Page.waitForFunction() method

The waitForFunction can be used to observe viewport size change:

const puppeteer = require('puppeteer');
(async () => {
const browser = await puppeteer.launch();
const page = await browser.newPage();
const watchDog = page.waitForFunction('window.innerWidth < 100');
await page.setViewport({ width: 50, height: 50 });
await watchDog;
await browser.close();
})();

To pass arguments from node.js to the predicate of page.waitForFunction function:

const selector = '.foo';
await page.waitForFunction(
(selector) => !!document.querySelector(selector),
{},
selector
);

The predicate of page.waitForFunction can be asynchronous too:

const username = 'github-username';
await page.waitForFunction(
async (username) => {
const githubResponse = await fetch(
 `https://api.github.com/users/${username}`
);
const githubUser = await githubResponse.json();
// show the avatar
const img = document.createElement('img');
img.src = githubUser.avatar_url;
// wait 3 seconds
await new Promise((resolve, reject) => setTimeout(resolve, 3000));
img.remove();
},
{},
username
);

Signature:

waitForFunction<Params extends unknown[], Func extends EvaluateFunc<Params> = EvaluateFunc<Params>>(pageFunction: Func | string, options?: {
        timeout?: number;
        polling?: string | number;
    }, ...args: Params): Promise<HandleFor<Awaited<ReturnType<Func>>>>;

Parameters

Parameter	Type	Description
pageFunction	Func | string	Function to be evaluated in browser context
options	{ timeout?: number; polling?: string | number; }	(Optional) Optional waiting parameters
args	Params	Arguments to pass to pageFunction

Returns:

Promise<HandleFor<Awaited<ReturnType<Func>>>>

Promise which resolves when the pageFunction returns a truthy value. It resolves to a JSHandle of the truthy value.

The optional waiting parameter can be:

• Polling: An interval at which the pageFunction is executed, defaults to raf. If polling is a number, then it is treated as an interval in milliseconds at which the function would be executed. If polling is a string, then it can be one of the following values:
  - raf: to constantly execute pageFunction in requestAnimationFrame callback. This is the tightest polling mode which is suitable to observe styling changes.
  - mutation: to execute pageFunction on every DOM mutation.

• timeout: maximum time to wait for in milliseconds. Defaults to 30000 (30 seconds). Pass 0 to disable timeout. The default value can be changed by using the page.setDefaultTimeout(timeout) method.