Open source

Socket.on(event, callback)


A module with a better and standard API exists.

The new k6/experimental/websockets API partially implements the WebSockets API living standard.

When possible, we recommend using the new API. It uses a global event loop for consistency with other k6 APIs and better performance.

Set up callback functions for various events on the WebSocket connection. Multiple handlers can be defined for the same event.

eventstringThe event name to define a callback for.
callbackfunctionThe function to call when the event happens.
Event nameDescription
openEmitted when the connection is established
messageEmitted when a message is received from the server.
pingEmitted when a ping is received from the server. The client will automatically send back a pong.
pongEmitted when a pong is received from the server.
closeEmitted when the connection is closed by the client Socket.close() or when the server sends the close event with code status 1000 (normal closure).
errorEmitted when an error occurs. Non-normal closure errors will be forwarded.


import ws from 'k6/ws';
import { check } from 'k6';

export default function () {
  const url = 'ws://';
  const params = { tags: { my_tag: 'hello' } };

  const response = ws.connect(url, params, function (socket) {
    socket.on('open', function open() {

      socket.setInterval(function timeout() {;
        console.log('Pinging every 1sec (setInterval test)');
      }, 1000);

    socket.on('ping', function () {

    socket.on('pong', function () {

    socket.on('pong', function () {
      // Multiple event handlers on the same event
      console.log('OTHER PONG!');

    socket.on('message', function (message) {
      console.log(`Received message: ${message}`);

    socket.on('close', function () {

    socket.on('error', function (e) {
      if (e.error() != 'websocket: close sent') {
        console.log('An unexpected error occured: ', e.error());

    socket.setTimeout(function () {
      console.log('2 seconds passed, closing the socket');
    }, 2000);

  check(response, { 'status is 101': (r) => r && r.status === 101 });