add wait method

mod.ts

@@ -9,7 +9,7 @@ export class EventEmitter<E extends Record<string, unknown[]>> {
   #listeners: {
     [K in keyof E]?: Array<{
       once: boolean;
-      cb: (...args: E[K]) => void;
+      cb: (...args: E[K]) => void | Promise<void>;
     }>;
   } = {};
   #globalWriters: WritableStreamDefaultWriter<Entry<E, keyof E>>[] = [];
@@ -183,6 +183,26 @@ export class EventEmitter<E extends Record<string, unknown[]>> {
     }
   }
 
+  /**
+   * Call and, if async, await the completion of the listeners registered for
+   * the event named eventName, in the order they were registered, passing the
+   * supplied arguments to each. Returns a Promise that resolves on the completion
+   * of the last listener.
+   *
+   * Does not support being awaited the same was as `emit`.
+   */
+  async wait<K extends keyof E>(eventName: K, ...args: E[K]): Promise<void> {
+    const listeners = this.#listeners[eventName]?.slice() ?? [];
+    for (const { cb, once } of listeners) {
+      const ret = cb(...args);
+      if (ret) await ret;
+
+      if (once) {
+        this.off(eventName, cb);
+      }
+    }
+  }
+
   [Symbol.asyncIterator]<K extends keyof E>(): AsyncIterableIterator<
     { [V in K]: Entry<E, V> }[K]
   > {

test.ts

@@ -171,7 +171,7 @@ Deno.test("closeMixed", async () => {
     }
   })();
 
-  for await (const x of ee) {
+  for await (const _ of ee) {
     await ee.off();
   }
 });
@@ -182,3 +182,17 @@ Deno.test("limitReached", () => {
   ee.on("foo", () => {});
   assertThrows(() => ee.on("foo", () => {}));
 });
+
+Deno.test("wait", async () => {
+  const ee = new EventEmitter<Events>();
+
+  ee.on("foo", async () => {
+    console.log("foo");
+    await new Promise((resolve) => setTimeout(resolve, 1000));
+  }).on("bar", () => {
+    console.log("bar");
+  });
+
+  await ee.wait("foo", "baz");
+  await ee.wait("bar", 0);
+});