How to get current master page and change it on the Site using javascript

 GET request

/_api/Web?$select=CustomMasterUrl,MasterUrl

function MastePageChange() {
    var clientcontext;
    var cweb;
    var customMasterURL = '/_catalogs/masterpage/NewMasterPage.master';
    clientcontext = new SP.ClientContext.get_current();
    cweb = clientcontext.get_web();
    cweb.set_customMasterUrl(masterPageUrl);
    cweb.set_masterUrl(masterPageUrl);
    cweb.update();
    clientcontext.executeQueryAsync(function() {
        alert("Master Page has been changed successfully \n" + customMasterURL);
    }, function(sender, args) {
        alert("Error: " + args.get_message());
    });
}

MastePageChange();