Using JavaScript to center a large background map

Get the width of the original
var yuantuwidth = 1920;
Gets the current one element
var dom = Document.queryselector (". Banner");
Functions that are called when the window size changes
Window.onresize = function () {
Get the width of the viewport with the current function
var viewwidth = document.documentElement.clientWidth;
If the viewport width is less than 1000, jump out of this loop
if (Viewwidth < 1000) {
Return
}
Calculate the offset required by the image center
var panduanwidth = (1920-viewwidth)/2;
The size of the style offset for the x axis of the picture
Dom.style.backgroundPositionX =-panduanwidth + "px";
}

Using JavaScript to center a large background map