var nodes = document.querySelectorAll('img[data-src]'),
  elem = nodes[0],
  rect = elem.getBoundingClientRect(),
  vpHeight = document.documentElement.clientHeight;
if(rect.top < vpHeight && rect.bottom>=0) {
 console.log('show')
}

<img src="loading.gif" alt="" data-src="1.gif">
...
<script data-filtered="filtered">
  var src = elem.dataset.src;
</script>

var img = new Image();
img.onload = function(){
 elem.src = img.src;
}
img.src = src;

var scrollElement = document.querySelector('.page'),
  viewH = document.documentElement.clientHeight;
function lazyload(){
 var nodes = document.querySelectorAll('img[data-src]');
 Array.prototype.forEach.call(nodes,function(item,index){
  var rect;
  if(item.dataset.src==='') return;
  rect = item.getBoundingClientRect();
  if(rect.bottom>=0 && rect.top < viewH){
    (function(item){
     var img = new Image();
     img.onload = function(){
      item.src = img.src;
     }
     img.src = item.dataset.src
     item.dataset.src = ''
    })(item)
  }
 })
}
lazyload();
scrollElement.addEventListener('scroll',throttle(lazyload,500,1000));
function throttle(fun, delay, time) {
  var timeout,
    startTime = new Date();
  return function() {
    var context = this,
      args = arguments,
      curTime = new Date();
    clearTimeout(timeout);
    if (curTime - startTime >= time) {
      fun.apply(context, args);
      startTime = curTime;
    } else {
      timeout = setTimeout(fun, delay);
    }
  };
};