image onload 判断图片加载进度

var newimages=[]
var imgArr = ['images/banner.jpg','images/grid-bg.png','images/grid-bg00.png','images/grid-bg01.png',
'images/grid-bg02.png','images/grid-bg03.png','images/grid-bg04.png'];
function loadImage(imgArr){
    var Num = 0;
    var imgArr = (typeof imgArr != 'object') ? [imgArr] : imgArr;////确保参数总是数组
    for(var i=0; i<imgArr.length; i++){
        newimages[i] = new Image();
        newimages[i].index = i;
        newimages[i].src = imgArr[i];
        newimages[i].onload = function(){
            Num++;
            console.log(Num);
        }
    }
}
loadImage(imgArr);