index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>BrowserExec</title>
  </head>
  <body>
    <h1>Ready to execute javascript!</h1>
    <h2>Websocket status: <span id="status"></span></h2>
    <script>
      const status = document.getElementById('status');

      const socket = new WebSocket(`ws://${window.location.host}`);
      socket.addEventListener('open', (event) => {
        status.innerText = 'open';
        socket.send(JSON.stringify({ event: 'connected' }));
      });
      socket.addEventListener('close', (event) => {
        status.innerText = 'close';
      });
      socket.addEventListener('error', (event) => {
        status.innerText = 'error';
      });
      socket.addEventListener('message', (event) => {
        const data = JSON.parse(event.data);
        console.log(data.exec);
        let error, res;
        try {
          res = eval(data.exec);
        } catch (e) {
          error = e;
        }
        console.log(res);
        socket.send(
          JSON.stringify({
            event: 'result',
            result: res || null,
            error: error || null,
            id: data.id,
          })
        );
      });
    </script>
  </body>
</html>