var img = "imgurl";//imgurl 就是你的图片路径  

function getBase64Image(img) {

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

     canvas.width = img.width;

     canvas.height = img.height;

     var ctx = canvas.getContext("2d");

     ctx.drawImage(img, 0, 0, img.width, img.height);

     var ext = img.src.substring(img.src.lastIndexOf(".")+1).toLowerCase();

     var dataURL = canvas.toDataURL("image/"+ext,0-1);(0-1　　　　表示图片的质量)

     return dataURL;

}  

var image = new Image();

image.src = img;

image.onload = function(){

  var base64 = getBase64Image(image);

  console.log(base64);

}