Puppeteer | Wait for all JavaScript is executed

async function takeScreenshot(browser, viewport, route) {
  return browser.newPage().then(async (page) => {
    const fileName = `${viewport.directory}/${getFilename(route)}`;

    await page.setViewport({
      width: viewport.width,
      height: 500,
    });
    await page.goto(
        `${config.server.master}${route}.html`,
        {
          waitUntil: 'networkidle0',
        }
    );
    await page.evaluate(() => {
      scroll(0, 99999)
    });
    await page.waitFor(5000);
    await page.screenshot({
      path: `screenshots/master/${fileName}.png`,
      fullPage: true,
    });

    await page.close();
    console.log(`Viewport "${viewport.name}", Route "${route}"`);
  });
}

The following waitForFunction might be useful for you, you can use it to wait for any arbitrary function to evaluate to true. If you have access to the page's code you can set the window status and use that to notify puppeteer it is safe to continue, or just rely on some sort of other ready state. Note: this function is a polling function, and re-evaluates at some interval which can be specified.

const watchDog = page.waitForFunction('<your function to evaluate to true>');

E.g.,

const watchDog = page.waitForFunction('window.status === "ready"');
await watchDog;

In your page's code you simply need to set the window.status to ready

To utilize multiple watchdogs in multiple asynchronous files you could do something like

index.js

...import/require file1.js;
...import/require file2.js;
...code...

file1.js:

var file1Flag=false; // global
...code...
file1Flag=true;

file2.js:

var file2Flag=false; // global
...code...
file2Flag=true;

main.js:

const watchDog = page.waitForFunction('file1Flag && file2Flag');
await watchDog;