Center and Scale background image using JQuery

background.css

.bgimgs{
  position:absolute;
  top:0;
  left:0;
  z-index:-1000;
  overflow:hidden;
}

background.js

$(document).ready(function() {
  //Initial load and size
  $("body").prepend('<div class="bgimgs" id="bgimgs"><div id="bgimg1" class="bgimg"><img src="img/bg.jpg"></div></div>');
  resizeBgDiv();
});

$(window).on('resize', function(event){
  resizeBgDiv();
});

var resizeBgDiv = function(){
  //Input background height, width here
  var bgh = 2560;
  var bgw = 1920;
  var height = window.innerHeight;
  var width = window.innerWidth;
  var hratio = height/bgh;
  var wratio = width/bgw;
  if(hratio < wratio){
    //negative margin up
    var hexpect = wratio * bgh;
    var hdiff = (hexpect - height)*0.5;
    $('#bgimgs').css({
      'margin-top': -hdiff,
      'margin-left': 0,
      'width': width,
      'height':hdiff+height});
    $('.bgimg').css({
      'width':width,
      'height':hexpect});
    $('.bgimg').children().css({
      'width':width,
      'height':hexpect});
  } else {
    //negative margin left
    var wexpect = hratio * bgw;
    var wdiff = (wexpect - width)/2;
    $('#bgimgs').css({
      'margin-top': 0,
      'margin-left': -wdiff,
      'width': width+wdiff,
      'height' : height});
    $('.bgimg').css({
      'width':wexpect,
      'height':height});
    $('.bgimg').children().css({
      'width':wexpect,
      'height':height});
  }
}