[Home](./index.md) > [puppeteer](./puppeteer.md) > [Page](./puppeteer.page.md) > [waitForFunction](./puppeteer.page.waitforfunction.md)
## 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:
```typescript
waitForFunction(pageFunction: Function | string, options?: {
timeout?: number;
polling?: string | number;
}, ...args: SerializableOrJSHandle[]): Promise;
```
## Parameters
| Parameter | Type | Description |
| --- | --- | --- |
| pageFunction | Function \| string | Function to be evaluated in browser context |
| options | { timeout?: number; polling?: string \| number; } | Optional waiting parameters |
| args | [SerializableOrJSHandle](./puppeteer.serializableorjshandle.md)\[\] | Arguments to pass to pageFunction |
Returns:
Promise<[JSHandle](./puppeteer.jshandle.md)>
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)](./puppeteer.page.setdefaulttimeout.md) method.