next.js/packages/next-server/server/serve-static.ts

import {IncomingMessage, ServerResponse} from 'http'
import send from 'send'

// since send doesn't support wasm yet
send.mime.define({ 'application/wasm': ['wasm'] })

export function serveStatic(req: IncomingMessage, res: ServerResponse, path: string): Promise<void> {
  return new Promise((resolve, reject) => {
    send(req, path)
      .on('directory', () => {
      // We don't allow directories to be read.
        const err: any = new Error('No directory access')
        err.code = 'ENOENT'
        reject(err)
      })
      .on('error', reject)
      .pipe(res)
      .on('finish', resolve)
  })
}