JavaScript HTML DOM Events: transitionend

The transitionend event occurs when a CSS transition (e.g., width, height, color, transform) on an element finishes. With this event you can synchronize JavaScript logic with the visual effect—for example, display a message, start a new animation, or remove temporary classes.

EXAMPLE

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>transitionend – interactive demo</title>
    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-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;
            background: #fff;
            color: #333;
        }

        button:hover {
            background: #e5e7eb;
        }

        .square {
            width: 60px;
            height: 60px;
            background: #ffcc00;
            border: 2px solid #333;
            border-radius: 6px;
            transition: width 2s ease, background-color 2s ease;
        }

        .square.expanded {
            width: 120px;
            background: #66c2ff;
        }

        .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;
        }
    </style>
</head>
<body>
<div class="box">
    <h2>JavaScript DOM Event: <code>transitionend</code></h2>
    <p>Click the square to trigger a CSS transition (width and color). When the transition finishes,
       the <code>transitionend</code> event fires and the details are written to the log.</p>

    <button id="toggleBtn">Toggle transition</button>

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

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

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

    // Elements
    const square = document.getElementById("square");
    const toggleBtn = document.getElementById("toggleBtn");

    // Button – toggle the transition class
    toggleBtn.addEventListener("click", () => {
        square.classList.toggle("expanded");
        log("Transition started (class toggled).");
    });

    // transitionend listener
    // Fires when any CSS transition on the element finishes.
    // Filter by the property we care about (width) to avoid duplicate logs.
    square.addEventListener("transitionend", (e) => {
        if (e.propertyName === "width") {
            log(`transitionend – property "${e.propertyName}" finished on <${e.target.id || e.target.className}>`);
        }
    });
</script>
</body>
</html>

RESULT