var $file = document.getElementById('files');
document.getElementById('btn').onclick = ()=> {
    let data = new FormData();
    data.append('file', $file.files[0]);
    data.append('foo', 'bar');

    var xhr = new XMLHttpRequest();
    xhr.open('post', '/upload');
    xhr.onload = (e) => {
        alert(e.currentTarget.response);
    }
    xhr.send(data);
};


//or use jQuery
$("#files").change(function () {
    let data = new FormData();
    data.append('files', $file.files[0]);
    $.ajax({
        type: "post",
        url: "/upload",
        processData: false,
        contentType: false,
        data: data,
        dataType: 'json',
        success: function (_data) {
            if (_data.code == "200") {
                //do something...
            } else {
               //do something...
            }
        }
    });
});