Guide

$0.004

JavaScript Rendering

Scrape dynamic websites that require JavaScript execution using our headless browser infrastructure.

Cost

JavaScript rendering uses Tier 4 (Browser). A basic JS-rendered scrape costs $0.004 per request.

When to Use JS Rendering

Most websites work fine with standard HTTP requests. Use JavaScript rendering only when needed:

Use JS Rendering For:

Single Page Applications (React, Vue, Angular)
Content loaded via AJAX/fetch
Infinite scroll pages
Sites requiring user interaction simulation
Pages with anti-bot JavaScript checks

Don't Need JS For:

Static HTML pages
Server-rendered content
APIs returning JSON
Most news/blog articles
Traditional e-commerce product pages

Auto Mode

Use mode: "auto" (default) and AlterLab will automatically detect if JS rendering is needed and escalate only when necessary.

Basic Usage

Enable JavaScript rendering by setting render_js: true in the advanced options:

Bash

curl-X POST https://api.alterlab.io/api/v1/scrape \-H"X-API-Key: YOUR_API_KEY"\-H"Content-Type: application/json"\-d'{
 "url": "https://example.com/spa-page",
 "advanced": {
 "render_js": true
 }
 }'

Try in Playground

Wait Conditions

Control when the page is considered "ready" for scraping. This is crucial for SPAs where content loads asynchronously.

Condition	Description	Best For
`domcontentloaded`	DOM is ready, external resources may still load	Fast pages, minimal JS
`load`	Page and all resources fully loaded	Image-heavy pages
`networkidle`	No network activity for 500ms (default)	SPAs, AJAX-heavy pages

Python

# Wait for network to be idle (default, best for SPAs)response = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://react-app.example.com","advanced":{"render_js":True,"wait_condition":"networkidle"}})
# Fast mode - don't wait for all resourcesresponse = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://simple-page.example.com","advanced":{"render_js":True,"wait_condition":"domcontentloaded"}})

Wait for Selector

Wait for a specific element to appear before capturing the page. This is the most reliable way to ensure dynamic content is loaded.

Python

# Wait for product grid to loadresponse = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://shop.example.com/products","advanced":{"render_js":True},"wait_for":".product-grid"# CSS selector})
# Wait for specific data attributeresponse = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://app.example.com/dashboard","advanced":{"render_js":True},"wait_for":"[data-loaded='true']"})

Timeout

The selector wait has a default timeout of 30 seconds. If the element doesn't appear, the request will return with whatever content is available.

Capturing Screenshots

Capture a full-page screenshot along with the HTML content. Screenshots are returned as base64-encoded PNG.

Python

import base64

response = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://example.com","advanced":{"render_js":True,"screenshot":True}})
data = response.json()
# Save screenshot to fileif data.get("screenshot"): screenshot_bytes = base64.b64decode(data["screenshot"])withopen("screenshot.png","wb")as f: f.write(screenshot_bytes)

Cost

Screenshots add +$0.0002 to your request.

PDF Generation

Generate a PDF of the rendered page. Useful for archiving or creating printable versions of web pages.

Python

import base64

response = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://example.com/report","advanced":{"render_js":True,"generate_pdf":True}})
data = response.json()
# Save PDF to fileif data.get("pdf"): pdf_bytes = base64.b64decode(data["pdf"])withopen("page.pdf","wb")as f: f.write(pdf_bytes)

Cost

PDF generation adds +$0.0004 to your request.

