Add flow definitions

Makefile

@@ -6,6 +6,9 @@ lint: node_modules/
 dist/fetch.umd.js: fetch.js rollup.config.js node_modules/
 	./node_modules/.bin/rollup -c
 
+dist/fetch.umd.js.flow: fetch.js.flow
+	cp $< $@
+
 node_modules/:
 	npm install
 

fetch.js.flow

@@ -0,0 +1,114 @@
+/* @flow strict */
+
+type CredentialsType = 'omit' | 'same-origin' | 'include'
+
+type ResponseType =  'default' | 'error'
+
+type BodyInit = string | URLSearchParams | FormData | Blob | ArrayBuffer | $ArrayBufferView
+
+type RequestInfo = Request | URL | string
+
+type RequestOptions = {|
+  body?: ?BodyInit;
+
+  credentials?: CredentialsType;
+  headers?: HeadersInit;
+  method?: string;
+  mode?: string;
+  referrer?: string;
+  signal?: ?AbortSignal;
+|}
+
+type ResponseOptions = {|
+  status?: number;
+  statusText?: string;
+  headers?: HeadersInit;
+|}
+
+type HeadersInit = Headers | {[string]: string}
+
+// https://github.com/facebook/flow/blob/f68b89a5012bd995ab3509e7a41b7325045c4045/lib/bom.js#L902-L914
+declare class Headers {
+  @@iterator(): Iterator<[string, string]>;
+  constructor(init?: HeadersInit): void;
+  append(name: string, value: string): void;
+  delete(name: string): void;
+  entries(): Iterator<[string, string]>;
+  forEach((value: string, name: string, headers: Headers) => any, thisArg?: any): void;
+  get(name: string): null | string;
+  has(name: string): boolean;
+  keys(): Iterator<string>;
+  set(name: string, value: string): void;
+  values(): Iterator<string>;
+}
+
+// https://github.com/facebook/flow/pull/6548
+interface AbortSignal {
+  aborted: boolean;
+  addEventListener(type: string, listener: (Event) => mixed, options?: EventListenerOptionsOrUseCapture): void;
+  removeEventListener(type: string, listener: (Event) => mixed, options?: EventListenerOptionsOrUseCapture): void;
+}
+
+// https://github.com/facebook/flow/blob/f68b89a5012bd995ab3509e7a41b7325045c4045/lib/bom.js#L994-L1018
+// unsupported in polyfill:
+// - cache
+// - integrity
+// - redirect
+// - referrerPolicy
+declare class Request {
+  constructor(input: RequestInfo, init?: RequestOptions): void;
+  clone(): Request;
+
+  url: string;
+
+  credentials: CredentialsType;
+  headers: Headers;
+  method: string;
+  mode: ModeType;
+  referrer: string;
+  signal: ?AbortSignal;
+
+  // Body methods and attributes
+  bodyUsed: boolean;
+
+  arrayBuffer(): Promise<ArrayBuffer>;
+  blob(): Promise<Blob>;
+  formData(): Promise<FormData>;
+  json(): Promise<any>;
+  text(): Promise<string>;
+}
+
+// https://github.com/facebook/flow/blob/f68b89a5012bd995ab3509e7a41b7325045c4045/lib/bom.js#L968-L992
+// unsupported in polyfill:
+// - body
+// - redirected
+// - trailer
+declare class Response {
+  constructor(input?: ?BodyInit, init?: ResponseOptions): void;
+  clone(): Response;
+  static error(): Response;
+  static redirect(url: string, status?: number): Response;
+
+  type: ResponseType;
+  url: string;
+  ok: boolean;
+  status: number;
+  statusText: string;
+  headers: Headers;
+
+  // Body methods and attributes
+  bodyUsed: boolean;
+
+  arrayBuffer(): Promise<ArrayBuffer>;
+  blob(): Promise<Blob>;
+  formData(): Promise<FormData>;
+  json(): Promise<any>;
+  text(): Promise<string>;
+}
+
+declare module.exports: {
+  fetch(input: RequestInfo, init?: RequestOptions): Promise<Response>;
+  Headers: typeof Headers;
+  Request: typeof Request;
+  Response: typeof Response;
+}

package.json

@@ -31,7 +31,7 @@
   ],
   "scripts": {
     "karma": "karma start ./test/karma.config.js --no-single-run --auto-watch",
-    "prepare": "make dist/fetch.umd.js",
+    "prepare": "make dist/fetch.umd.js dist/fetch.umd.js.flow",
     "pretest": "make",
     "test": "karma start ./test/karma.config.js && karma start ./test/karma-worker.config.js"
   }