@netlify/config/src/build_dir.js

Netlify config module

import { isDirectory } from 'path-type'

import { throwUserError } from './error.js'

// Retrieve the build directory used to resolve most paths.
// This is (in priority order):
//  - `build.base`
//  - `--repositoryRoot`
//  - the current directory (default value of `--repositoryRoot`)
export const getBuildDir = async function (repositoryRoot, base) {
  const buildDir = base === undefined ? repositoryRoot : base
  await checkBuildDir(buildDir, repositoryRoot)
  return buildDir
}

// The build directory is used as the current directory of build commands and
// build plugins. Therefore it must exist.
// We already check `repositoryRoot` earlier in the code, so only need to check
// `buildDir` when it is the base directory instead.
const checkBuildDir = async function (buildDir, repositoryRoot) {
  if (buildDir === repositoryRoot || (await isDirectory(buildDir))) {
    return
  }

  throwUserError(`Base directory does not exist: ${buildDir}`)
}