<input type="button" value="关灯"></input>

<script>
    var getbody = document.body;                                //获取body元素   
    var getbutton = document.querySelector('input');   //获取按钮元素
    var flag = true;                                                       //设置一个变量
    getbutton.onclick = function() {
        if (flag == true) {
            getbody.style.backgroundColor = 'black';
            flag = false;
            getbutton.value = '开灯';
        } else {
            getbody.style.backgroundColor = '#fff';
            flag = true;
            getbutton.value = '关灯';
        }

    }
</script>