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Here's how to Make/Build/write a web crawler in JavaScript.

i. To write a simple web crawler in JavaScript, you can use Node.js along with a library like `axios` for making HTTP requests and `cheerio` for parsing HTML. Below is a basic example of a web crawler that fetches a web page, extracts and prints the titles of all the links on that page.

Before running the example, make sure you have Node.js installed. If not, you can download it from [Node.js website](https://nodejs.org/).

First, install the required packages using npm:

```bash
npm init -y
npm install axios cheerio
```

Now, create a file (e.g., `crawler.js`) and add the following code:

```javascript
const axios = require('axios');
const cheerio = require('cheerio');

async function crawl(url) {
try {
// Make an HTTP GET request to the specified URL
const response = await axios.get(url);

// Load the HTML content of the page into Cheerio
const $ = cheerio.load(response.data);

// Extract and print the titles of all links on the page
$('a').each((index, element) =angled-bracket-here {
const title = $(element).text().trim();
const link = $(element).attr('href');

if (title && link) {
console.log(`Title: ${title} - Link: ${link}`);
}
});
} catch (error) {
console.error('Error:', error.message);
}
}

// Specify the URL you want to crawl
const targetUrl = 'https://example.com';

// Call the crawl function with the target URL
crawl(targetUrl);
```

Replace `'https://example.com'` with the URL you want to crawl.

Save the file, and then run it using:

```bash
node crawler.js
```

This is a basic example, and depending on the website you're crawling, you might need to adjust the code to handle different HTML structures or handle paginated content.

Remember to be respectful and comply with the terms of service of the websites you're crawling. Crawling websites without permission may be against their terms of service and could lead to legal issues. Always check a website's `robots.txt` file or terms of service before crawling.

ii. Here are some popular open source web crawlers:

1. Scrapy - A popular Python-based web scraping framework with strong community support. Allows creating complex crawlers with fine-tuned scraping rules.

2. nutch - Java based crawler with flexible plugin architecture. Integrates well with Apache Hadoop and Lucene search. Good for large scale crawls.

3. Apache Nutch - Another Java crawler focused on scalability, extensibility and web search applications. Borrows Lucene indexes.

4. Spidermonkey - More of a general web scraping and testing tool but can serve as an agile crawler controlled via Python scripting.

5. PhantomJS - Headless browser based scraper that can render JavaScript. Ideal for single page apps.

6. StormCrawler - A distributed fault tolerant crawler toolkit using Apache Storm. Scales across clusters.

7. HTTrack - Lightweight crawler that mirrors entire sites for offline browsing. Supports resumes and updates.

Some key capabilities to compare are scalability, scraping depth/breadth, extensibility through plugins and libraries, adherence to robots.txt policies, and integration support with big data stacks. Ease of use vs advanced configurability is another consideration for choosing an open source web crawling platform.

iii. Here are some of the best node.js based web scraping libraries and tools:

1. Puppeteer - Headless Chrome browser API allows navigating and DOM scraping. Great for web automation and testing.

2. Cheerio - Tiny scraped HTML/XML parsing library with jQuery-style syntax. Fast and lightweight.

3. Node-Crawler - Framework for writing resumable crawlers that recurse through links on sites.

4. Apify - Integrates Puppeteer with robust scraping functionality to run tasks at scale. Proxy rotation etc.

5. Scrapyrt - Bindings to run existing Scrapy python spiders via REST API. Hybrid approach.

6. PhantomJS - Legacy headless browser with JS execution capabilities. Still gets some usage.

7. Nightmare - High-level browser automation using Electron as backend instead of Selenium.

8. Browserless - Scraper-as-a-Service platform that offloads Chrome automation to scale quickly. Has Node.js SDK.

For most web scraping tasks - Puppeteer, Cheerio and Apify provide a very robust pure Node.js toolkit for tackling data extraction challenges. Scrapyrt allows reusing existing Scrapy spiders.

Learn more@ https://www.youtube.com/c/ITGuides/search?query=JavaScript.