打开模态框之后背景不可以滑动

<script>
var _rulecli = document.getElementById('rulecli');
var _closeChess = document.getElementById("closeChess");
var _moImg = document.getElementById("middmodal");
var Top = [];
_rulecli.onclick = function(){
var top = 0;
var length = Top.length;
if (length == 0) {
top = 0;
} else {
top = -Top[length - 1];/*最后一次滚动过的距离*/
}
//_tables.style.position = 'fixed';
_moImg.style.display = "block";
}
_closeChess.onclick = function () {
var length = Top.length;
//_tables.style.position = 'relative';
_moImg.style.display = "none";
}

</script>