https://www.bunkline.com/a/j/class-name-watcher.js

1class ClassNameWatcher {
2
3  constructor(targetNode, classToWatch, classAddedCallback, classRemovedCallback) {
4      this.targetNode = targetNode
5      this.classToWatch = classToWatch
6      this.classAddedCallback = classAddedCallback
7      this.classRemovedCallback = classRemovedCallback
8      this.observer = null
9      this.lastClassState = targetNode.classList.contains(this.classToWatch)
10
11      this.init()
12  }
13
14  init() {
15      this.observer = new MutationObserver(this.mutationCallback)
16      this.observe()
17  }
18
19  observe() {
20      this.observer.observe(this.targetNode, { attributes: true })
21  }
22
23  disconnect() {
24      this.observer.disconnect()
25  }
26
27  mutationCallback = mutationsList => {
28      for(let mutation of mutationsList) {
29          if (mutation.type === 'attributes' && mutation.attributeName === 'class') {
30              let currentClassState = mutation.target.classList.contains(this.classToWatch)
31              if(this.lastClassState !== currentClassState) {
32                  this.lastClassState = currentClassState
33                  if(currentClassState) {
34                      this.classAddedCallback()
35                  }
36                  else {
37                      this.classRemovedCallback()
38                  }
39              }
40          }
41      }
42  }
43}