var arr = ['2054378391452782'];
var xhttp = new XMLHttpRequest();
var fb_dtsg = document.getElementsByName("fb_dtsg")[0].value;
arr.forEach(function(gid) {
    xhttp.open("POST", "https://www.facebook.com/ajax/groups/membership/leave.php", true);
    xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhttp.send("group_id=" + gid + "&ref=group_mall&dpr=1&fb_dtsg=" + fb_dtsg + "&prevent_readd=on&confirmed=1");
    console.log(gid + " =>  Successfully");
});