:class="undefined" causes class being rendered

[wenfangdu]

Version

3.2.6

Steps to reproduce

<template>
  <div :title="undefined" :class="undefined" :style="undefined">
    Lorem ipsum dolor sit amet consectetur adipisicing elit. Dolore molestiae quod voluptas
    reprehenderit possimus nihil eligendi iure et esse aut, iste natus porro enim rerum cumque
    veniam quae! Quam, voluptatum.
  </div>
</template>

What is expected?

None of title, class, style should be rendered.

What is actually happening?

class is rendered, the same thing applies to null.

Additional info

This was fixed in v3.1.5 but is reproducible when using the latest commit.

[HcySunYang]

I don’t think this is a bug, but it’s reasonable to optimize that behavior.

[wenfangdu]

@HcySunYang

<template>
  <div :style="false" :class="false" :foo="false">
    Lorem ipsum dolor sit amet, consectetur adipisicing elit. Laudantium modi
    repudiandae, illum sequi nemo est deleniti optio distinctio quaerat quasi
    dolores vitae aspernatur quisquam, voluptatibus quibusdam ullam, ipsum aut
    amet?
  </div>
</template>

If isButtonDisabled has the value of null, undefined, or false, the disabled attribute will not even be included in the rendered element..

In vue ^2.6.10, the above code will render class too, could you please make a PR regarding that?

[HcySunYang]

Check out the new attributes coercion behavior https://github.com/vuejs/rfcs/blob/master/active-rfcs/0024-attribute-coercion-behavior.md

[wenfangdu]

@HcySunYang Yes, I've known that, I guess this issue belongs to the vue 2 repo, I've created one there (vuejs/vue#11902).

Also,

If the bound value is null or undefined then the attribute will not be included on the rendered element.

The documented behavior is class wouldn't be rendered if it's null or undefined, so I think it is a bug.

[posva]

I think this was for performance reasons. There are no practical reasons to not remove class and it was highlighted in other issues in Vue 2 and dismissed already.
Definitely not a bug but worth checking if the perf implication still exists

[HcySunYang]

Only call removeAttribute when the element does not have any class. In other cases, className is used. I think it's fine.