Add json-rpc-over-ws example

examples/json-rpc-over-websocket/web_root/rpc-over-websocket.js

@@ -0,0 +1,36 @@
+// JSON-RPC over Websocket implementation
+var JSONRPC_TIMEOUT_MS = 1000;
+
+var jsonrpc = function(url, onopen, onclose, onnotification) {
+  var rpcid = 0, pending = {}, ws = new WebSocket(url);
+  if (!ws) return null;
+  ws.onclose = onclose;
+  ws.onmessage = function(ev) {
+    const frame = JSON.parse(ev.data);
+    console.log('rcvd', frame, 'pending:', pending);
+    if (frame.id !== undefined) {
+      if (pending[frame.id] !== undefined) pending[frame.id](frame);  // Resolve
+      delete (pending[frame.id]);
+    } else {
+      if (onnotification) onnotification(frame);
+    }
+  };
+  if (onopen) onopen();
+  return {
+    close: () => ws.close(),
+    call: function(method, params) {
+      const id = rpcid++, request = {id, method, params};
+      ws.send(JSON.stringify(request));
+      console.log('sent', request);
+      return new Promise(function(resolve, reject) {
+        setTimeout(JSONRPC_TIMEOUT_MS, function() {
+          if (pending[id] === undefined) return;
+          log('Timing out frame ', JSON.stringify(request));
+          delete (pending[id]);
+          reject();
+        });
+        pending[id] = x => resolve(x);
+      });
+    },
+  };
+};