npr.js

const npx = require('libnpx')
const which = require('which-promise')
const path = require('path')

const actualPackageName = req => req.split('/', req[0] === '@' ? 2 : 1).join('/')

module.exports = (requireName, version = 'latest') => {
  try {
    return Promise.resolve(require(requireName))
  } catch(_) {
    return which('npm').then(npm => {
      const args = npx.parseArgs([], npm)
      const packageName = actualPackageName(requireName)
      const requirePath = requireName.replace(new RegExp(`^${packageName}(/|$)`), '')
      
      return npx._ensurePackages(`${packageName}@${version}`, args).then(({added, updated}) => {
        const pkg = added.concat(updated).find(({name}) => name === packageName)
        return require(path.join(pkg.path, requirePath))
      })
    })
  }
}