electron/lib/browser/api/auto-updater/squirrel-update-win.js

const fs = require('fs')
const path = require('path')
const spawn = require('child_process').spawn

// i.e. my-app/app-0.1.13/
const appFolder = path.dirname(process.execPath)

// i.e. my-app/Update.exe
const updateExe = path.resolve(appFolder, '..', 'Update.exe')

const exeName = path.basename(process.execPath)

// Spawn a command and invoke the callback when it completes with an error
// and the output from standard out.
var spawnUpdate = function (args, detached, callback) {
  var error, errorEmitted, spawnedProcess, stderr, stdout
  try {
    spawnedProcess = spawn(updateExe, args, {
      detached: detached
    })
  } catch (error1) {
    error = error1

    // Shouldn't happen, but still guard it.
    process.nextTick(function () {
      return callback(error)
    })
    return
  }
  stdout = ''
  stderr = ''
  spawnedProcess.stdout.on('data', function (data) {
    stdout += data
  })
  spawnedProcess.stderr.on('data', function (data) {
    stderr += data
  })
  errorEmitted = false
  spawnedProcess.on('error', function (error) {
    errorEmitted = true
    return callback(error)
  })
  return spawnedProcess.on('exit', function (code, signal) {
    // We may have already emitted an error.
    if (errorEmitted) {
      return
    }

    // Process terminated with error.
    if (code !== 0) {
      return callback('Command failed: ' + (signal != null ? signal : code) + '\n' + stderr)
    }

    // Success.
    return callback(null, stdout)
  })
}

// Start an instance of the installed app.
exports.processStart = function () {
  return spawnUpdate(['--processStart', exeName], true, function () {})
}

// Download the releases specified by the URL and write new results to stdout.
exports.download = function (updateURL, callback) {
  return spawnUpdate(['--download', updateURL], false, function (error, stdout) {
    var json, ref, ref1, update
    if (error != null) {
      return callback(error)
    }
    try {
      // Last line of output is the JSON details about the releases
      json = stdout.trim().split('\n').pop()
      update = (ref = JSON.parse(json)) != null ? (ref1 = ref.releasesToApply) != null ? typeof ref1.pop === 'function' ? ref1.pop() : void 0 : void 0 : void 0
    } catch (jsonError) {
      return callback('Invalid result:\n' + stdout)
    }
    return callback(null, update)
  })
}

// Update the application to the latest remote version specified by URL.
exports.update = function (updateURL, callback) {
  return spawnUpdate(['--update', updateURL], false, callback)
}

// Is the Update.exe installed with the current application?
exports.supported = function () {
  try {
    fs.accessSync(updateExe, fs.R_OK)
    return true
  } catch (error) {
    return false
  }
}