puppeteer/website/versioned_docs/version-19.4.0/api/puppeteer.page.exposefunction.md

---
sidebar_label: Page.exposeFunction
---

# 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.

:::note

#### Signature:

```typescript
class Page {
  exposeFunction(
    name: string,
    pptrFunction:
      | Function
      | {
          default: Function;
        }
  ): Promise<void>;
}
```

## Parameters

| Parameter    | Type                               | Description                                                    |
| ------------ | ---------------------------------- | -------------------------------------------------------------- |
| name         | string                             | Name of the function on the window object                      |
| pptrFunction | Function \| { default: Function; } | Callback function which will be called in Puppeteer's context. |

**Returns:**

Promise&lt;void&gt;

## Example 1

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

```ts
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();
})();
```

## Example 2

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

```ts
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();
})();
```