Single Page Apps (SPAs)

Modern SPAs built with React, Vue, or Angular require special handling. Here's a reliable pattern:

Python

# Scraping a React applicationresponse = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://react-store.example.com/products/123","advanced":{"render_js":True,"wait_condition":"networkidle"},# Wait for the main content container"wait_for":"[data-testid='product-details']","timeout":60# Allow more time for complex apps})
# Scraping a Vue application with lazy loadingresponse = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://vue-app.example.com/dashboard","advanced":{"render_js":True,"wait_condition":"networkidle"},"wait_for":".dashboard-loaded"})

Tips for SPAs:

1. Find a reliable selector: Look for elements that only appear after data loads
2. Use networkidle: This waits for all AJAX calls to complete
3. Increase timeout: Complex apps may need 60-90 seconds
4. Check for loading states: Wait for spinners/skeletons to disappear

Timeout & Scroll Configuration

Request Timeout

Control the maximum time (in seconds) for a JS rendering request with the timeout parameter:

Parameter	Range	Default
`timeout`	1 - 300 seconds	90 seconds

Scroll Configuration

Use the enable_scroll parameter to control automatic page scrolling for lazy-loaded images and content:

Value	Behavior	Overhead
`true`	Always scroll to capture lazy-loaded images	+5-10 seconds
`false`	Never scroll (faster, may miss dynamic images)	None
`null` (default)	Auto-detect: scrolls unless the site is a social media platform	Varies

JSON

{"url":"https://example.com/gallery","advanced":{"render_js":true},"enable_scroll":true,"timeout":120}

Infinite Scroll Pages

For pages with infinite scroll, you'll typically get the initial viewport content. For full content, consider using pagination or the async API with custom scroll handling.

Python

# Get initial content from infinite scroll pageresponse = requests.post("https://api.alterlab.io/api/v1/scrape", headers={"X-API-Key":"YOUR_API_KEY"}, json={"url":"https://social-feed.example.com","advanced":{"render_js":True,"wait_condition":"networkidle"},# Wait for initial items to load"wait_for":".feed-item:nth-child(10)"})
# For more content, look for pagination APIs# Many "infinite scroll" sites have underlying REST/GraphQL APIs# that you can call directly without JS rendering

Pro Tip

Many infinite scroll sites have underlying APIs that return JSON. Use browser DevTools to find these APIs, then scrape them directly without JS rendering for better performance and lower costs.

Cost Optimization

JS rendering costs 4x more than basic scraping. Here's how to minimize costs:

1. Use Auto Mode

Set mode: "auto" and let AlterLab detect when JS is needed. We'll try cheaper methods first.

2. Set Cost Controls

Limit how much you're willing to spend per request:

JSON

{"url":"https://example.com","cost_controls":{"max_tier":"3",// Don't escalate beyond stealth"max_cost":0.001,// Cap at $0.001 per request"prefer_cost":true// Optimize for lowest cost}}

3. Cache Results

Enable caching for pages that don't change frequently:

JSON

{"url":"https://example.com","cache":true,"cache_ttl":3600// Cache for 1 hour}

Troubleshooting

Content is empty or incomplete

Try using wait_for with a specific selector
Increase the timeout value
Use wait_condition: "networkidle"

Page shows "Please enable JavaScript"

Make sure render_js: true is set in advanced options
The site may have additional anti-bot measures - try using a higher tier

Request times out

Increase timeout (max 300 seconds)
Use sync: false for long-running scrapes
Check if the selector in wait_for actually exists

Getting blocked or CAPTCHAs

AlterLab automatically handles most anti-bot measures
For persistent blocks, we'll escalate to CAPTCHA-solving tier automatically
Consider using your own proxies with use_own_proxy: true

Node.js Output Formats

Last updated: June 2026

URL: https://alterlab.io/docs/guides/javascript-rendering

⇱

JavaScript Rendering

When to Use JS Rendering

Use JS Rendering For:

Don't Need JS For:

Basic Usage

Wait Conditions

Wait for Selector

Capturing Screenshots

PDF Generation

Single Page Apps (SPAs)

Tips for SPAs:

Timeout & Scroll Configuration

Request Timeout

Scroll Configuration

Infinite Scroll Pages

Cost Optimization

1. Use Auto Mode

2. Set Cost Controls

3. Cache Results

Troubleshooting

Content is empty or incomplete

Page shows "Please enable JavaScript"

Request times out

Getting blocked or CAPTCHAs