Custom directive's deep doesn't work when add key to a json object

[brucesong08]

I want to custom a directive with a json Object. When I add new key to the json object, the directive doesn't work.
Example:

props: {
node: {
attr:{}
}
},

directives:{
updAttr:{
deep: true,
update:function{
/_do something_**/
}
}
////add a key it doesn't work
node.attr['a'] = "test";

////if clone a new obj, it works
var attr = jQuery.extend(true, {}, node.attr);
attr["a"] = "test";
node.attr = attr;

[azamat-sharapov]

http://vuejs.org/guide/reactivity.html#Change_Detection_Caveats

[brucesong08]

Thank you for your comment.
But I get the error:

Uncaught TypeError: exp.trim is not a function
parseExpression @ vue.common.js:2923
Vue.$set @ vue.common.js:8569

[azamat-sharapov]

Can you show how you used it? It's best to make repro on jsfiddle (or somewhere else)

[brucesong08]

I think the way is same as my way.
this.someObject = Object.assign({}, this.someObject, { a: 1, b: 2 })

I used the way instead jQuery.extend(true, {}, node.attr);

Thanks again.