Injecting HTML before script evaluation with puppeteer

It appears that this is actually a very popular request and I perhaps should have searched more thoroughly before posting my question.

Nevertheless, I settled on the solution proposed by aslushnikov here.

The following code is just what I produced to test the idea, I'm sure there's significant room for improvement.

I made a simple function to perform XHRs:

const requestPage = async (url) => {
  return new Promise(function (resolve, reject) {
    let xhr = new XMLHttpRequest();
    xhr.open('GET', url);
    xhr.setRequestHeader('Ignore-Intercept', 'Value');
    xhr.onload = function () {
      if (this.status >= 200 && this.status < 300) {
        const response = {};
        xhr.getAllResponseHeaders()
          .trim()
          .split(/[\r\n]+/)
          .map(value => value.split(/: /))
          .forEach(keyValue => {
              response[keyValue[0].trim()] = keyValue[1].trim();
          });
        resolve({ ...response, body: xhr.response });
      } else {
        reject({
            status: this.status,
            statusText: xhr.statusText
        });
      }
    };
    xhr.onerror = function () {
      reject({
          status: this.status,
          statusText: xhr.statusText
      });
    };
    xhr.send();
  });
};

I then exposed this function to the page.

I then used this function to perform an XHR instead of allowing the request to go ahead and used the result of that as the response to the request.

await page.setRequestInterception(true);
page.on('request', async (request) => {
  if (
    request.url() === url
    && (
      typeof request.headers()['access-control-request-headers'] === 'undefined'
      || !request.headers()['access-control-request-headers'].match(/ignore-intercept/gi)
    ) && typeof request.headers()['ignore-intercept'] === 'undefined'
  ) {
    const response = await page.evaluate(`requestPage('${url}')`);
    response.body += "hello";
    request.respond(response);
  } else {
    request.continue();
  }
});

await page.goto(`data:text/html,<iframe style='width:100%; height:100%' src=${url}></iframe>`);

Annoyingly, it didn't seem possible to use page.evaluate unless the desired page was in an iframe. (hence the await page.goto(`data:text/html....

With the following snippet I was able to augment the body. I use this for mocking purposes.

const browser = await puppeteer.launch();
browser.on('targetchanged', async target => {
  const targetPage = await target.page();
  const client = await targetPage.target().createCDPSession();
  await client.send('Runtime.evaluate', {
    expression: `
      window.document.addEventListener("DOMContentLoaded", function () {
        const container = window.document.createElement('span');
        container.innerText = "Hello World!";
        window.document.body.appendChild(container);
      });
    `,
  });
});

I'm not entirely sure what targetchanged is. My assumption from fiddling with it is that its when the browser goes to a particular page "target" but I could be wrong.

Other Resources

• Set localstorage items before page loads in puppeteer?
• browser.on('targetcreated') and browser.on('targetdestroyed') are executed repeatedly