switch (res.status) {
case 200: return res.json();
case 451: return (async () => {
const data = await res.json();
if(data.field === 'email') form.controls['email'].setErrors({'email_busy': true});
return data;
})();
default: return res.status;
}
fetch('http://localhost:4000').then(res => res.json()).then(console.log);