text: import { MutationObs, isElement } from './dom'
  import { warnNoMutationObserverSupport } from './warn'
  
  /**
   * Observe a DOM element changes, falls back to eventListener mode
   * @param {Element} el The DOM element to observe
   * @param {Function} callback callback to be called on change
   * @param {object} [options={childList: true, subtree: true}] observe options
   * @see https://stackoverflow.com/questions/3219758
   */
  const observeDom = (
    el,
    callback,
    options
  ) => /* istanbul ignore next: difficult to test in JSDOM */ {
    // Handle cases where we might be passed a Vue instance
    el = el ? el.$el || el : null
  
    // Early exit when we have no element
    /* istanbul ignore next: difficult to test in JSDOM */
    if (!isElement(el)) {
      return null
    }
  
    // Exit and throw a warning when `MutationObserver` isn't available
    if (warnNoMutationObserverSupport('observeDom')) {
      return null
    }
  
    // Define a new observer
    const obs = new MutationObs(mutations => {
      let changed = false
  
      // A mutation can contain several change records, so we loop
      // through them to see what has changed
      // We break out of the loop early if any "significant" change
      // has been detected
      for (let i = 0; i < mutations.length && !changed; i++) {
        // The mutation record
        const mutation = mutations[i]
        // Mutation type
        const type = mutation.type
        // DOM node (could be any DOM node type - HTMLElement, Text, comment, etc.)
        const target = mutation.target
  
        // Detect whether a change happened based on type and target
        if (type === 'characterData' && target.nodeType === Node.TEXT_NODE) {
          // We ignore nodes that are not TEXT (i.e. comments, etc)
          // as they don't change layout
          changed = true
        } else if (type === 'attributes') {
          changed = true
        } else if (
          type === 'childList' &&
          (mutation.addedNodes.length > 0 || mutation.removedNodes.length > 0)
        ) {
          // This includes HTMLElement and text nodes being
          // added/removed/re-arranged
          changed = true
        }
      }
  
      // We only call the callback if a change that could affect
      // layout/size truely happened
      if (changed) {
        callback()
      }
    })
  
    // Have the observer observe foo for changes in children, etc
    obs.observe(el, { childList: true, subtree: true, ...options })
  
    // We return a reference to the observer so that `obs.disconnect()`
    // can be called if necessary
    // To reduce overhead when the root element is hidden
    return obs
  }
  
  export default observeDom
  