图片压缩预览（具体上传请看另外一篇文章，讲解的更详细图片压缩上传）

html结构

<div class="preview" id="preview">
      ![](img.png)
</div>
<input type="file" name="file" id="imgInput" 
            onchange="previewImage(this,'preview','img','imgInput')" class="dis">

js代码

//图片上传预览    IE是用了滤镜。
function previewImage(file, previewDiv, imgId, imgInputId) {
    var MAXWIDTH = document.getElementById(imgId).width;
    var MAXHEIGHT = document.getElementById(imgId).height;
    var div = document.getElementById(previewDiv);
    if (file.files && file.files[0]) {

        // 格式不正确的错误提示
        var error = document.createElement('p');
        error.className = 'pic-error';

        //如果不是图片格式，不往下执行
        if (file.files[0].type !== 'image/jpeg' && file.files[0].type !== 'image/jpg' && file.files[0].type !== 'image/png' && file.files[0].type !== 'image/gif') {
            error.innerHTML = '请上传图片格式的文件（jpeg/jpg/png/gif）';
            div.appendChild(error);
            return;
        }

        //限制6M以下的图片
        if(file.size > 6*1024*1026){
            error.innerHTML = '请上传大小在6M以下的图片';
            div.appendChild(error);
            return;
        }

        //移除错误提示
        if(div.childNodes[1]){
            div.removeChild(div.childNodes[1]);
        }

        div.innerHTML = '<img id="' + imgId + '" onclick=$("#' + imgInputId + '").click()>';
        var img = document.getElementById(imgId);
        img.onload = function () {
            var rect = clacImgZoomParam(MAXWIDTH, MAXHEIGHT, img.offsetWidth, img.offsetHeight);
            img.width = rect.width;
            img.height = rect.height;
            img.style.marginTop = rect.top + 'px';
        }
        var reader = new FileReader();
        reader.onload = function (evt) {
            img.src = evt.target.result;
        }
        reader.readAsDataURL(file.files[0]);
    }
    else //兼容IE
    {
        var sFilter = 'filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale,src="';
        file.select();
        var src = document.selection.createRange().text;
        div.innerHTML = '<img id=' + imgId + '>';
        var img = document.getElementById(imgId);
        img.filters.item('DXImageTransform.Microsoft.AlphaImageLoader').src = src;
        var rect = clacImgZoomParam(MAXWIDTH, MAXHEIGHT, img.offsetWidth, img.offsetHeight);
        status = ('rect:' + rect.top + ',' + rect.left + ',' + rect.width + ',' + rect.height);
        div.innerHTML = "<div id=divhead style='width:" + rect.width + "px;height:" + rect.height + "px;margin-top:" + rect.top + "px;" + sFilter + src + "\"'></div>";
    }
}
// 计算根据原有img标签宽高计算裁剪后的图片宽高
function clacImgZoomParam(maxWidth, maxHeight, width, height) {
    var param = {top: 0, left: 0, width: width, height: height};
    if (width > maxWidth || height > maxHeight) {
        rateWidth = width / maxWidth;
        rateHeight = height / maxHeight;

        if (rateWidth > rateHeight) {
            param.width = maxWidth;
            param.height = Math.round(height / rateWidth);
        } else {
            param.width = Math.round(width / rateHeight);
            param.height = maxHeight;
        }
    }
    param.left = Math.round((maxWidth - param.width) / 2);
    param.top = Math.round((maxHeight - param.height) / 2);
    return param;
}

css代码

.dis{
    display: none;
}
.preview{
    display: table-cell;
    width: 300px;
    height: 200px;
    border: 1px solid #ccc;
    background-color: #ccc;
    border-radius: 5px;
    text-align: center;
    vertical-align: middle;
}
.preview img{
    border: 0;
}
            

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感谢阅读，如有错误欢迎指正。