为什么原生js在调用函数时 用this

Question

    <script >
        var allCheck = document.getElementById("allCheck");
        allCheck.addEventListener("click", handler)
        for (var i = 0; i < 3; i++) {
            var check = document.getElementById("Check" + i);
            check.addEventListener("click", handler);
        }

        function handler() {
          if (tihs === allCheck) {
                for (var i = 0; i < 3; i++) {
                    var check = document.getElementById("Check" + i);
                    check.checked = allCheck.checked;    //让所有多选框都和全选一样
                }
                return;
            } 
            var bool = true;
            for (var j = 0; j < 3; j++) {
                var checks = document.getElementById("Check" + j);
                if (!checks.checked) {
                    bool = false;
                }
            }
            allCheck.checked = bool;
        }
 

    </script>

**上面是在html文件中的部分代码 为何运行时 会显示this 未定义呢？

**点进去显示这条语句中的this 未定义

Answer 1

 if (tihs === allCheck) 

写错了，写成了tihs