fix: use binaryType correctly

The `.binaryType` field on the `RTCDataChannel` class defines what
type the `.data` field on emitted `MessageEvent`s will be.

It's either `arraybuffer` (the default) or `blob`.

Docs: https://developer.mozilla.org/en-US/docs/Web/API/RTCDataChannel/binaryType

1. Switches the default to `arraybuffer`
2. Converts incoming data to the correct type based on the `binaryType`

I've done my best to assert the right types/values in the "P2P test"
in `polyfill.test.ts`.

Because getting the binary data out of a `Blob` is an async operation
I had to make the test async, but it doesn't fit very well with the
current nature of the test.

TBH that test is very complicated and would be much better off split
into multiple smaller tests, each testing one thing.

Author: achingbrain

src/polyfill/RTCDataChannel.ts

@@ -2,6 +2,8 @@
 import * as exceptions from './Exception';
 import { DataChannel } from '../lib/index';
 
+const encoder = new TextEncoder()
+
 export default class RTCDataChannel extends EventTarget implements globalThis.RTCDataChannel {
     #dataChannel: DataChannel;
     #readyState: globalThis.RTCDataChannelState;
@@ -26,7 +28,7 @@ export default class RTCDataChannel extends EventTarget implements globalThis.RT
         super();
 
         this.#dataChannel = dataChannel;
-        this.#binaryType = 'blob';
+        this.#binaryType = 'arraybuffer';
         this.#readyState = this.#dataChannel.isOpen() ? 'open' : 'connecting';
         this.#bufferedAmountLowThreshold = 0;
         this.#maxPacketLifeTime = opts.maxPacketLifeTime || null;
@@ -70,15 +72,33 @@ export default class RTCDataChannel extends EventTarget implements globalThis.RT
             this.dispatchEvent(new Event('bufferedamountlow'));
         });
 
-        this.#dataChannel.onMessage((data) => {
-            if (ArrayBuffer.isView(data)) {
-                if (this.binaryType == 'arraybuffer')
-                    data = data.buffer;
-                else
-                    data = Buffer.from(data.buffer);
+        this.#dataChannel.onMessage((message) => {
+            if (typeof message === 'string') {
+                message = encoder.encode(message);
+            }
+
+            let data: Blob | ArrayBuffer;
+
+            if (message instanceof ArrayBuffer) {
+                data = message;
+            } else  {
+                data = message.buffer;
+
+                if (message.byteOffset !== 0 || message.byteLength !== message.buffer.byteLength) {
+                    // message is view on underlying buffer, must create new
+                    // ArrayBuffer that only contains message data
+                    data = new ArrayBuffer(message.byteLength)
+                    new Uint8Array(data, 0, message.byteLength).set(message)
+                }
+            }
+
+            if (this.#binaryType === 'blob') {
+                data = new Blob([data]);
             }
 
-            this.dispatchEvent(new MessageEvent('message', { data }));
+            this.dispatchEvent(new MessageEvent('message', {
+                data
+            }));
         });
 
         // forward events to properties