyState == 4 && xhr.status == 200)
    {
             //some code
    }
};
}

But in my php file, I can't access this uploaded file. The $_POST array seems to be empty. I did a print_r for $_POST and it gave Array(). What am I doing wrong?

Answer 1

You are using FormData which works very much the same way a normal form does.

First of all, in PHP files will not be in $_POST but instead in $_FILES, see the documentation.

What that documentation does mention, along with the $_FILES buffer, is that you need to use the multipart/form-data encoding, any FormData transferred through an XMLHttpRequest will have this set by default, though you may want to check it if the $_FILES remain empty. You should remove the xhr.setRequestHeader("Content-type","image"); and let the XHR object handle it, of - if that doesn't work - add

xhr.setRequestHeader("Content-type","multipart/form-data");

There is a pretty nice example right here at stackoverflow