support fetch wrapper chaining

static/WebStorage.mjs

@@ -1,16 +1,30 @@
+/**
+ * @callback fetch
+ * @param {RequestInfo} resource
+ * @param {RequestInit} options
+ * @returns {Promise<Response>}
+ */
+
 export default class WebStorage {
 
-    #origin=new URL(import.meta.url).origin
+    #origin = new URL(import.meta.url).origin
     /** @type {import('@twurple/auth').AuthProvider} */
     #auth_provider = null;
     /** @type {Cache} */
     #cache = null
+    /** @type {fetch} */
+    #fetch = null
 
     /**
-     * 
-     * @param {import('@twurple/auth').AuthProvider} auth_provider 
+     * Creates a fetch request wrapper that returns cached responses when the server can't be reached.
+     * When only a path is provided, the origin defaults to the same origin this module was loaded from.
+     * When the origin matches this script, authorization headers are added automatically.
+     * You can also use this for GET requests to any orgigin, but other methods may have unknown behavior.
+     * @param {import('@twurple/auth').AuthProvider} auth_provider used to add the authentication header to requests for web storage
+     * @param {fetch} fetch defaults to `globalThis.fetch`, allows you to further customize fetch behavior via chaining, for example with `fetch-retry`
      */
-    constructor(auth_provider) {
+    constructor(auth_provider, fetch = globalThis.fetch) {
+        this.#fetch = fetch
         this.#auth_provider = auth_provider
         auth_provider.getAccessTokenForUser()
             .then(token => caches.open(token.userId + '/' + auth_provider.clientId))
@@ -19,21 +33,23 @@ export default class WebStorage {
 
     /**
      * 
-     * @param {String} resource 
+     * @param {String | URL} resource 
      * @param {RequestInit} options 
      */
     async fetch(resource, options) {
+        resource = new URL(resource, this.#origin)
+        if (resource.origin == this.#origin) {
             const token = await this.#auth_provider.getAccessTokenForUser()
             if (!options.headers) {
                 options.headers = {}
             }
             options.headers['authorization'] = 'OAuth ' + token.accessToken
-        resource = new URL(resource, this.#origin)
+        }
         if ('method' in options) {
             switch (options.method) {
                 case 'PUT':
                 case 'POST': {
-                    const response = await fetch(resource, options)
+                    const response = await this.#fetch(resource, options)
                     if (!response.ok) {
                         return response
                     }
@@ -42,7 +58,7 @@ export default class WebStorage {
                     return response
                 }
                 case 'DELETE': {
-                    const response = await fetch(resource, options)
+                    const response = await this.#fetch(resource, options)
                     if (!response.ok) {
                         return response
                     }
@@ -51,7 +67,7 @@ export default class WebStorage {
                 }
             }
         }
-        return fetch(resource, options).then(
+        return this.#fetch(resource, options).then(
             response => {
                 this.#cache.put(resource, response)
                 return response