node_modules/npm/lib/config/set-user.js

module.exports = setUser

var assert = require("assert")
var path = require("path")
var fs = require("fs")
var mkdirp = require("mkdirp")

function setUser (cb) {
  var defaultConf = this.root
  assert(defaultConf !== Object.prototype)

  // If global, leave it as-is.
  // If not global, then set the user to the owner of the prefix folder.
  // Just set the default, so it can be overridden.
  if (this.get("global")) return cb()
  if (process.env.SUDO_UID) {
    defaultConf.user = +(process.env.SUDO_UID)
    return cb()
  }

  var prefix = path.resolve(this.get("prefix"))
  mkdirp(prefix, function (er) {
    if (er) return cb(er)
    fs.stat(prefix, function (er, st) {
      defaultConf.user = st && st.uid
      return cb(er)
    })
  })
}