如果我正确理解了你的问题，你想在javascript中找到你的元素的高度，那么下面是你的选择：

const elm = document.getElementById("sect");

// Returns an object with left, x, top, y, right, bottom, width, height properties of the element.
// width and height includes the border and padding. Others are relative to the top/left of the viewport
elm.getBoundingClientRect();

 // Viewable width/height of the element including padding, but NOT border, scrollbar or margin (excludes overflow)
elm.clientWidth;
elm.clientHeight;

// Viewable width/height of the element including padding, border, scrollbar, but NOT margin (excludes overflow)
elm.offsetWidth;
elm.offsetHeight;

// Entire width/height of the element including padding, but NOT border, scrollbar or margin (includes overflow)
elm.scrollWidth;
elm.scrollHeight;

所以你的函数可能看起来像：

function footer()
{
  var h = parseFloat(document.getElementById("sect").offsetHeight);
  document.getElementById("foot").style.top = h + "px";
}