next.js/packages/next/build/write-build-id.ts

import fs from 'fs'
import {promisify} from 'util'
import {join} from 'path'
import {BUILD_ID_FILE} from 'next-server/constants'

const writeFile = promisify(fs.writeFile)

export async function writeBuildId (distDir: string, buildId: string): Promise<void> {
  const buildIdPath = join(distDir, BUILD_ID_FILE)
  await writeFile(buildIdPath, buildId, 'utf8')
}