Feature request: class directives

[jacwright]

Proposal for a directive to toggle classes.

<div class:active="thing.isActive()" class:selected="selectedItem === thing">
  ...
</div>

Adding and removing a class based off of boolean state is a very common pattern. This directive would only be a nice-to-have since there are several alternatives currently in use.

Alternatives:

• the class attribute, e.g. <div class="thing {{getClasses()}}"> with a bunch of class logic in a component method—my preference is to put the logic in the template
• data attributes, e.g. <div data-active="{{thing.isActive()}}">—cons, many existing libraries (ahem bootstrap) already use classes, and the css is marginally nicer with classes (.active vs [data-active="true"])

The existing solutions are not bad, so this is very much a nice-to-have. But the little things do make a difference. Thoughts?

[Rich-Harris]

I'm ambivalent about this. Normally I use ternary expressions in these situations. On the one hand a class: directive does result in slightly leaner code, which I'm a fan of:

<div class:active="thing.isActive()"></div>
<div class="{{thing.isActive() ? 'active' : ''}}"></div>

On the other, it's another extra piece of syntax to learn, which is potentially off-putting — the ternary expression is immediately clear to anyone who knows JS.

I'd be interested to know what everyone else reckons. Yay or nay?

[tivac]

We've been doing these as computed properties to keep the view code easier to read.

[coussej]

I'd be interested to know what everyone else reckons. Yay or nay?

YAY!

[jacwright]

I'm also a YAY for a class directive. It is one more thing, but an optional one, and one which would be used in every SPA I've ever worked on. Toggling classes is very common, and I still prefer it over attribute selectors in my CSS.

[eyeseast]

I've been doing computed properties and terneries, and it hasn't been a problem. The only thing that's been weird is having this sort of thing:

{{#each things as t, i }}
<p class="{{ i == 2 ? 'active' : 'inactive' }}">{{ thing }}</p>
{{/each}}

I end up with either this unused class name (inactive in this case) or class="" on a bunch of elements. Neither is really a problem, but both are sort of weird.

[jacwright]

There are work arounds, for sure. And they're not awful. But <div class:active="thing.isActive()"></div> is just clean and readable and a great improvement for a common use-case.

[tivac]

Could this format be tweaked to allow multiple classes per check? Or to allow dynamic values for the class? Right now it seems really limited.

[jacwright]

It is limited to the most common use-case. If you have suggestions that allow more flexibility without making the syntax more difficult to read, I would be interested to hear your thoughts. We don't want to provide a syntax that is no cleaner than what we have now, just more to learn without the benefit.

[jacwright]

If the directive uses elem.classList.toggle('active', expr()) then you could have both.
<div class="list-item" class:selected="selected"></div>

[jacwright]

Please vote on this comment whether you'd like to see a class directive as described above added to Svelte using 👍 and 👎 .

[arxpoetica]

I say nay to directives, yay to computed properties. Attitude about it: feature/scope creep. Less is more.

[silentworks]

I think this is the beginning of something we won't like in the future. You can already do what you want this syntax to do in current codebase, why introduce this shortcut with this syntax which is only going to confuse folks?

<MyComponent class:active="this.isActive()" :foo :bar hello="World" class:selected="this.isSelected()" />

To me the above would leave me in such confusion and if class: can't work on components I would then start to wonder why is this thing so inconsistent. One of the reasons why the above wouldn't work is that Svelte components does not require 1 root level item, so how would Svelte work out where to apply this class?

[fiskgrodan]

What about something similar to Vue object syntax?: https://vuejs.org/v2/guide/class-and-style.html#Object-Syntax

Object syntax:
<div :class="{ active: thing.isActive(), error: thing.hasError() }"></div>

[jacwright]

@silentworks I think the transition directive only works on components as well. I hope this isn't the beginning of something we don't like. I would like to just see this one directive added for a very common use-case, and assuming it is documented, it shouldn't confused anyone any more than any other binding.

@fiskgrodan I think that is a decent syntax as well, though it collides with the 1-way binding syntax.

[opus131]

If it's only to set a class or not, I'd usually use {{thing.isActive() && 'active'}} rather than a ternary.

IMHO the difference in terms of typed chars is in that case not big enough to warrant new syntactic sugar:
<div class:active="thing.isActive()"></div>
<div class="{{thing.isActive() && 'active'}}"></div>

Plus, if you later realise that you need more than just an on/off toggle, you'll have more to refactor if you used the proposed new directive. So I'll vote nay.

[jacwright]

Looks like the nays have it. Thank you for all your input!

[jacwright]

You know, the vote was pretty even. It wasn't a landslide in either direction. And other frameworks have support like this such as ractive's class-*. What if it were supported but not documented? 😄

Isn't one of the benefits of Svelte that your app won't bloat with features you don't use, but those who do want to use them can? Would a class:active="isSelected" directive be something that could be added as a 3rd party plugin? (Svelte doesn't handle 3rd-party plugins for the compilation phase yet, does it?)

