<input type="file" id="image"><br/>

var reader = new FileReader();
var AllowImgFileSize = 2100000; //上傳圖片最大值(單位字節(jié))（ 2 M = 2097152 B ）超過(guò)2M上傳失敗
var file = $("#image")[0].files[0];
var imgUrlBase64;
if (file) {
  //將文件以Data URL形式讀入頁(yè)面
  imgUrlBase64 = reader.readAsDataURL(file);
  reader.onload = function (e) {
   //var ImgFileSize = reader.result.substring(reader.result.indexOf(",") + 1).length;//截取base64碼部分（可選可不選，需要與后臺(tái)溝通）
   if (AllowImgFileSize != 0 && AllowImgFileSize < reader.result.length) {
      alert( '上傳失敗，請(qǐng)上傳不大于2M的圖片！');
      return;
    }else{
      //執(zhí)行上傳操作
      alert(reader.result);
    }
  }
 }

function(){
    var url = "static/img/js1.jpg";//這是站內(nèi)的一張圖片資源，采用的相對(duì)路徑
    convertImgToBase64(url, function(base64Img){
    //轉(zhuǎn)化后的base64
    alert(base64Img);
    });
}
//實(shí)現(xiàn)將項(xiàng)目的圖片轉(zhuǎn)化成base64
function convertImgToBase64(url, callback, outputFormat){
    var canvas = document.createElement('CANVAS'),
  　　ctx = canvas.getContext('2d'),
  　　img = new Image;
  　　img.crossOrigin = 'Anonymous';
  　　img.onload = function(){
    　　canvas.height = img.height;
    　　canvas.width = img.width;
    　　ctx.drawImage(img,0,0);
    　　var dataURL = canvas.toDataURL(outputFormat || 'image/png');
    　　callback.call(this, dataURL);
    　　canvas = null;
    };
  　　img.src = url;
}

function(){
  　　//這是網(wǎng)上的一張圖片鏈接
  　　var url="http://p1.pstatp.com/large/435d000085555bd8de10";
    getBase64(url)
      .then(function(base64){
         console.log(base64);//處理成功打印在控制臺(tái)
      },function(err){
         console.log(err);//打印異常信息
      });
}
//傳入圖片路徑，返回base64
function getBase64(img){
    function getBase64Image(img,width,height) {//width、height調(diào)用時(shí)傳入具體像素值，控制大小 ,不傳則默認(rèn)圖像大小
     var canvas = document.createElement("canvas");
     canvas.width = width ? width : img.width;
     canvas.height = height ? height : img.height;
     var ctx = canvas.getContext("2d");
     ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
     var dataURL = canvas.toDataURL();
     return dataURL;
    }
    var image = new Image();
    image.crossOrigin = '';
    image.src = img;
    var deferred=$.Deferred();
    if(img){
     image.onload =function (){
      deferred.resolve(getBase64Image(image));//將base64傳給done上傳處理
     }
     return deferred.promise();//問(wèn)題要讓onload完成后再return sessionStorage['imgTest']
    }
}

reader.result.substring(reader.result.indexOf(",") + 1)

var AllowImgFileSize = 2100000;  //上傳圖片最大值(單位字節(jié))（ 2 M = 2097152 B ）
if (AllowImgFileSize != 0 && AllowImgFileSize < reader.result.length) {
   alert( '上傳失敗，請(qǐng)上傳不大于2M的圖片！');
   return;
}