[Unhandled promise rejection: TypeError: Body not allowed for GET or HEAD requests]
// Please note that you cannot use body in GET Request. Please change it to POST.
var url = new URL("https://apitest.kuveytturk.com.tr/prep/v1/data/fecs"),
params = {firstParam: 'isoCode ',
secondParam: 'internationalCode',
thirdParam: 'name',
fourthParam: 'code',
FifthParam: 'group',
SixthParam: 'id'}
Object.keys(params).forEach(key => url.searchParams.append(key, params[key]))
fetch(url).then(/* … */)