source: frontend/node_modules/signal-exit/index.js

// Note: since nyc uses this module to output coverage, any lines
// that are in the direct sync flow of nyc's outputCoverage are
// ignored, since we can never get coverage for them.
// grab a reference to node's real process object right away
var process = global.process

const processOk = function (process) {
  return process &&
    typeof process === 'object' &&
    typeof process.removeListener === 'function' &&
    typeof process.emit === 'function' &&
    typeof process.reallyExit === 'function' &&
    typeof process.listeners === 'function' &&
    typeof process.kill === 'function' &&
    typeof process.pid === 'number' &&
    typeof process.on === 'function'
}

// some kind of non-node environment, just no-op
/* istanbul ignore if */
if (!processOk(process)) {
  module.exports = function () {
    return function () {}
  }
} else {
  var assert = require('assert')
  var signals = require('./signals.js')
  var isWin = /^win/i.test(process.platform)

  var EE = require('events')
  /* istanbul ignore if */
  if (typeof EE !== 'function') {
    EE = EE.EventEmitter
  }

  var emitter
  if (process.__signal_exit_emitter__) {
    emitter = process.__signal_exit_emitter__
  } else {
    emitter = process.__signal_exit_emitter__ = new EE()
    emitter.count = 0
    emitter.emitted = {}
  }

  // Because this emitter is a global, we have to check to see if a
  // previous version of this library failed to enable infinite listeners.
  // I know what you're about to say.  But literally everything about
  // signal-exit is a compromise with evil.  Get used to it.
  if (!emitter.infinite) {
    emitter.setMaxListeners(Infinity)
    emitter.infinite = true
  }

  module.exports = function (cb, opts) {
    /* istanbul ignore if */
    if (!processOk(global.process)) {
      return function () {}
    }
    assert.equal(typeof cb, 'function', 'a callback must be provided for exit handler')

    if (loaded === false) {
      load()
    }

    var ev = 'exit'
    if (opts && opts.alwaysLast) {
      ev = 'afterexit'
    }

    var remove = function () {
      emitter.removeListener(ev, cb)
      if (emitter.listeners('exit').length === 0 &&
          emitter.listeners('afterexit').length === 0) {
        unload()
      }
    }
    emitter.on(ev, cb)

    return remove
  }

  var unload = function unload () {
    if (!loaded || !processOk(global.process)) {
      return
    }
    loaded = false

    signals.forEach(function (sig) {
      try {
        process.removeListener(sig, sigListeners[sig])
      } catch (er) {}
    })
    process.emit = originalProcessEmit
    process.reallyExit = originalProcessReallyExit
    emitter.count -= 1
  }
  module.exports.unload = unload

  var emit = function emit (event, code, signal) {
    /* istanbul ignore if */
    if (emitter.emitted[event]) {
      return
    }
    emitter.emitted[event] = true
    emitter.emit(event, code, signal)
  }

  // { <signal>: <listener fn>, ... }
  var sigListeners = {}
  signals.forEach(function (sig) {
    sigListeners[sig] = function listener () {
      /* istanbul ignore if */
      if (!processOk(global.process)) {
        return
      }
      // If there are no other listeners, an exit is coming!
      // Simplest way: remove us and then re-send the signal.
      // We know that this will kill the process, so we can
      // safely emit now.
      var listeners = process.listeners(sig)
      if (listeners.length === emitter.count) {
        unload()
        emit('exit', null, sig)
        /* istanbul ignore next */
        emit('afterexit', null, sig)
        /* istanbul ignore next */
        if (isWin && sig === 'SIGHUP') {
          // "SIGHUP" throws an `ENOSYS` error on Windows,
          // so use a supported signal instead
          sig = 'SIGINT'
        }
        /* istanbul ignore next */
        process.kill(process.pid, sig)
      }
    }
  })

  module.exports.signals = function () {
    return signals
  }

  var loaded = false

  var load = function load () {
    if (loaded || !processOk(global.process)) {
      return
    }
    loaded = true

    // This is the number of onSignalExit's that are in play.
    // It's important so that we can count the correct number of
    // listeners on signals, and don't wait for the other one to
    // handle it instead of us.
    emitter.count += 1

    signals = signals.filter(function (sig) {
      try {
        process.on(sig, sigListeners[sig])
        return true
      } catch (er) {
        return false
      }
    })

    process.emit = processEmit
    process.reallyExit = processReallyExit
  }
  module.exports.load = load

  var originalProcessReallyExit = process.reallyExit
  var processReallyExit = function processReallyExit (code) {
    /* istanbul ignore if */
    if (!processOk(global.process)) {
      return
    }
    process.exitCode = code || /* istanbul ignore next */ 0
    emit('exit', process.exitCode, null)
    /* istanbul ignore next */
    emit('afterexit', process.exitCode, null)
    /* istanbul ignore next */
    originalProcessReallyExit.call(process, process.exitCode)
  }

  var originalProcessEmit = process.emit
  var processEmit = function processEmit (ev, arg) {
    if (ev === 'exit' && processOk(global.process)) {
      /* istanbul ignore else */
      if (arg !== undefined) {
        process.exitCode = arg
      }
      var ret = originalProcessEmit.apply(this, arguments)
      /* istanbul ignore next */
      emit('exit', process.exitCode, null)
      /* istanbul ignore next */
      emit('afterexit', process.exitCode, null)
      /* istanbul ignore next */
      return ret
    } else {
      return originalProcessEmit.apply(this, arguments)
    }
  }
}