fetch.ts

type Todo = {userId: string, title: string, completed: boolean};

const apiUrl = 'any url';

//Issues
// 1. No Generic Responsetype
// 2. Typesafe method
// 3. Body not always allowed
// 4. type safe headers
const resp = fetch<Todo>(apiUrl, {
  method: 'PUT',
  body: 'asdf',
  headers: {
    'Content-Type': 'application/json',
    'Accept': 'application/json',
  }
})
  .then(resp => resp.json());

type Type = string;

const a =

fetch<Type>(apiUrl)
.then(r => r.json())
//          ^? Promise<Type>

type TypedHeaders = RequestInit['headers'] & PreparedHeaders;

type PreparedHeaders = Partial<{
  'Content-Type': MimeTypes,
  'Accept': MimeTypes,
  'Authorization': `Bearer ${string}`
}>;

declare function fetch<ResponseType = any>(
  input: RequestInfo | URL, init?: TypedRequestInit
): Promise<TypedResponse<ResponseType>>;

type HttpVerbs = 'POST' | 'PUT' | 'DELETE' | 'UPDATE' | 'GET' | 'CONNECT' | 'HEAD' |
  'OPTIONS';

type WithBody = Extract<HttpVerbs, 'POST' | 'PUT' | 'DELETE' | 'UPDATE'>;
type NonBody = Exclude<HttpVerbs, WithBody>;

type MethodBodyCombination = {method?: WithBody, body?: RequestInit['body']} |
  {method?: NonBody, body?: never}

type TypedRequestInit = RequestInit & MethodBodyCombination & {headers?: TypedHeaders};



interface TypedResponse<T> extends Response {
  json(): Promise<T>;
}

type MimeTypes =
  ".jpg" |
  ".midi" |
  "XML" |
  "application/epub+zip" |
  "application/gzip" |
  "application/java-archive" |
  "application/json" |
  "application/ld+json" |
  "application/msword" |
  "application/octet-stream" |
  "application/ogg" |
  "application/pdf" |
  "application/php" |
  "application/rtf" |
  "application/vnd.amazon.ebook" |
  "application/vnd.apple.installer+xml" |
  "application/vnd.mozilla.xul+xml" |
  "application/vnd.ms-excel" |
  "application/vnd.ms-fontobject" |
  "application/vnd.ms-powerpoint" |
  "application/vnd.oasis.opendocument.presentation" |
  "application/vnd.oasis.opendocument.spreadsheet" |
  "application/vnd.oasis.opendocument.text" |
  "application/vnd.openxmlformats-officedocument.presentationml.presentation" |
  "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" |
  "application/vnd.openxmlformats-officedocument.wordprocessingml.document" |
  "application/vnd.rar" |
  "application/vnd.visio" |
  "application/x-abiword" |
  "application/x-bzip" |
  "application/x-bzip2" |
  "application/x-csh" |
  "application/x-freearc" |
  "application/x-sh" |
  "application/x-shockwave-flash" |
  "application/x-tar" |
  "application/x-7z-compressed" |
  "application/xhtml+xml" |
  "application/zip" |
  "audio/aac" |
  "audio/mpeg" |
  "audio/ogg" |
  "audio/opus" |
  "audio/wav" |
  "audio/webm" |
  "font/otf" |
  "font/ttf" |
  "font/woff" |
  "font/woff2" |
  "image/bmp" |
  "image/gif" |
  "image/png" |
  "image/svg+xml" |
  "image/tiff" |
  "image/vnd.microsoft.icon" |
  "image/webp" |
  "text/calendar" |
  "text/css" |
  "text/csv" |
  "text/html" |
  "text/javascript" |
  "text/plain" |
  "video/3gpp" |
  "video/3gpp2" |
  "video/mp2t" |
  "video/mpeg" |
  "video/ogg" |
  "video/webm" |
  "video/x-msvideo";