var file = $(event.target)[0].files[i];
var reader = new FileReader();
reader.onload = function ( event ) {
var txt = event.target.result; //base64
document.getElementById("result").src = txt;//将base64赋予src
};
reader.readAsDataURL(file);
原文：https://www.cnblogs.com/yuyedaocao/p/9987996.html