next.js/packages/next/src/lib/helpers/get-pkg-manager.ts

import fs from 'fs'
import path from 'path'
import { execSync } from 'child_process'

export type PackageManager = 'npm' | 'pnpm' | 'yarn'

export function getPkgManager(baseDir: string): PackageManager {
  try {
    for (const { lockFile, packageManager } of [
      { lockFile: 'yarn.lock', packageManager: 'yarn' },
      { lockFile: 'pnpm-lock.yaml', packageManager: 'pnpm' },
      { lockFile: 'package-lock.json', packageManager: 'npm' },
    ]) {
      if (fs.existsSync(path.join(baseDir, lockFile))) {
        return packageManager as PackageManager
      }
    }
    const userAgent = process.env.npm_config_user_agent
    if (userAgent) {
      if (userAgent.startsWith('yarn')) {
        return 'yarn'
      } else if (userAgent.startsWith('pnpm')) {
        return 'pnpm'
      }
    }
    try {
      execSync('yarn --version', { stdio: 'ignore' })
      return 'yarn'
    } catch {
      execSync('pnpm --version', { stdio: 'ignore' })
      return 'pnpm'
    }
  } catch {
    return 'npm'
  }
}