api doc: explicitly tell `set` only works when the prop doesn't exist yet

Note: Here I’m all talking about a prop that doesn’t exist in data in the first place.

The api says:

If the object is reactive, ensure the property is created as a reactive property and trigger view updates.

At least for me, it’s not clear “does set work if the prop isn’t in data, and I’ve already added it diredtly without set?”. So I tried the source code and the comment made it clear:

Adds the new property and triggers change notification if the property doesn’t already exist.

Then I altered my code to use set at prop addition and the not-properly-re-rendering problem was fixed.

So I think it may be better to explicitly tell in the api that set only works when the prop doesn’t exist yet? Like the comment does?

[chrisvfritz]

Excellent! I just made that update to the docs. I also left a comment on your issue. I don’t think Vue.set should work to make existing properties reactive, because not doing it initially leaves open a window for bugs. I do agree we can improve the experience, however, and suggested that Vue emit a warning instead so that users know what they’ve done wrong. 🙂

[JJPandari]

I think this is more clear if anyone has the same doubt as me:

…This must be used at new property addition, using it after normal property addition (e.g. myObject.newProperty = 'hi') won’t make the property reactive.

Somehow I feel this behavior itself is not intuitive. I’ve opened an issue for vue, let’s see what people say about that then.