let frmUpload = document.getElementById('frmUpload');
let btnUpload = document.getElementById('btnUpload');

btnUpload.addEventListener('click', event => { btnUploadClick(event) });

function btnUploadClick(event) {

    event.preventDefault();
    
    fetch('/upload', {
        method: 'POST',
        body  : new FormData(frmUpload)
    }).then(
        //...
    )
}