handle onload/onfail callback on script tag

load-script.js

function loadScript (src, cb) {
  var userAgent = navigator.userAgent.toLowerCase()
  var isIE = (userAgent.indexOf('msie') !== -1) ? parseInt(userAgent.split('msie')[1], 10) : false
  var doc = document
  var tag = 'script'
  var el = doc.createElement(tag)

  el.done = false
  el.async = true
  el.charset = 'utf-8'

  if (isIE && isIE < 9) {
    el.onreadystatechange = function () { // use readyState for handling onload/onfail
      if (!el.done && /loaded|complete/.test(this.readyState)) {
        _handleIE()
      }
    }
  }

  el.onload = function () {
    _handleOnload()
  }

  el.onerror = function (err) {
    _handleOnError(err || new Error('failed load: ' + src))
  }

  el.src = src

  if (!isIE || isIE >= 9) { // non-IE or IE 9 above browser can request src when it comes to append to document
    _appendTo(document.head || document.getElementsByTagName('head')[0])
  }

  // use code from http://stackoverflow.com/a/18840568
  function _handleIE () {
    if (el.readyState === 'complete') {
      _appendTo(document.head || document.getElementsByTagName('head')[0])
      el.onreadystatechange = null

      _handleOnload()
      return
    }

    var firstState = el.readyState

    // hack: calling 'children' property changes node's readyState from 'loaded' to complete
    // (if script was loaded normally) or to 'loading' - if error detected
    el.children

    // error detected
    if (el.readyState === 'loading' && firstState === 'loaded') {
      // custom error code
      _handleOnError(new Error('failed load: ' + src))
    }
  }

  function _handleOnload () {
    el.done = true
    cb()
    _destroyTag()
  }

  function _handleOnError (err) {
    cb(err)
    _destroyTag()
  }

  function _destroyTag () {
    // Handle memory leak in IE
    el.onreadystatechange = null
    el.onload = null
    el.onerror = null
    el.parentNode.removeChild(el)
  }

  function _appendTo (base) {
    base.appendChild(el)
  }
}

if (typeof module === 'object' && module.exports) {
  module.exports = loadScript
} else {
  exports.loadScript = loadScript
}

load-script.spec.js

/* global describe, it, require */

var assert = require('assert')
var loadScript = require('./load-script')

describe('loadScript', function () {
  it('should be failed', function (done) {
    this.timeout(5 * 1000)
    loadScript('http://cdnjs.cloudflare.com/ajax/libs/jquery/1.10.x/jquery.min.js', function (err) {
      if (err) {
        assert.notEqual(err, null)
        done()
      }
    })
  })

  it('should be loaded', function (done) {
    this.timeout(5 * 1000)
    loadScript('http://cdnjs.cloudflare.com/ajax/libs/jquery/1.10.2/jquery.min.js', function (err) {
      if (err) {
        done(err)
        return
      }
      assert.notEqual(window.jQuery, null)
      done()
    })
  })
})