const dynamicLoadScript = (src, callback, id) => {
  const existingScript = document.getElementById(src)
  const cb = callback || function() {}

  if (!existingScript) {
    const script = document.createElement('script')
    script.src = src // src url for the third-party library being loaded.
    script.id = id || src
    script.async = true
    document.body.appendChild(script)

    const onEnd = 'onload' in script ? stdOnEnd : ieOnEnd
    onEnd(script, cb)
  }

  if (existingScript && cb) cb(null, existingScript)

  function stdOnEnd(script, cb) {
    script.onload = function() {
      // this.onload = null here is necessary
      // because even IE9 works not like others
      this.onerror = this.onload = null
      cb(null, script)
    }
    script.onerror = function() {
      this.onerror = this.onload = null
      cb(new Error('Failed to load ' + src), script)
    }
  }

  function ieOnEnd(script, cb) {
    script.onreadystatechange = function() {
      if (this.readyState !== 'complete' && this.readyState !== 'loaded') return
      this.onreadystatechange = null
      cb(null, script) // there is no way to catch loading errors in IE8
    }
  }
}

export default dynamicLoadScript