只执行一次的方法。

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<head>
    <meta charset="UTF-8">
    <title>Document</title>
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<body>
    
    <script>
        window.onload = function () {
            var flag = true;

            function once() {
                if (flag) {
                    alert("我被调用");
                    flag = false;
                } else {
                    return;
                }
            }
            once();
            
        }
    </script>
</body>
</html>

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