js图片预加载

var img=document.getElementById('img');
function add(isrc){
      var imgn=new Image();
       imgn.src=isrc;
       imgn.onload=function(){
             img.src=this.src;
             img.style.width=50+'px';
             img.style.height=50+'px';
}        
}
add('1.png');
add('5.png);

 最近用到一种方法：

<div style="border:2px dashed red;">
        <p>
            图片上传前预览：<input type="file" id="xdaTanFileImg" onchange="xmTanUploadImg(this)" accept="image/*"/>
            <input type="button" value="隐藏图片" onclick="document.getElementById('xmTanImg').style.display = 'none';"/>
            <input type="button" value="显示图片" onclick="document.getElementById('xmTanImg').style.display = 'block';"/>
        </p>
        <img id="xmTanImg"/>
        <div id="xmTanDiv"></div>
    </div>

　　//判断浏览器是否支持FileReader接口
    if (typeof FileReader == 'undefined') {
        document.getElementById("xmTanDiv").InnerHTML = "<h1>当前浏览器不支持FileReader接口</h1>";
        //使选择控件不可操作
        document.getElementById("xdaTanFileImg").setAttribute("disabled", "disabled");
    }

    //选择图片，马上预览
    function xmTanUploadImg(obj) {
        var file = obj.files[0];
        
        console.log(obj);
        console.log(file);
        console.log("file.size = " + file.size);  //file.size 单位为byte

        var reader = new FileReader();

        //读取文件过程方法
        reader.onloadstart = function (e) {
            console.log("开始读取....");
        }
        reader.onprogress = function (e) {
            console.log("正在读取中....");
        }
        reader.onabort = function (e) {
            console.log("中断读取....");
        }
        reader.onerror = function (e) {
            console.log("读取异常....");
        }
        reader.onload = function (e) {
            console.log("成功读取....");

            var img = document.getElementById("xmTanImg");
            console.log(e.target.result.length)
            img.src = e.target.result;
            console.log(e.target.result)
            //或者 img.src = this.result;  //e.target == this
        }

        reader.readAsDataURL(file)
    }