puppeteer/website/versioned_docs/version-10.1.0/puppeteer.page.exposefunction.md

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[Home](./index.md) &gt; [puppeteer](./puppeteer.md) &gt; [Page](./puppeteer.page.md) &gt; [exposeFunction](./puppeteer.page.exposefunction.md)

## Page.exposeFunction() method

The method adds a function called `name` on the page's `window` object. When called, the function executes `puppeteerFunction` in node.js and returns a `Promise` which resolves to the return value of `puppeteerFunction`.

If the puppeteerFunction returns a `Promise`, it will be awaited.

NOTE: Functions installed via `page.exposeFunction` survive navigations.

<b>Signature:</b>

```typescript
exposeFunction(name: string, puppeteerFunction: Function): Promise<void>;
```

## Parameters

|  Parameter | Type | Description |
|  --- | --- | --- |
|  name | string | Name of the function on the window object |
|  puppeteerFunction | Function | Callback function which will be called in Puppeteer's context. |

<b>Returns:</b>

Promise&lt;void&gt;

## Example

An example of adding an `md5` function into the page:

```js
const puppeteer = require('puppeteer');
const crypto = require('crypto');

(async () => {
const browser = await puppeteer.launch();
const page = await browser.newPage();
page.on('console', (msg) => console.log(msg.text()));
await page.exposeFunction('md5', (text) =>
crypto.createHash('md5').update(text).digest('hex')
);
await page.evaluate(async () => {
// use window.md5 to compute hashes
const myString = 'PUPPETEER';
const myHash = await window.md5(myString);
console.log(`md5 of ${myString} is ${myHash}`);
});
await browser.close();
})();

```
An example of adding a `window.readfile` function into the page:

```js
const puppeteer = require('puppeteer');
const fs = require('fs');

(async () => {
const browser = await puppeteer.launch();
const page = await browser.newPage();
page.on('console', (msg) => console.log(msg.text()));
await page.exposeFunction('readfile', async (filePath) => {
return new Promise((resolve, reject) => {
fs.readFile(filePath, 'utf8', (err, text) => {
   if (err) reject(err);
   else resolve(text);
 });
});
});
await page.evaluate(async () => {
// use window.readfile to read contents of a file
const content = await window.readfile('/etc/hosts');
console.log(content);
});
await browser.close();
})();

```