working_puppeteer_prototype_override.js

const puppeteer = require("puppeteer");

(async () => {
  const browser = await puppeteer.launch({ headless: false });
  const page = await browser.newPage();

  await page.evaluateOnNewDocument(() => {
    Object.defineProperty(HTMLCanvasElement.prototype, "toBlob", {
      value: () => {
        console.log("Hey there");
      },
    });
  });

  await page.goto("https://example.com");

  await page.evaluate(() => {
    console.log(HTMLCanvasElement.prototype.toBlob.toString());
  });

  // await browser.close();
})();