[Home](./index.md) > [puppeteer](./puppeteer.md) > [Page](./puppeteer.page.md) > [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.
Signature:
```typescript
exposeFunction(name: string, puppeteerFunction: Function): Promise;
```
## 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. |
Returns:
Promise<void>
## 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();
})();
```