Configuring a Global Proxy for Undici That Kind of Supports the `no_proxy` Environment Variable.

configureGlobalFetch.ts

/**
 * Importing this modules will configure the global undici agent, which is used to provide
 * global `fetch()` support, to use an http proxy if present during development. Unfortunately
 * undici doesn't come with this functionality so we implement a solution that is not fully
 * correct as to the way that proxy environment variables are supposed to be parsed.
 * 
 * This only goes into effect during development, though, and it's functional enough for that
 * purpose.
 */

import { Agent, ProxyAgent, Dispatcher, setGlobalDispatcher } from 'undici';

// If we are in development mode and there is an `http_proxy` set
if (process.env.NODE_ENV == 'development' && process.env.http_proxy) {
	// Collect the list of domains that we should not use a proxy for
	const noProxyList = (process.env.no_proxy && process.env.no_proxy.split(',')) || [];
	// Parse the proxy URL
	const proxyUrl = new URL(process.env.http_proxy);
	// Create an access token if the proxy requires authentication
	const token = proxyUrl.username && proxyUrl.password ? 
		`Basic ${Buffer.from(`${proxyUrl.username}:${proxyUrl.password}`).toString('base64')}` : undefined;

	// Create a default agent that will be used for no_proxy origins
	const defaultAgent = new Agent();

	// Create an interceptor that will use the appropriate agent based on the origin and the no_proxy
	// environment variable.
	const noProxyInterceptor = (dispatch: Dispatcher['dispatch']): Dispatcher['dispatch'] => {
		return (opts, handler) => {
			let noProxy = false;
			for (const exclusion of noProxyList) {
				if (opts.origin?.toString().search(exclusion) != -1) {
					noProxy = true;
					break;
				}
			}
			return noProxy ?
				defaultAgent.dispatch(opts, handler) :
				dispatch(opts, handler);
		}
	};

	// Create a proxy agent that will send all requests through the configured proxy, unless the
	// noProxyInterceptor bypasses it.
	const proxyAgent = new ProxyAgent({
		uri: proxyUrl.protocol + proxyUrl.host,
  	token,
		interceptors: {
      Client: [noProxyInterceptor]
		}
	});
  
	// Make sure our configured proxy agent is used for all `fetch()` requests globally.
	setGlobalDispatcher(proxyAgent); 
}