Lifecycle hooks/methods: Add note not to use arrow functions for them.

[LinusBorg]

Arrow functions don't work for hook functions and component methods because Vue can't use .bind() on them to bind them to the current vm instance, so this will be undefined

But people who don't know the mechanics of Vue here often miss this.

I see a question about this on the forum or in the github issues every week at the least.

[chrisvfritz]

@LinusBorg What do you think about this kind of warning?

Note that you should not use arrow functions on an instance property or callback (e.g. vm.$watch('a', newVal => this.myMethod())). The reason is arrow functions bind the parent context, so this will not be the Vue instance as you expect and this.myMethod will be undefined.

Do you think that will make sense to the people you see make this mistake?

[LinusBorg]

I think the text is good, but using $watch for the example is not.

It's not an option everyone uses, and not one that beginners become familiar with early on. We should pick something more basic, like mounted or a computed prop.

Also, it may be worth to note that inside of these "first-level" functions, array functions are fine?

[chrisvfritz]

Sounds good - and I agree about $watch. I only used it because it's what's in the example under Properties and Methods, which is the first place I was thinking of introducing the warning (I'd like to add versions of it with different examples to a few different places).

Do you think that's a good place to start? I was worried about adding it to the introduction, as I don't want to scare new users by mentioning Babel/ES2015 right away. Or should it only be added to the API section, under data, computed, methods, and watch?

[LinusBorg]

Sounds good - and I agree about $watch. I only used it because it's what's in the example under Properties and Methods, which is the first place I was thinking of introducing the warning (I'd like to add versions of it with different examples to a few different places).
Do you think that's a good place to start?

Oh, in that context I guess $watch is fine.

Or should it only be added to the API section, under data, computed, methods, and watch?

We should add it in those places as well.