Introduction to `pac-resolver`

`pac-resolver` is a Node.js library that enables the evaluation of Proxy Auto-Config (PAC) files. It is a powerful tool for determining the appropriate proxy for a given URL during web traffic routing. PAC files are a way to define how web browsers and other agents can automatically choose the proxy server for fetching a given URL.

Key APIs and Their Uses

1. Creating a PAC Resolver from a String

Use the createPacResolver function to create a PAC resolver from a PAC script string.

  const createPacResolver = require('pac-resolver');
  const pacScript = 'function FindProxyForURL(url, host) { return "PROXY proxy.example.com:8080; DIRECT"; }';
  const findProxyForURL = createPacResolver(pacScript);

2. Using the PAC Resolver to Get a Proxy for a URL

With the PAC resolver, call the findProxyForURL function to determine the proxy for a given URL.

  findProxyForURL('http://example.com', (err, proxy) => {
   if (err) {
     console.error('Error occurred:', err);
     return;
   }
   console.log('Proxy for example.com:', proxy);
  });

3. Handling Asynchronous PAC Files

For PAC files that involve asynchronous operations, such as DNS resolutions, `pac-resolver` natively supports asynchronous execution. Simply use the asynchronous version of the PAC resolver:

  (async () => {
    try {
      const proxy = await findProxyForURL('http://example.com');
      console.log('Proxy for example.com:', proxy);
    } catch (err) {
      console.error('Error occurred:', err);
    }
  })();

Example Application Using `pac-resolver`

Let’s create a simple Node.js application that uses `pac-resolver` to route web traffic.

  const http = require('http');
  const createPacResolver = require('pac-resolver');
  
  const pacScript = 'function FindProxyForURL(url, host) { return "PROXY proxy.example.com:8080; DIRECT"; }';
  const findProxyForURL = createPacResolver(pacScript);
  
  const server = http.createServer(async (req, res) => {
     try {
        const proxy = await findProxyForURL(req.url);
        res.writeHead(200, { 'Content-Type': 'text/plain' });
        res.end('Proxy for ' + req.url + ': ' + proxy);
     } catch (err) {
        res.writeHead(500, { 'Content-Type': 'text/plain' });
        res.end('Error occurred: ' + err.message);
     }
  });
  
  server.listen(3000, () => {
     console.log('Server started on http://localhost:3000');
  });

Conclusion

In this guide, we have explored the `pac-resolver` library, covered its primary functions, and demonstrated how to integrate it into a Node.js application to route web traffic efficiently using PAC files. Understanding and utilizing `pac-resolver` can lead to more dynamic and flexible network configurations.

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