//此方法为file input元素的change事件

 function change(){

  var file = this.files[0];

  var orientation;

  //EXIF js 可以读取图片的元信息 https://github.com/exif-js/exif-js

  EXIF.getData(file,function(){

    orientation=EXIF.getTag(this,'Orientation');

  });

  var reader = new FileReader();

  reader.onload = function(e) {

    getImgData(this.result,orientation,function(data){

      //这里可以使用校正后的图片data了

    });

  }

  reader.readAsDataURL(file);

}

// @param {string} img 图片的base64

// @param {int} dir exif获取的方向信息

// @param {function} next 回调方法，返回校正方向后的base64

function getImgData(img,dir,next){

 var image=new Image();

 image.onload=function(){

  var degree=0,drawWidth,drawHeight,width,height;

  drawWidth=this.naturalWidth;

  drawHeight=this.naturalHeight;

  //以下改变一下图片大小

  var maxSide = Math.max(drawWidth, drawHeight);

  if (maxSide > 1024) {

    var minSide = Math.min(drawWidth, drawHeight);

    minSide = minSide / maxSide * 1024;

    maxSide = 1024;

    if (drawWidth > drawHeight) {

      drawWidth = maxSide;

      drawHeight = minSide;

    } else {

      drawWidth = minSide;

      drawHeight = maxSide;

    }

  }

  var canvas=document.createElement('canvas');

  canvas.width=width=drawWidth;

  canvas.height=height=drawHeight;

  var context=canvas.getContext('2d');

  //判断图片方向，重置canvas大小，确定旋转角度，iphone默认的是home键在右方的横屏拍摄方式

  switch(dir){

    //iphone横屏拍摄，此时home键在左侧

    case 3:

      degree=180;

      drawWidth=-width;

      drawHeight=-height;

      break;

    //iphone竖屏拍摄，此时home键在下方(正常拿手机的方向)

    case 6:

      canvas.width=height;

      canvas.height=width;

      degree=90;

      drawWidth=width;

      drawHeight=-height;

      break;

    //iphone竖屏拍摄，此时home键在上方

    case 8:

      canvas.width=height;

      canvas.height=width;

      degree=270;

      drawWidth=-width;

      drawHeight=height;

      break;

  }

  //使用canvas旋转校正

  context.rotate(degree*Math.PI/180);

  context.drawImage(this,0,0,drawWidth,drawHeight);

  //返回校正图片

  next(canvas.toDataURL("image/jpeg",.8));

 }

 image.src=img;

}