Open source


Adds a script which will be evaluated in one of the following scenarios:

The script is evaluated after the document is created but before any of its scripts are run. This is useful to amend the JavaScript environment, for example, to override Math.random.


Promise<void>A Promise that fulfills when the script has been added to the browser context.


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

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

export default async function () {
  const browserContext = await browser.newContext();

  // This will execute and override the existing implementation of Math.random.
  await browserContext.addInitScript('Math.random = function(){return 0}');

  const page = await browserContext.newPage();

  // In this example we are going to set the content manually, so we first
  // navigate to about:blank which will execute the init script, before setting
  // the content on the page.
  await page.goto('about:blank');

  await page.setContent(`
      <div id="random">waiting...</div>
        document.getElementById('random').textContent = Math.random();

  const text = await p.locator('#random').textContent();
  check(page, {
    zero: () => text == '0',

  await page.close();