JavaScript HTML DOM Events: animationiteration

The animationiteration event occurs when an animation defined with @keyframes finishes one cycle and begins the next. In the example below, each animation repeat increments a counter and logs the time and iteration number.

EXAMPLE

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>animationiteration – simplified button</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">

    <style>
        body {
            font-family: Arial,Helvetica,sans-serif;
            padding: 20px;
        }
        .box {
            max-width: 800px;
            margin: 30px auto;
            border: 1px solid #e5e7eb;
            border-radius: 8px;
            padding: 16px;
        }
        h2 {
            margin-top: 0;
            border-bottom: 1px solid #eee;
            padding-bottom: 10px;
            margin-bottom: 15px;
        }
        button {
            margin: 4px 0;
            padding: 8px 16px;
            border: 1px solid #777;
            border-radius: 4px;
            cursor: pointer;
        }
        button:hover {
            background: #e5e7eb;
            color: #333;
        }
        .log {
            margin-top: 12px;
            padding: 10px;
           border: 1px solid #e5e7eb;
            border-radius: 4px;
            max-height: 180px;
            overflow: auto;
            font-family: monospace;
            font-size: 13px;
            background: #f3f4f6;
            color: #333;
        }
        .animatable {
            margin-top: 12px;
            font-size: 1.4rem;
        }
        @keyframes glava {
            from {
                color: red;
            }
            to { 
                color: lightgreen;
            }
        }
    </style>
</head>
<body>
<div class="box">
    <h2>animationiteration – simplified button</h2>

    <button id="startBtn">START ANIMATION</button>

    <h1 class="animatable" id="title">Welcome to the JavaScript tutorial!</h1>

    <div class="log" id="log"></div>
</div>

<script>
    const logBox = document.getElementById("log");
    const log = (msg) => { const l = document.createElement("div"); 
        l.textContent = new Date().toLocaleTimeString() + ": " + msg;
        logBox.appendChild(l);
        logBox.scrollTop = logBox.scrollHeight;
    };

    let iterationCount = 0;

    window.addEventListener("animationiteration", (e) => {
        iterationCount++;
        const el = e.target;
        const name = el.id || el.className || el.tagName;
        log(`animationiteration on <${name}> – repeat #${iterationCount}`);
    });

    const startBtn = document.getElementById("startBtn");
    let started = false;
    startBtn.addEventListener("click", () => {
        if (started) return;
        started = true;
        iterationCount = 1;
        log(`Animation started – repeat #${iterationCount}`);
        const title = document.getElementById("title");
        title.style.animation = "glava 2s linear 2";
        startBtn.disabled = true;
        startBtn.style.opacity = "0.6";
    });
</script>
</body>
</html>

RESULT

Every iteration triggers this event, allowing you to use it for:

counting how many times the animation has already run;
updating the UI (e.g., changing text, colour, or other properties) between iterations;
creating sequential effects that fire after each iteration.