html5 本地预览图片
if (typeof FileReader === 'undefined') {
alert('Your browser does not support FileReader...');
return;
}
var reader = new FileReader();
reader.onload = function(e) {
var img = document.getElementById(targetId);
img.src = this.result;
}
reader.readAsDataURL(files);
objectURL = window.URL.createObjectURL(blob);
blob参数是一个file对象或者Blob对象