next.js/packages/next-server/lib/asset.js

let assetPrefix = typeof window !== 'undefined' ? (window.__NEXT_DATA__.assetPrefix) : undefined

export default function asset (path) {
  // If the URL starts with http, we assume it's an
  if (/^https?:\/\//.test(path)) {
    return path
  }

  const pathWithoutSlash = path.replace(/^\//, '')
  return `${assetPrefix || ''}/static/${pathWithoutSlash}`
}

export function setAssetPrefix (url) {
  assetPrefix = url
}