JS判断输入框值是否为空

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>JS判断输入框的值是否为空</title>
</head>
<body>
<input type="text" id="txt"/>
<input type="button" value="检测" id="btn">
<script>
    var oTxt = document.getElementById('txt');

    var oBtn = document.getElementById('btn');
    function isnull(val) {

        var str = val.replace(/(^s*)|(s*$)/g, '');//去除空格;

        if (str == '' || str == undefined || str == null) {
            //return true;
            console.log('空')
        } else {
            //return false;
            console.log('非空');
        }
    }

    oBtn.onclick = function () {

        isnull( oTxt.value );

    }

</script>
</body>
</html>