As I've been doing real app work with Svelte (refactoring an existing app), I'm really missing the cleanliness of my template code with this feature, and I would love it if it were something I could have somehow without negatively affecting those who do not want it.

[arxpoetica]

I actually changed my vote to yay after thinking long and hard about it. Your point about it being additive—people can still do ternaries!—without it breaking anything. I suspect I would actually end up using this a lot.

However, I do think it should be documented if we do it, even if it's in some remote corner of doculand.

[jacwright]

A problem with {{thing.isActive() && 'active'}} is you get class names like false or undefined depending on the output of isActive. Svelte doesn't try and turn falsey bindings into empty strings so we have to use ternaries or add || '' at the end which is about the same.

The real benefit is when you have 2+ dynamic classes on an element. Having many ternaries gets messy.

Computed doesn't help for elements inside #each blocks. And creating new components to support this case can explode the number of components you have making your application more complex to find your way around.

[jacwright]

I've implemented this feature in #1685. It uses classList.toggle and should have minimal size impact in apps that use it and no impact in apps that don't.

[Rich-Harris]

Thanks @jacwright! Released in 2.13

[garygreen]

The real benefit is when you have 2+ dynamic classes on an element. Having many ternaries gets messy.

This is something we're discovering whilst checking out possibility of migrating away from Vue app to Svelte.

We use a utility-first css approach which means we have many classes applied to an element. In vue this was convenient because we could just do:

<div
    class="block py-4 px-6 hover:bg-blue hover:text-white cursor"
    :class="{ 'active bg-blue text-white': i == activeIndex }"
>

However in Svelte we need to add each utility separately which is a bit clunky:

<div
    class="block py-4 px-6 hover:bg-blue hover:text-white cursor"
    class={ i == activeIndex ? 'active' : '' }
    class={ i == activeIndex ? 'bg-blue' : '' }
    class={ i == activeIndex ? 'text-white' : '' }
>

We're possibly missing some other trick to simplify things though, like a computed property or something. But ultimately if your coming from Vue background the workflow does feel a bit fiddly in this regard.

Also if your adding classes through the class.active= style shortcut I'm sceptical what kind of special characters it will allow - in the case of Tailwind it uses : in classnames, so would it allow something like this? Technically in CSS you can have any characters as long as you escape them, I believe.

class.hover:bg-blue={ active }

[kylecordes]

@garygreen Take a look at the pull request merged above; after the comment earlier about "the nays have it", nonetheless a class directive has been added, and is very nice. (I don't know whether it handles class names that include the full breadth of characters allowed by CSS.)

https://svelte.dev/tutorial/classes

[mrkishi]

@garygreen In addition, if you have suggestions on how to improve the class directive without conflicting with the current use cases I'm sure everyone would love to hear them, so feel free to open an issue on that.

For now, if you have a single conditional responsible for multiple classes, you could use a reactive declaration to update your class string once instead of using multiple class directives. Check out an example.

Regarding special characters: they work, but with the class: directive there's currently a bug with scoped styles. I've opened #2929 for this, but you can still reference outside styles such as those from Tailwind even while that's not fixed.

[narenderv7]

The real benefit is when you have 2+ dynamic classes on an element. Having many ternaries gets messy.

This is something we're discovering whilst checking out possibility of migrating away from Vue app to Svelte.

We use a utility-first css approach which means we have many classes applied to an element. In vue this was convenient because we could just do:

<div
    class="block py-4 px-6 hover:bg-blue hover:text-white cursor"
    :class="{ 'active bg-blue text-white': i == activeIndex }"
>

However in Svelte we need to add each utility separately which is a bit clunky:

<div
    class="block py-4 px-6 hover:bg-blue hover:text-white cursor"
    class={ i == activeIndex ? 'active' : '' }
    class={ i == activeIndex ? 'bg-blue' : '' }
    class={ i == activeIndex ? 'text-white' : '' }
>

We're possibly missing some other trick to simplify things though, like a computed property or something. But ultimately if your coming from Vue background the workflow does feel a bit fiddly in this regard.

Also if your adding classes through the class.active= style shortcut I'm sceptical what kind of special characters it will allow - in the case of Tailwind it uses : in classnames, so would it allow something like this? Technically in CSS you can have any characters as long as you escape them, I believe.

class.hover:bg-blue={ active }

When we add multiple classes in elements, as you did, svelte complaining duplicates attributes.

you can use something like this,

 <div class="class="block py-4 px-6 hover:bg-blue hover:text-white cursor { i == activeIndex ? 'active bg-blue text-white' : '' }"></div >