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

# Fetching JavaScript Pages

> When to use render_js, how JS rendering works, and how to control it.

## When to use `render_js`

Set `"render_js": true` when the content you need is injected by JavaScript after the initial HTML loads. Common cases:

* Single-page applications (React, Vue, Angular)
* Infinite-scroll feeds where items appear after JS executes
* Pages behind login redirects that trigger on load
* Dynamic price or inventory widgets

If the page's HTML already contains the content (SSR pages, Wikipedia, static sites), skip `render_js` — it saves credits and latency.

## How it works

When `render_js` is `true`, the API launches a headless Chromium browser, navigates to the URL, waits for the network to settle, and captures the fully-rendered DOM. The browser is isolated per request and discarded afterwards.

Rendering typically adds 2–5 seconds of latency.

## Waiting for specific conditions

Use `wait_for` to hold rendering until a specific condition is met:

```json theme={null}
{
  "url": "https://example.com/dashboard",
  "render_js": true,
  "wait_for": { "network_idle": true },
  "output": ["html"]
}
```

`network_idle: true` waits until there are no in-flight network requests for 500 ms. This is useful for pages that fire background API calls after initial load.

## Setting a timeout

Use `timeout_ms` to cap how long the browser waits. The default is 15,000 ms (15 s) for inline requests. Async requests allow up to 300,000 ms (5 min).

```json theme={null}
{
  "url": "https://example.com/slow-app",
  "render_js": true,
  "timeout_ms": 30000,
  "output": ["markdown"]
}
```

If the timeout is exceeded, the API returns a `timeout` error code.

## Device emulation

Emulate mobile or tablet viewports to reach responsive content:

```json theme={null}
{
  "url": "https://m.example.com",
  "render_js": true,
  "device": "mobile",
  "output": ["html"]
}
```

Supported values: `"desktop"` (default), `"mobile"`, `"tablet"`.

## Sessions

Reuse a browser session across multiple fetches — cookies, local storage, and auth state are preserved:

```json theme={null}
{
  "url": "https://app.example.com/page-2",
  "render_js": true,
  "session": { "id": "sess_abc123" },
  "output": ["markdown"]
}
```

See [Identity & Sessions](/guides/identity-and-sessions) for the full session lifecycle.

## Cost

JS-rendered fetches cost 2 credits vs 1 for static fetches.