feat(client): url option accepts a function or a Promise

docs/interfaces/client.clientoptions.md

@@ -213,10 +213,18 @@ ___
 
 ### url
 
-• **url**: *string*
+• **url**: *string* \| () => *string* \| *Promise*<string\>
 
 URL of the GraphQL over WebSocket Protocol compliant server to connect.
 
+If the option is a function, it will be called on every WebSocket connection attempt.
+Returning a promise is supported too and the connecting phase will stall until it
+resolves with the URL.
+
+A good use-case for having a function is when using the URL for authentication,
+where subsequent reconnects (due to auth) may have a refreshed identity token in
+the URL.
+
 ___
 
 ### webSocketImpl

src/client.ts

@@ -81,8 +81,18 @@ export type EventListener<E extends Event> = E extends EventConnecting
 
 /** Configuration used for the GraphQL over WebSocket client. */
 export interface ClientOptions {
-  /** URL of the GraphQL over WebSocket Protocol compliant server to connect. */
-  url: string;
+  /**
+   * URL of the GraphQL over WebSocket Protocol compliant server to connect.
+   *
+   * If the option is a function, it will be called on every WebSocket connection attempt.
+   * Returning a promise is supported too and the connecting phase will stall until it
+   * resolves with the URL.
+   *
+   * A good use-case for having a function is when using the URL for authentication,
+   * where subsequent reconnects (due to auth) may have a refreshed identity token in
+   * the URL.
+   */
+  url: string | (() => Promise<string> | string);
   /**
    * Optional parameters, passed through the `payload` field with the `ConnectionInit` message,
    * that the client specifies when establishing a connection with the server. You can use this
@@ -330,7 +340,10 @@ export function createClient(options: ClientOptions): Client {
           }
 
           emitter.emit('connecting');
-          const socket = new WebSocketImpl(url, GRAPHQL_TRANSPORT_WS_PROTOCOL);
+          const socket = new WebSocketImpl(
+            typeof url === 'function' ? await url() : url,
+            GRAPHQL_TRANSPORT_WS_PROTOCOL,
+          );
 
           socket.onerror = (err) => {
             // we let the onclose reject the promise for correct retry handling

src/tests/client.ts

@@ -140,6 +140,19 @@ it('should use the provided WebSocket implementation', async () => {
   await server.waitForClient();
 });
 
+it('should accept a function for the url', async () => {
+  const { url, ...server } = await startTServer();
+
+  createClient({
+    url: () => Promise.resolve(url),
+    retryAttempts: 0,
+    onNonLazyError: noop,
+    lazy: false,
+  });
+
+  await server.waitForClient();
+});
+
 it('should not accept invalid WebSocket implementations', async () => {
   const { url } = await startTServer();