It provides an abstract class that must define a connect() function, which is responsible for creating the underlying socket that the HTTP client requests will use.

The connect() function may return an arbitrary Duplex stream, or another http.Agent instance to delegate the request to, and may be asynchronous (by defining an async function).

Instances of this agent can be used with the http and https modules. To differentiate, the options parameter in the connect() function includes a secureEndpoint property, which can be checked to determine what type of socket should be returned.

Some subclasses:

Here are some more interesting uses of agent-base. Send a pull request to list yours!

http-proxy-agent: An HTTP(s) proxy http.Agent implementation for HTTP endpoints
https-proxy-agent: An HTTP(s) proxy http.Agent implementation for HTTPS endpoints
pac-proxy-agent: A PAC file proxy http.Agent implementation for HTTP and HTTPS
socks-proxy-agent: A SOCKS proxy http.Agent implementation for HTTP and HTTPS

Example

Here's a minimal example that creates a new net.Socket or tls.Socket based on the secureEndpoint property. This agent can be used with both the http and https modules.

import * as net from 'net';
import * as tls from 'tls';
import * as http from 'http';
import { Agent } from 'agent-base';

class MyAgent extends Agent {
  connect(req, opts) {
    // `secureEndpoint` is true when using the "https" module
    if (opts.secureEndpoint) {
      return tls.connect(opts);
    } else {
      return net.connect(opts);
    }
  }
});

// Keep alive enabled means that `connect()` will only be
// invoked when a new connection needs to be created
const agent = new MyAgent({ keepAlive: true });

// Pass the `agent` option when creating the HTTP request
http.get('http://nodejs.org/api/', { agent }, (res) => {
  console.log('"response" event!', res.headers);
  res.pipe(process.stdout);
});

README.md

agent-base

Turn a function into an http.Agent instance

Some subclasses:

Example

Turn a function into an `http.Agent` instance