> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vortexpdf.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Evaluate JavaScript

> Execute JavaScript code in the page context before rendering to manipulate the DOM or wait for dynamic content

The `evaluate_js` parameter allows you to run JavaScript code in the page context before rendering occurs.

## Overview

When rendering a webpage with dynamic content, you may need to manipulate the DOM, trigger actions, or wait for content to load. The `evaluate_js` parameter lets you execute custom JavaScript code during the rendering process.

## Parameter details

<ResponseField name="evaluate_js" type="object">
  Allows you to run JavaScript code in the page context before rendering occurs

  <Expandable title="Properties">
    <ResponseField name="expression" type="string" required>
      JavaScript code to execute in the page context
    </ResponseField>
  </Expandable>
</ResponseField>

## Usage

```javascript theme={null}
{
  "parts": "https://example.com",
  "evaluate_js": {
    "expression": "document.querySelector('.load-more-button').click()"
  }
}
```

## Behavior

The JavaScript expression is evaluated in the page context before rendering begins. If the expression returns a Promise, the rendering process will wait for the Promise to resolve before proceeding.

## Use cases

This parameter is particularly useful for:

1. **Loading dynamic content**: Trigger actions that load additional content before capturing the page.

2. **DOM manipulation**: Modify page elements, styles, or attributes before rendering.

3. **Waiting for conditions**: Ensure specific elements or data are available before rendering.

## Examples

### Clicking a Button

```javascript theme={null}
{
  "parts": "https://example.com",
  "evaluate_js": {
    "expression": "document.querySelector('#load-more').click()"
  }
}
```

### Waiting for content to load

```javascript theme={null}
{
  "parts": "https://example.com",
  "evaluate_js": {
    "expression": "new Promise(resolve => { 
      const checkContent = () => {
        if (document.querySelector('.dynamic-content')) {
          resolve();
        } else {
          setTimeout(checkContent, 100);
        }
      };
      checkContent();
    })"
  }
}
```

### Modifying page content

```javascript theme={null}
{
  "parts": "https://example.com",
  "evaluate_js": {
    "expression": "document.querySelectorAll('.advertisement').forEach(ad => ad.remove())"
  }
}
```

### Scrolling the page

```javascript theme={null}
{
  "parts": "https://example.com",
  "evaluate_js": {
    "expression": "window.scrollTo(0, document.body.scrollHeight)"
  }
}
```