Decode inlined sounds directly instead of fetching them, with the test.

src/soundUtils.test.ts

@@ -0,0 +1,77 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import { prefetchSounds } from "./soundUtils";
+
+const bytesOf = (buffer: ArrayBuffer): number[] => [...new Uint8Array(buffer)];
+
+describe("prefetchSounds", () => {
+  const fetchSpy = vi.fn<typeof fetch>();
+
+  beforeEach(() => {
+    vi.stubGlobal("fetch", fetchSpy);
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    fetchSpy.mockReset();
+  });
+
+  it("fetches sounds given as ordinary URLs", async () => {
+    fetchSpy.mockResolvedValue(
+      new Response(new Uint8Array([1, 2, 3]), { status: 200 }),
+    );
+    const sounds = await prefetchSounds({
+      beep: { ogg: "https://example.org/beep.ogg" },
+    });
+    expect(fetchSpy).toHaveBeenCalledWith("https://example.org/beep.ogg");
+    expect(bytesOf(sounds.beep)).toEqual([1, 2, 3]);
+  });
+
+  it("decodes base64 data: URLs without fetching them", async () => {
+    // "OggS" — the bytes 0x4f 0x67 0x67 0x53
+    const sounds = await prefetchSounds({
+      beep: { ogg: "data:audio/ogg;base64,T2dnUw==" },
+    });
+    expect(fetchSpy).not.toHaveBeenCalled();
+    expect(bytesOf(sounds.beep)).toEqual([0x4f, 0x67, 0x67, 0x53]);
+  });
+
+  it("decodes percent-encoded data: URLs without fetching them", async () => {
+    const sounds = await prefetchSounds({
+      beep: { ogg: "data:audio/ogg,Ogg%53" },
+    });
+    expect(fetchSpy).not.toHaveBeenCalled();
+    expect(bytesOf(sounds.beep)).toEqual([0x4f, 0x67, 0x67, 0x53]);
+  });
+
+  it("leaves out a sound whose response is not okay", async () => {
+    fetchSpy.mockResolvedValue(new Response(null, { status: 404 }));
+    const sounds = await prefetchSounds({
+      beep: { ogg: "https://example.org/missing.ogg" },
+    });
+    expect(sounds).toEqual({});
+  });
+
+  it("leaves out a sound that fails to load, keeping the others", async () => {
+    fetchSpy.mockRejectedValue(new TypeError("Failed to fetch"));
+    const sounds = await prefetchSounds({
+      beep: { ogg: "https://example.org/beep.ogg" },
+      boop: { ogg: "data:audio/ogg;base64,T2dnUw==" },
+    });
+    expect(Object.keys(sounds)).toEqual(["boop"]);
+  });
+
+  it("leaves out a sound with a malformed data: URL", async () => {
+    const sounds = await prefetchSounds({
+      beep: { ogg: "data:audio/ogg;base64" },
+    });
+    expect(sounds).toEqual({});
+  });
+});

src/soundUtils.ts

@@ -32,6 +32,43 @@ function getPreferredAudioFormat(): "ogg" | "mp3" {
 
 const preferredFormat = getPreferredAudioFormat();
 
+/**
+ * Decode a `data:` URL into the bytes it carries.
+ *
+ * The component build inlines the sound files as `data:` URLs. Those could be
+ * `fetch`ed like any other URL, but that fetch is subject to the host page's
+ * Content Security Policy, and `connect-src` lists rarely allow `data:` (a `*`
+ * source does not cover it). Reading the bytes out of the URL directly needs no
+ * network access, so no policy can stop it.
+ * @param url The `data:` URL to decode.
+ * @returns The decoded bytes.
+ */
+function decodeDataUrl(url: string): ArrayBuffer {
+  const separator = url.indexOf(",");
+  if (!url.startsWith("data:") || separator === -1)
+    throw new Error("Not a valid data: URL");
+  const header = url.slice("data:".length, separator);
+  const payload = url.slice(separator + 1);
+  const decoded = header.split(";").includes("base64")
+    ? atob(payload)
+    : decodeURIComponent(payload);
+  // `decoded` is a binary string: one char per byte
+  const bytes = Uint8Array.from(decoded, (c) => c.charCodeAt(0));
+  return bytes.buffer;
+}
+
+/**
+ * Load the bytes of a single sound file.
+ * @param url Where the sound is: an ordinary URL to fetch, or a `data:` URL.
+ * @returns The bytes, or null if the sound could not be loaded.
+ */
+async function loadSound(url: string): Promise<ArrayBuffer | null> {
+  if (url.startsWith("data:")) return decodeDataUrl(url);
+  const response = await fetch(url);
+  if (isFailure(response)) return null;
+  return await response.arrayBuffer();
+}
+
 /**
  * Prefetch sounds to be used by the AudioContext. This can
  * be called outside the scope of a component to ensure the
@@ -47,18 +84,21 @@ export async function prefetchSounds<S extends string>(
     Object.entries(sounds).map(async ([name, file]) => {
       const { mp3, ogg } = file as SoundDefinition;
       // Use preferred format, fallback to ogg if no mp3 is provided.
-      // Load an audio file
-      const response = await fetch(
-        preferredFormat === "ogg" ? ogg : (mp3 ?? ogg),
-      );
-      if (isFailure(response)) {
+      const url = preferredFormat === "ogg" ? ogg : (mp3 ?? ogg);
+      let buffer: ArrayBuffer | null = null;
+      try {
+        buffer = await loadSound(url);
+      } catch (e) {
+        logger.warn(`Could not load sound ${name}`, e);
+        return;
+      }
+      if (buffer === null) {
         // If the sound doesn't load, it's not the end of the world. We won't play
         // the sound when requested, but it's better than failing the whole application.
         logger.warn(`Could not load sound ${name}, response was not okay`);
         return;
       }
-      // Decode it
-      buffers[name] = await response.arrayBuffer();
+      buffers[name] = buffer;
     }),
   );
   return buffers as Record<S, ArrayBuffer>;