点击头像上传时if（mui.os.plus）这个判断一直不通过

//上传头像  

document.getElementById('image-list').addEventListener('tap', function() {
alert("11");
if(mui.os.plus) {
alert("22");
var a = [{
title: "拍照"
}, {
title: "从手机相册选择"
}];
plus.nativeUI.actionSheet({
title: "修改用户头像",
cancel: "取消",
buttons: a
}, function(b) { /actionSheet 按钮点击事件/
switch(b.index) {
case 0:
break;
case 1:
getImage(); /拍照/
break;
case 2:
galleryImg(); /打开相册/
break;
default:
break;
}
})
}
}, false);