puppeteer/website/versioned_docs/version-21.3.6/api/puppeteer.page.waitforfunction.md

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---
sidebar_label: Page.waitForFunction
---
# Page.waitForFunction() method
Waits for a function to finish evaluating in the page's context.
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#### Signature:
```typescript
class Page {
waitForFunction<
Params extends unknown[],
Func extends EvaluateFunc<Params> = EvaluateFunc<Params>,
>(
pageFunction: Func | string,
options?: FrameWaitForFunctionOptions,
...args: Params
): Promise<HandleFor<Awaited<ReturnType<Func>>>>;
}
```
## Parameters
| Parameter | Type | Description |
| ------------ | ------------------------------------------------------------------------- | ------------------------------------------------------ |
| pageFunction | Func \| string | Function to be evaluated in browser context |
| options | [FrameWaitForFunctionOptions](./puppeteer.framewaitforfunctionoptions.md) | _(Optional)_ Options for configuring waiting behavior. |
| args | Params | |
**Returns:**
Promise&lt;[HandleFor](./puppeteer.handlefor.md)&lt;Awaited&lt;ReturnType&lt;Func&gt;&gt;&gt;&gt;
## Example 1
The [Page.waitForFunction()](./puppeteer.page.waitforfunction.md) can be used to observe viewport size change:
```ts
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import puppeteer from '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();
})();
```
## Example 2
To pass arguments from node.js to the predicate of [Page.waitForFunction()](./puppeteer.page.waitforfunction.md) function:
```ts
const selector = '.foo';
await page.waitForFunction(
selector => !!document.querySelector(selector),
{},
selector
);
```
## Example 3
The predicate of [Page.waitForFunction()](./puppeteer.page.waitforfunction.md) can be asynchronous too:
```ts
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
);
```