鼠标略过图片放大js效果

<script type="text/javascript">
            
exid = 0;
exstep = 0;
exwdth = 0;
exht = 0;
extp = 0;
exlft = 0;
extot = 0;
extotst = 15;
            
function expandthumb(thumbid, fullwidth, fullheight) {
  if (extot != 0) {
    clearTimeout(extot);
  }
  if (exid > 0 && exid != thumbid) {
    restorethumb();
  }
  if (exid != thumbid) {
    img = document.getElementById("screen" + thumbid);
    img.style.display = 'block';
    exid = thumbid;
    exstep = 1;
    exwdth = fullwidth;
    exht = fullheight;
    extp = img.offsetTop;
    exlft = img.offsetLeft;
  } else if (exstep < 1) {
    exstep = 1;
  }
  expandstep();
}
            
function doexpand() {
  img = document.getElementById("screen" + exid);
  thumb = document.getElementById("thumb" + exid);
  myscroll = getScroll();
  if (extp + thumb.height > myscroll.top + myscroll.height) {
    finaltop = myscroll.top + myscroll.height - exht;
  } else {
    finaltop = extp + thumb.height - exht;
  }
  if (finaltop < myscroll.top) { finaltop = myscroll.top; }
  img.style.top = finaltop + ((extp - finaltop) * 
                              (extotst - exstep) / extotst) + 'px';
            
  if (exlft + thumb.width > myscroll.left + myscroll.width) {
    finalleft = myscroll.left + myscroll.width - exwdth;
  } else {
    finalleft = exlft + thumb.width - exwdth;
  }
  if (finalleft < myscroll.left) { finalleft = myscroll.left; }
  img.style.left = finalleft + ((exlft - finalleft) * 
                                (extotst - exstep) / extotst) + 'px';
            
  img.width = thumb.width + ((exwdth - thumb.width) * exstep / extotst);
  img.height = thumb.height + ((exht - thumb.height) * exstep / extotst);
}
            
function restorethumb() {
  img = document.getElementById("screen" + exid);
  img.style.top = '';
  img.style.left = '';
  img.style.display = 'none';
  exid = 0;
}
            
function expandstep() {
  extot = 0;
  doexpand();
  if (exstep < extotst) {
    exstep++;
    extot = setTimeout("expandstep()", 20);
  }
}
            
function reducestep() {
  extot = 0;
  doexpand();
  if (exstep > 0) {
    exstep--;
    extot = setTimeout("reducestep()", 20);
  } else {
    restorethumb();
  }
}
            
function reducethumb(thumbid) {
  if (extot != 0) {
    clearTimeout(extot);
  }
  if (exstep > 0) {
    reducestep();
  }
}
            
// returns the scroll position and size of the browser
function getScroll() {
  if (document.all && typeof document.body.scrollTop != "undefined") {  
    // IE model
    var ieBox = document.compatMode != "CSS1Compat";
    var cont = ieBox ? document.body : document.documentElement;
    return {
      left:   cont.scrollLeft,
      top:    cont.scrollTop,
        cont.clientWidth,
      height: cont.clientHeight
    };
  } else {
    return {
      left:   window.pageXOffset,
      top:    window.pageYOffset,
        window.innerWidth,
      height: window.innerHeight
    };
  }
}
</script>

<body>
   <img src='1.jpg' width=400 height=400 alt='img' border=0
     id='screen142' onmouseout='reducethumb(142); return false;'
     class='position: absolute; display: none;'>
  <img src='1.jpg' width=100 height=100 alt='mis' border=0
     id='thumb142' onmouseover='expandthumb(142, 500, 449);'></a>
</body>