let callbacks = []
  
  function loadedTinymce() {
    // to fixed https://github.com/PanJiaChen/vue-element-admin/issues/2144
    // check is successfully downloaded script
    return window.tinymce
  }
  
  const dynamicLoadScript = (src, callback) => {
    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 = src
      document.body.appendChild(script)
      callbacks.push(cb)
      const onEnd = 'onload' in script ? stdOnEnd : ieOnEnd
      onEnd(script)
    }
  
    if (existingScript && cb) {
      if (loadedTinymce()) {
        cb(null, existingScript)
      } else {
        callbacks.push(cb)
      }
    }
  
    function stdOnEnd(script) {
      script.onload = function() {
        // this.onload = null here is necessary
        // because even IE9 works not like others
        this.onerror = this.onload = null
        for (const cb of callbacks) {
          cb(null, script)
        }
        callbacks = null
      }
      script.onerror = function() {
        this.onerror = this.onload = null
        cb(new Error('Failed to load ' + src), script)
      }
    }
  
    function ieOnEnd(script) {
      script.onreadystatechange = function() {
        if (this.readyState !== 'complete' && this.readyState !== 'loaded') return
        this.onreadystatechange = null
        for (const cb of callbacks) {
          cb(null, script) // there is no way to catch loading errors in IE8
        }
        callbacks = null
      }
    }
  }
  
  export default dynamicLoadScript