: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.

[Liwoj]

I think this should be reopend as the fix in 7013e8f does not work. Problem is the value argument of patchClass is never null because all class values (including null, undefined etc) are converted into strings using normalizeClass by the compiler

[wenfangdu]

@Liwoj Just tried, reproducible for me as well. Ping @yyx990803

[edison1105]

@Liwoj Just tried, reproducible for me as well. Ping @yyx990803

I made a repro

the value is empty string not null

[wenfangdu]

@edison1105 For :class="undefined" and :class="null", @vue/runtime-dom v3.1.5 removed it, whereas @vue/runtime-dom v3.2.4 kept it, which seems like a regression bug.

[edison1105]

@edison1105 For :class="undefined" and :class="null", @vue/runtime-dom v3.1.5 removed class, whereas @vue/runtime-dom v3.2.4 kept it, which seems like a regression bug.

Good catch. Would you mind commit a new issue with a working repro?

[wenfangdu]

@edison1105 I've updated the issue to reflect that, since the playground doesn't seem to keep the vue version, I included the repro for the latest commit.

[LinusBorg]

The issue is that normalizeClass will always coerce the :class value to a string.

https://github.com/vuejs/vue-next/blob/cac6ab5bdb712a3872dfa2b1cd4751d80cb4f38c/packages/shared/src/normalizeProp.ts#L65-L66

and patchClass will only remove the attribute when it's nullish:

https://github.com/vuejs/vue-next/blob/cac6ab5bdb712a3872dfa2b1cd4751d80cb4f38c/packages/runtime-dom/src/modules/class.ts#L16-L17

[gokaybiz]

The issue still exists, does anyone have any solution?

https://github.com/vuejs/rfcs/blob/master/active-rfcs/0024-attribute-coercion-behavior.md#drawbacks
https://vuejs.org/guide/essentials/template-syntax.html#attribute-bindings

when I use null, empty class property remains in the tag.

Vue version: 3.3.4