on(event, handler)

Registers a handler to be called whenever the specified event occurs.

Parameter	Type	Default	Description
event	string	`''`	Event to attach the handler to. Currently, `'console'` and `'metric'` events are supported.
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.
`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.

Console event Example

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

export const options = {
  scenarios: {
    ui: {
      executor: 'shared-iterations',
      options: {
        browser: {
          type: 'chromium',
        },
      },
    },
  },
  thresholds: {
    checks: ['rate==1.0'],
  },
};

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

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();
  }
}