esbuild.build.js

const { build } = require('esbuild');
const manifestPlugin = require('esbuild-plugin-manifest');

const options = {
  entryPoints: ['src/index.js'],
  entryNames: '[dir]/[name]-[hash]',
  outdir: 'dist',
  bundle: true,
  minify: true,
  plugins: [manifestPlugin()],
}

build(options).catch(() => {
  process.exit(1)
})

esbuild.serve.js

const { serve } = require('esbuild')
const http = require('http');

const define = {}
for (const k in process.env) {
  define[`process.env.${k}`] = JSON.stringify(process.env[k])
}

// Start esbuild's server on a random local port
serve({
  servedir: 'dist',
}, {
  // ... your build options go here ...
  entryPoints: ['src/index.js'],
  outdir: 'dist/js',
  bundle: true,
  minify: false,
  sourcemap: true,
  define,
}).then(result => {
  // The result tells us where esbuild's local server is
  const { host, port } = result

  // Then start a proxy server on a custom port
  http.createServer((req, res) => {
    const options = {
      hostname: host,
      port: port,
      path: req.url,
      method: req.method,
      headers: req.headers,
    }

    // Forward each incoming request to esbuild's root
    const proxyReq = http.request(options, proxyRes => {
      date = new Date();
      console.log(`${date.toISOString()} - ${req.method} to ${req.url}`)
      if (proxyRes.statusCode === 404) {
        const redirectReq = http.request({ ...options, path: "/" }, (proxyRes) => {
          console.log(`  ↳ redirected to /`)
          // Forward the response from esbuild to the client
          res.writeHead(proxyRes.statusCode, proxyRes.headers);
          proxyRes.pipe(res, { end: true });
        });
        redirectReq.end();
      } else {
        // Forward the response from esbuild to the client
        res.writeHead(proxyRes.statusCode, proxyRes.headers);
        proxyRes.pipe(res, { end: true });
      }
    });

    // Forward the body of the request to esbuild
    req.pipe(proxyReq, { end: true });
  }).listen(1274);
});