JS判断文本框是否获取光标

<html>
    <head>
    <script type="text/javascript">
        var t1=document.getElementById("txt1");
        function isFocus(){
            if(document.activeElement.id=='txt2'){
                alert('txt2获得焦点');
            }
            else{
                alert('txt2未获得焦点');
            }
        }
    </script>
    </head>
    <input >
    </html>

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