JS实现未上传图片预览

function checkType(upload){

    var p_w_picpath = document.getElementById(upload).value;

    var extStart = p_w_picpath.lastIndexOf(".");

    var ext = p_w_picpath.substring(extStart, p_w_picpath.length).toUpperCase();

    if ( ext != ".PNG" && ext != ".GIF" && ext != ".JPG" && ext != ".JPEG") {

        alert("图片限于png，gif，jpeg，jpg格式！");

        return false;

    }

    return true;

}

function setImagePreview(upload,preview) {

    if(checkType(upload)){

        showTips('result','图片预览：',0);

        var docObj=document.getElementById(upload);

        var imgObjPreview=document.getElementById(preview);

        if(docObj.files &&    docObj.files[0]){

            //火狐下，直接设img属性

            imgObjPreview.style.width = '160px';

            imgObjPreview.style.height = '120px';

            try{

                imgObjPreview.src = docObj.files[0].getAsDataURL();

            }catch(e){

                //火狐7以上版本不能用上面的getAsDataURL()方式获取，需要一下方式

                imgObjPreview.src = window.URL.createObjectURL(docObj.files[0]);

            }

  

  

        }else{

            //IE下，使用滤镜

            docObj.select();

            var imgSrc = document.selection.createRange().text;

            var localImagId = document.getElementById("localImag");

            //必须设置初始大小

            localImagId.style.width = "160px";

            localImagId.style.height = "120px";

            //图片异常的捕捉，防止用户修改后缀来伪造图片

            try{

                localImagId.style.filter="progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale)";

                localImagId.filters.item("DXImageTransform.Microsoft.AlphaImageLoader").src = imgSrc;

            }catch(e){

                alert("您上传的图片格式不正确，请重新选择!");

                return false;

            }

            imgObjPreview.style.display = 'block';

            document.selection.empty();

        }

        return true;

    }

  

}

　摘自：http://www.oschina.net/code/snippet_187608_34433