Clarify "falsy" values

[Justineo]

https://vuejs.org/v2/guide/syntax.html#Attributes

It also works for boolean attributes - the attribute will be removed if the condition evaluates to a falsy value

In fact, binding an attribute to 0 or NaN won't result in removing the attribute. It's not a normal case but the doc should be more precise on this IMO.

via vuejs/v2.cn.vuejs.org#586

[chrisvfritz]

Interesting. I'm glad you brought this up, but I'm wondering if it's a documentation issue or a bug. @yyx990803 Is this intentional and if so, why the deviation?

[yyx990803]

Yeah we should be more precise here - it only removes the value if it is null, undefined or false: https://github.com/vuejs/vue/blob/dev/src/platforms/web/util/attrs.js#L41-L43

[chrisvfritz]

@yyx990803 Thanks! Can you share the reasoning for that? I'd like to provide a justification for the behavior in the more precise explanation.

[yyx990803]

Well, if the value is 0, most likely the user does want to display 0. The case for NaN is subject to debate, but usually it is a result of error and probably better to have that shown in the output so the user can actually spot it.

[chrisvfritz]

I see. Thanks!

[sdras]

Fixed!