Intercepting WebSockets in the browser using JavaScript

websocket_proxy.js

const WebSocketProxy = new Proxy(window.WebSocket, {
  construct(target, args) {
    console.log("Proxying WebSocket connection", ...args);
    const ws = new target(...args);
    
    // Configurable hooks
    ws.hooks = {
      beforeSend: () => null,
      beforeReceive: () => null
    };

    // Intercept send
    const sendProxy = new Proxy(ws.send, {
      apply(target, thisArg, args) {
        if (ws.hooks.beforeSend(args) === false) {
          return;
        }
        return target.apply(thisArg, args);
      }
    });
    ws.send = sendProxy;

    // Intercept events
    const addEventListenerProxy = new Proxy(ws.addEventListener, {
      apply(target, thisArg, args) {
        if (args[0] === "message" && ws.hooks.beforeReceive(args) === false) {
          return;
        }
        return target.apply(thisArg, args);
      }
    });
    ws.addEventListener = addEventListenerProxy;

    Object.defineProperty(ws, "onmessage", {
      set(func) {
        const onmessage = function onMessageProxy(event) {
          if (ws.hooks.beforeReceive(event) === false) {
            return;
          }
          func.call(this, event);
        };
        return addEventListenerProxy.apply(this, [
          "message",
          onmessage,
          false
        ]);
      }
    });

    // Save reference
    window._websockets = window._websockets || [];
    window._websockets.push(ws);
    
    return ws;
  }
});

window.WebSocket = WebSocketProxy;

// Usage
// After a websocket connection has been created:
window._websockets[0].hooks = {
  // Just log the outgoing request
  beforeSend: data => console.log(data),
  // Return false to prevent the on message callback from being invoked
  beforeReceive: data => false
};

for iframe:

var iframe=document.createElement('iframe');

document.body.append(iframe);

new iframe.contentWindow.WebSocket(...);