1.     fetch(req)
2.          .then((res) => res.json())
3.          .then((data) => {
4.            console.log('Response from server');
5.            console.log(data);
6.            $tmt = data;
7.            document.getElementById("myData").innerHTML = $tmt;
8.          })
9.          .catch(console.warn);

[object Object]

data

function mydump(arr,level) {
    var dumped_text = "";
    if(!level) level = 0;

    var level_padding = "";
    for(var j=0;j<level+1;j++) level_padding += "    ";

    if(typeof(arr) == 'object') {  
        for(var item in arr) {
            var value = arr[item];

            if(typeof(value) == 'object') {
                dumped_text += level_padding + "'" + item + "' ...\n";
                dumped_text += mydump(value,level+1);
            } else {
                dumped_text += level_padding + "'" + item + "' => \"" + value + "\"\n";
            }
        }
    } else {
        dumped_text = "===>"+arr+"<===("+typeof(arr)+")";
    }
    return dumped_text;
}

0: Object { ID_Dostawcy: 1, Nazwa: null, adres: "ul. alamakota 37" }

for(const item in arr) {
                        var value = arr[item];

                         if(value === null) {
                            dumped_text += "<td>";
                            dumped_text += "balwanek";
                            dumped_text += "</td>";
                        }else if (typeof(value) == 'object') {
                            dumped_text += mydump(value,level+1);
                        }else{
                            dumped_text += "<td>";
                            dumped_text += value;
                            dumped_text += "</td>";
                        }
}