本文实例讲述了JavaScript生成的动态下雨背景效果实现方法。分享给大家供大家参考。具体实现方法如下：

for(i=0; i<rainsize; ++i) {
  initRain();
  if (i == 0) {
    document.write(\"<div id=\\\"dot\"+ i +\"\\\" style=\\\"POSITION: \");
    document.write(\"absolute; Z-INDEX: \"+ i +\"; VISIBILITY: \");
    document.write(\"visible; TOP: 15px; LEFT: 15px;\\\"><font color=\\\"blue\\\">\");
    document.write(\",</font></div>\"); }
  else {
    document.write(\"<div id=\\\"dot\"+ i +\"\\\" style=\\\"POSITION: \");
    document.write(\"absolute; Z-INDEX: \"+ i +\"; VISIBILITY: \");
    document.write(\"visible; TOP: 15px; LEFT: 15px;\\\"><font color=\\\"blue\\\">\");
    document.write(\",.</font></div>\");  }
}

function initRain() {
  a = 6;
  r[i] = 1;
  sn = Math.sin(a);
  cs = Math.cos(a);
  cx[i] = Math.random() * doc_width + 1;
  cy[i] = Math.random() * doc_height + 1;
  x[i] = r[i] * sn + cx[i];
  y[i] = cy[i];
}

function raindropIE() {
  for (i = 0; i < rainsize; ++ i) {
     updateRain();
     if ((x[i] <= 1) || (x[i] >= (doc_width - 20)) || (y[i] >= (doc_height - 20))) {
         makeRain();
         doc_width = document.body.clientWidth;
         doc_height = document.body.clientHeight; }
     document.all[\"dot\"+i].style.pixelTop = y[i];
     document.all[\"dot\"+i].style.pixelLeft = x[i]; }
  setTimeout(\"raindropIE()\", speed);
}
function updateRain() {
  r[i] += 10;
  x[i] = r[i] * sn + cx[i];
  y[i] = r[i] * cs + cy[i];
}
function makeRain() {
  r[i] = 1;
  cx[i] = Math.random() * doc_width + 1;
  cy[i] = 1;
  x[i] = r[i] * sn + cx[i];
  y[i] = r[i] * cs + cy[i];
}
raindropIE();
</script>
</BODY>
</HTML>