Event trigger on a class change

Question

I'd like my event to be triggered when a div tag containing a trigger class is changed.

I have no idea how to make it listen to the class' adding event.

<div id="test">test</div>

<script type="text/javascript">
    document.getElementById.setAttribute("class", "trigger");

    function workOnClassAdd() {
       alert("I'm triggered");
    }
</script>

Answer 1

The future is here, and you can use the MutationObserver interface to watch for a specific class change.

let targetNode = document.getElementById('test')

function workOnClassAdd() {
    alert("I'm triggered when the class is added")
}

function workOnClassRemoval() {
    alert("I'm triggered when the class is removed")
}

// watch for a specific class change
let classWatcher = new ClassWatcher(targetNode, 'trigger', workOnClassAdd, workOnClassRemoval)

// tests:
targetNode.classList.add('trigger') // triggers workOnClassAdd callback
targetNode.classList.add('trigger') // won't trigger (class is already exist)
targetNode.classList.add('another-class') // won't trigger (class is not watched)
targetNode.classList.remove('trigger') // triggers workOnClassRemoval callback
targetNode.classList.remove('trigger') // won't trigger (class was already removed)
targetNode.setAttribute('disabled', true) // won't trigger (the class is unchanged)

I wrapped MutationObserver with a simple class:

class ClassWatcher {

    constructor(targetNode, classToWatch, classAddedCallback, classRemovedCallback) {
        this.targetNode = targetNode
        this.classToWatch = classToWatch
        this.classAddedCallback = classAddedCallback
        this.classRemovedCallback = classRemovedCallback
        this.observer = null
        this.lastClassState = targetNode.classList.contains(this.classToWatch)

        this.init()
    }

    init() {
        this.observer = new MutationObserver(this.mutationCallback)
        this.observe()
    }

    observe() {
        this.observer.observe(this.targetNode, { attributes: true })
    }

    disconnect() {
        this.observer.disconnect()
    }

    mutationCallback = mutationsList => {
        for(let mutation of mutationsList) {
            if (mutation.type === 'attributes' && mutation.attributeName === 'class') {
                let currentClassState = mutation.target.classList.contains(this.classToWatch)
                if(this.lastClassState !== currentClassState) {
                    this.lastClassState = currentClassState
                    if(currentClassState) {
                        this.classAddedCallback()
                    }
                    else {
                        this.classRemovedCallback()
                    }
                }
            }
        }
    }
}

Answer 2

Well there were mutation events, but they were deprecated and the future there will be Mutation Observers, but they will not be fully supported for a long time. So what can you do in the mean time?

You can use a timer to check the element.

function addClassNameListener(elemId, callback) {
    var elem = document.getElementById(elemId);
    var lastClassName = elem.className;
    window.setInterval( function() {   
       var className = elem.className;
        if (className !== lastClassName) {
            callback();   
            lastClassName = className;
        }
    },10);
}

Running example: jsFiddle

Answer 3

Here's a simple, basic example on how to trigger a callback on Class attribute change
MutationObserver API

const attrObserver = new MutationObserver((mutations) => {
  mutations.forEach(mu => {
    if (mu.type !== "attributes" && mu.attributeName !== "class") return;
    console.log("class was modified!");
  });
});

const ELS_test = document.querySelectorAll(".test");
ELS_test.forEach(el => attrObserver.observe(el, {attributes: true}));


// Example of Buttons toggling several .test classNames
document.querySelectorAll(".btn").forEach(btn => {
  btn.addEventListener("click", () => ELS_test.forEach(el => el.classList.toggle(btn.dataset.class)));
});

.blue {background: blue;}
.gold {color: gold;}

<div class="test">TEST DIV</div>
<button class="btn" data-class="blue">BACKGROUND</button>
<button class="btn" data-class="gold">COLOR</button>

Answer 4

Can use this onClassChange function to watch whenever classList of an element changes

function onClassChange(element, callback) {
  const observer = new MutationObserver((mutations) => {
    mutations.forEach((mutation) => {
      if (
        mutation.type === 'attributes' &&
        mutation.attributeName === 'class'
      ) {
        callback(mutation.target);
      }
    });
  });
  observer.observe(element, { attributes: true });
  return observer.disconnect;
}

var itemToWatch = document.querySelector('#item-to-watch');
onClassChange(itemToWatch, (node) => {
  node.classList.contains('active')
    ? alert('class added')
    : alert('class removed');
  node.textContent = 'Item to watch. classList: ' + node.className;
});

function addClass() {
  itemToWatch.classList.add('active');
}

function removeClass() {
  itemToWatch.classList.remove('active');
}

<div id="item-to-watch">Item to watch</div>
<button onclick="addClass();">Add Class</button>
<button onclick="removeClass();">Remove Class</button>

Answer 5

I needed a class update listener for a project, so I whipped this up. I didn’t end up using it, so it’s not fully tested, but should be fine on browsers supporting Element.classList DOMTokenList.

Bonus: allows “chaining” of the 4 supported methods, for example el.classList.remove(“inactive”).remove(“disabled”).add(“active”)

function ClassListListener( el ) {
  const ecl = el.classList;
  ['add','remove','toggle','replace'].forEach(prop=>{
    el.classList['_'+prop] = ecl[prop]
    el.classList[prop] = function() {
      const args = Array.from(arguments)
      this['_'+prop].apply(this, args)
      el.dispatchEvent(new CustomEvent(
        'classlistupdate',
        { detail: { method: prop, args } }
      ))
      return this
    }
  })
  return el
}

Useage:

const el = document.body

ClassListListener(el).addEventListener('classlistupdate', e => {
    const args = e.detail.args.join(', ')
    console.log('el.classList.'+e.detail.method+'('+args+')')
}, false)

el.classList
  .add('test')
  .replace('test', 'tested')