此demo为大于1M对图片进行压缩上传
若小于1M则原图上传，可以根据自己实际需求更改。
demo源码如下：
<!DOCTYPE html><html><head><meta charset="UTF-8"> ???<title>XMLHttpRequest上传文件</title> ???<script type="text/javascript"> ???????/* ???????三个参数 ???????file：一个是文件(类型是图片格式)， ???????w：一个是文件压缩的后宽度，宽度越小，字节越小 ???????objDiv：一个是容器或者回调函数 ???????photoCompress() ????????*/ ???????function photoCompress(file,w,objDiv){ ???????????var ready=new FileReader(); ???????????????/*开始读取指定的Blob对象或File对象中的内容. 当读取操作完成时,readyState属性的值会成为DONE,如果设置了onloadend事件处理程序,则调用之.同时,result属性中将包含一个data: URL格式的字符串以表示所读取文件的内容.*/ ???????????????ready.readAsDataURL(file); ???????????????ready.onload=function(){ ???????????????????var re=this.result; ???????????????????canvasDataURL(re,w,objDiv) ???????????????} ???????} ???????function canvasDataURL(path, obj, callback){ ????????????var img = new Image(); ????????????img.src = path; ????????????img.onload = function(){ ?????????????var that = this; ?????????????// 默认按比例压缩 ?????????????var w = that.width, ??????????????h = that.height, ??????????????scale = w / h; ??????????????w = obj.width || w; ??????????????h = obj.height || (w / scale); ?????????????var quality = 0.7; ?// 默认图片质量为0.7 ?????????????//生成canvas ?????????????var canvas = document.createElement(‘canvas‘); ?????????????var ctx = canvas.getContext(‘2d‘); ?????????????// 创建属性节点 ?????????????var anw = document.createAttribute("width"); ?????????????anw.nodeValue = w; ?????????????var anh = document.createAttribute("height"); ?????????????anh.nodeValue = h; ?????????????canvas.setAttributeNode(anw); ?????????????canvas.setAttributeNode(anh); ??????????????ctx.drawImage(that, 0, 0, w, h); ?????????????// 图像质量 ?????????????if(obj.quality && obj.quality <= 1 && obj.quality > 0){ ??????????????quality = obj.quality; ?????????????} ?????????????// quality值越小，所绘制出的图像越模糊 ?????????????var base64 = canvas.toDataURL(‘image/jpeg‘, quality); ?????????????// 回调函数返回base64的值 ?????????????callback(base64); ???????????} ???????} ???????/** ??????????* 将以base64的图片url数据转换为Blob ??????????* @param urlData ??????????* ???????????用url方式表示的base64图片数据 ??????????*/ ?????????function convertBase64UrlToBlob(urlData){ ?????????????var arr = urlData.split(‘,‘), mime = arr[0].match(/:(.*?);/)[1], ???????????????bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n); ???????????while(n--){ ???????????????u8arr[n] = bstr.charCodeAt(n); ???????????} ???????????return new Blob([u8arr], {type:mime}); ???????} ????????????????????????var xhr; ???????//上传文件方法 ???????function UpladFile() { ???????????var fileObj = document.getElementById("file").files[0]; // js 获取文件对象 ???????????var url = "后台图片上传接口"; // 接收上传文件的后台地址 ????????????????????????var form = new FormData(); // FormData 对象 ???????????????????????if(fileObj.size/1024 > 1025) { //大于1M，进行压缩上传 ???????????????photoCompress(fileObj, { ???????????????????quality: 0.2 ???????????????}, function(base64Codes){ ???????????????????//console.log("压缩后：" + base.length / 1024 + " " + base); ???????????????????var bl = convertBase64UrlToBlob(base64Codes); ???????????????????form.append("file", bl, "file_"+Date.parse(new Date())+".jpg"); // 文件对象 ???????????????????xhr = new XMLHttpRequest(); ?// XMLHttpRequest 对象 ???????????????????xhr.open("post", url, true); //post方式，url为服务器请求地址，true 该参数规定请求是否异步处理。 ???????????????????xhr.onload = uploadComplete; //请求完成 ???????????????????xhr.onerror = ?uploadFailed; //请求失败 ???????????????????xhr.send(form); //开始上传，发送form数据 ???????????????}); ???????????}else{ //小于等于1M 原图上传 ???????????????form.append("file", fileObj); // 文件对象 ???????????????xhr = new XMLHttpRequest(); ?// XMLHttpRequest 对象 ???????????????xhr.open("post", url, true); //post方式，url为服务器请求地址，true 该参数规定请求是否异步处理。 ???????????????xhr.onload = uploadComplete; //请求完成 ???????????????xhr.onerror = ?uploadFailed; //请求失败 ???????????????xhr.send(form); //开始上传，发送form数据 ???????????} ???????} ???????//上传成功响应 ???????function uploadComplete(evt) { ????????//服务断接收完文件返回的结果 ?????????????????????????var data = JSON.parse(evt.target.responseText); ????????????if(data.success) { ????????????????alert("上传成功！"); ????????????}else{ ????????????????alert("上传失败！"); ????????????} ????????????????????} ???????//上传失败 ???????function uploadFailed(evt) { ???????????alert("上传失败！"); ???????} ?????????//取消上传 ???????function cancleUploadFile(){ ???????????xhr.abort(); ???????} ???</script></head><body> ???<progress id="progressBar" value="0" max="100" style="width: 300px;"></progress> ???<span id="percentage"></span><span id="time"></span> ???<br /><br /> ???<input type="file" id="file" name="myfile" /> ???<input type="button" onclick="UpladFile()" value="上传" /> ???<!--<input type="button" onclick="cancleUploadFile()" value="取消" />--></body></html>前端图片压缩上传(纯js的质量压缩，非大小压缩)
原文地址：http://www.cnblogs.com/007sx/p/7583202.html