on(event, handler)

Registers a handler to be called whenever the specified event occurs. This method can also be used to prevent the too many time series error when using the k6 browser module. For more details, refer to Prevent too many time series error.

Parameter	Type	Default	Description
event	string	`''`	Event to attach the handler to. See the supported events.
handler	function	`null`	A function to be called every time the specified event is emitted.

Events

Event	Description
`console`	Emitted every time the console API methods are called from within the page JavaScript context. The arguments passed into the handler are defined by the `ConsoleMessage` class. See the example.
`metric`	Emitted every time a metric is measured and emitted for the page. The arguments passed into the handler are defined by the `MetricMessage` object. See the example.
`request`	Emitted every time a request made by the page. The handler gets a `Request` object that contains information about the request. See the example.
`response`	Emitted every time a response is received by the page. The handler gets a `Response` object that contains information about the response. See the example.

Console event example

JavaScript
import { browser } from 'k6/browser';
import { check } from 'k6';

export const options = {
  scenarios: {
    ui: {
      executor: 'shared-iterations',
      options: {
        browser: {
          type: 'chromium',
        },
      },
    },
  },
};

export default async function () {
  const page = await browser.newPage();

  try {
    await page.goto('https://test.k6.io/');

    page.on('console', (msg) => {
      check(msg, {
        assertConsoleMessageType: (msg) => msg.type() == 'log',
        assertConsoleMessageText: (msg) => msg.text() == 'this is a console.log message 42',
        assertConsoleMessageArgs0: (msg) =>
          msg.args()[0].jsonValue() == 'this is a console.log message',
        assertConsoleMessageArgs1: (msg) => msg.args()[1].jsonValue() == 42,
      });
    });

    await page.evaluate(() => console.log('this is a console.log message', 42));
  } finally {
    await page.close(); // required so iteration can end
  }
}

Metric event example

JavaScript
import { browser } from 'k6/browser';

export const options = {
  scenarios: {
    ui: {
      executor: 'shared-iterations',
      options: {
        browser: {
          type: 'chromium',
        },
      },
    },
  },
};

export default async function () {
  const page = await browser.newPage();

  // We first register a handler using page.on('metric'). Calling page.on('metric')
  // multiple times is allowed. The registered handlers will be executed in the
  // order page.on was called.
  page.on('metric', (metric) => {
    // Using metric.tag finds a match between the current metric url and name
    // tags against the supplied regular expression in `url`.
    //
    // At the moment metric.tag is the only method on the metricMessage object.
    metric.tag({
      // This is the new name value that will replace the existing value in the
      // url and name tags when a match is found.
      name: 'test',
      // You can provide multiple matches here.
      matches: [
        {
          url: /^https:\/\/test\.k6\.io\/\?q=[0-9a-z]+$/,
          // When a method is defined it will also need to match on that too. If a
          // method is not provided it will match on all method types.
          method: 'GET',
        },
      ],
    });
  });

  try {
    // This is only for illustrative purposes, the q query param doesn't affect
    // the website.
    await page.goto('https://test.k6.io/?q=abc123');
    await page.goto('https://test.k6.io/?q=def456');
  } finally {
    await page.close();
  }
}

Request and response events example

JavaScript
import { browser } from 'k6/browser';

export const options = {
  scenarios: {
    ui: {
      executor: 'shared-iterations',
      options: {
        browser: {
          type: 'chromium',
        },
      },
    },
  },
};

export default async function () {
  const page = await browser.newPage();

  // registers a handler that logs all requests made by the page
  page.on('request', async (request) => console.log(request.url()));
  // registers a handler that logs all responses received by the page
  page.on('response', async (response) => console.log(response.url()));

  await page.goto('https://quickpizza.grafana.com/', { waitUntil: 'networkidle' });
  await page.close();
}

Output:

bash
INFO[0000] https://quickpizza.grafana.com/                  source=console
INFO[0001] https://quickpizza.grafana.com/api/tools         source=console
INFO[0001] https://quickpizza.grafana.com/images/pizza.png  source=console
...