rsnext/packages/next/build/generate-build-id.ts

export async function generateBuildId (generate: () => string|null, fallback: () => string): Promise<string> {
  let buildId = await generate()
  // If there's no buildId defined we'll fall back
  if (buildId === null) {
    // We also create a new buildId if it contains the word `ad` to avoid false 
    // positives with ad blockers
    while (!buildId || /ad/i.test(buildId)) {
      buildId = fallback()
    }
  }

  if (typeof buildId !== 'string') {
    throw new Error('generateBuildId did not return a string. https://err.sh/zeit/next.js/generatebuildid-not-a-string')
  }

  return buildId.trim()
}