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say I have <ul>, I need to listen/watch when an <li> was removed and added.

2

Answers


  1. Sure, you can use a MutationObserver to watch for changes in a DOM element.

    The implementation of it is a little complex to describe in this answer, but the MDN article should provide you with all info you need.

    Here’s a contrived, and partially stolen example, to give you an idea:

    const btnAdd = document.getElementById('btn-add');
    const btnRemove = document.getElementById('btn-remove');
    
    // Select the node that will be observed for mutations
    const targetNode = document.getElementById('some-id');
    
    btnAdd.addEventListener('click', () => {
      const li = document.createElement('li');
      targetNode.appendChild(li);
    });
    
    btnRemove.addEventListener('click', () => {
      targetNode.removeChild(targetNode.children[0]);
    });
    
    
    // Options for the observer (which mutations to observe)
    const config = { attributes: true, childList: true, subtree: true };
    
    // Callback function to execute when mutations are observed
    const callback = function(mutationList, observer) {
        // Use traditional 'for loops' for IE 11
        for (const mutation of mutationList) {
            if (mutation.type === 'childList') {
                console.log('A child node has been added or removed.');
            }
            else if (mutation.type === 'attributes') {
                console.log(`The ${mutation.attributeName} attribute was modified.`);
            }
        }
    };
    
    // Create an observer instance linked to the callback function
    const observer = new MutationObserver(callback);
    
    // Start observing the target node for configured mutations
    observer.observe(targetNode, config);
    <button id="btn-add">Add Item</button>
    <button id="btn-remove">Remove Item</button>
    <ul id="some-id"></ul>
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  2. Actually, i can’t understand your question totally but there’s an answer in my mind:

    let ul = [];
    ul[0] = Array.from(document.getElementById("myUl").children);
    ul[1] = ul[0].length;
    //Make an event to when to listen it e.g. setInterval
    if (ul[0] < ul[1].length) { console.log("an li removed"); }
    else if (ul[0] > ul[1].length) { console.log("an li added"); }
    
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