How to use composition in elements without className? about css-modules HOT 25 CLOSED

@geelen the information you shared here is really valuable. Maybe add a "constraints" section with some of these information and maybe another section like "CSS Modules is NOT for..." This would help set expectations for newcomers ☺️

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Hey @NekR, I really don't appreciate your tone (what is it with this particular topic and people behaving like jerks?). You had a chance to propose something, I spent considerable time explaining the constraints of the system and the relative merits of your approach, and we landed on a potential implementation and both agreed to open it up for feedback. Now, when that feedback indicates that what you want is already possible without needing to invent new syntax, you're now "extremely disappointed"? Were you trying to find a solution or trying to win an argument? Because only one of those is acceptable behaviour on this issue tracker.

CSS Modules is not designed to replicate Sass, it's designed to hugely reduce the need for it. It offers a solution to a problem (global names in CSS) and a powerful new technique (composition) for styling reuse. There's a huge bunch of us trying to figure out what more if anything we really need, and the point of issue threads like this is to find that out.

So to answer the original question, and given the way composes works (as I described at the top), the proposed solutions are the idea of using a <Application> component suggested by @sokra, or to continue to use Sass in your pipeline as suggested by @nkbt. So I'm going to close this issue.

However, I do think this is one of the most difficult aspects of CSS Modules for newcomers to understand, so I'm going to raise another issue around adding that to the documentation.

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@geelen composes is really good idea and having classes for everything is mostly fine. But, some times it's enough. For example I may want to style html/body with default brand styles, but this won't work since :global does not support composing as far as I understand. Of course, everything can be done and detected in JS, but there is a good reason why people prefer writing basic things in CSS--they are basic, easier and probably faster when are written in CSS.

Adding dynamic classes to default elements such as html/body can really be a pain. It means that main html file for SPA should go through the save manager as JS/CSS (let's say webpack) and it's not easy now-days. You probably should have merges/extends/copies/whatever which only works for tag selectors (although one may want to apply them to IDs).

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Btw, I forgot to mention that @nkbt suggested before the use the :global which I already added and works fine if I'm dealing with only one context e.g. the docs page.

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@nkbt I hear you and as I told you before I don't mind at all 😊 after start using React I drop the html tags styling.

It's just because I use markdown a lot and I can see other cases where I'd use a loader e.g. YAML which will return a "pure" HTML and I'd have again to overwrite the loader and parse it. Just wondering if we could achieve or even hack something around only for this cases.

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By the way that sounds like a good idea to PR yaml-loader to add classes to each element ;)

<h1 class="h1">

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@nkbt yup! Nice one!

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oh sorry that would not work for obvious reasons.. disregard previous comment.

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@nkbt 👍 going to wait for Glen to see if he has any suggestions if not I'm happy to close this issue.

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Ok, so. Yes. This is a problem. Let me give you some background (maybe you're already across this but for other readers of this issue):

composes works by exporting multiple classnames from the CSS. So, when you have

.foo {
  color: red;
}
.bar {
  composes: foo;
  background-color: blue;
}

import styles from "./foo.css";

return "<p class='${styles.bar}'>Content</p>"

You get the following ICSS:

:export {
  foo: foo_98812cadf;
  bar: foo_98812cadf bar_312cbca63;
}
.foo_98812cadf {
  color: red;
}
.bar_312cbca63 {
  background-color: blue;
}

So, your HTML renders out as <p class='foo_98812cadf bar_312cbca63'>Content</p> and you get all of .foo and .bar styles. But the only way that works is through classes. Think of composes as an instruction to JavaScript to include this extra class whenever you reference this one. It relies on dynamic markup to work.

So if you don't have control over the markup being injected then composition doesn't work.
.foo :global(p) will still give you localisation, of course, but if you're building your whole site using composition that's not ideal.

I actually faced this exact problem and wrote a monstrous JSPM loader for it. It parses the markdown as YAML (for frontmatter), then markdown then JSX, then returns an ES6 function that accepts a styles object. I think. I can never remember, I wanted to tidy it up and release it but it breaks if I touch it...

It gets used here and lets me do cool stuff with inline JSX in markdown:

But I digress, back to the issue at hand.

Theoretically, you could write a runtime JS component that gave you this more advanced behaviour, where it watches the live DOM and attaches classes based on these more advanced rules. That could be kind of neat, but at this stage it's not something I'm going to work on. For one, I am trying to avoid using descendent selectors & bare tags as much as possible, except for contextual overrides that don't care about the nature of the things they're overriding. E.g:

.MyComponent {
  > * {
    margin-top: 1rem;
  }
  > :first-child {
    margin-top: 0;
  }
}

Secondly, if you add a runtime dependency you need to be damn careful not to break server-side rendering. Because all the React SSR stuff already evaluates the render tree, injecting classnames there works really nicely. And since we're using plain-old-CSS, we get pseudo-selectors like :hover for free (Radium has problems with that). I think we can do it, but I'm not sure if it's worth it. And if we want to support traditional stuff like Rails (which we should!)

Probably should also point out that the following usages of composes don't work for the same reasons:

.foo .bar {
  composes: different-when-nested;
}

@media (max-width: 599px) {
  .bar {
    composes: something-on-small-breakpoint;
  }
}

The media query one, at least, has a decent fallback:

/* A class can use media queries then get composed */
.headline {
  font-size: 4rem;
}

@media (max-width: 599px) {
  .headline {
    font-size: 3rem;
  }
}

/* You can use suffixes like Tachyons/Basscss */
.foo {
  color: red;
}

@media (max-width: 599px) {
  .foo--small {
    color: red;
  }
}

.bar {
  composes: headline foo--small;
}

The thing I like about this is that we're building our own abstractions on top of the basic CSS mechanisms. I see composes as constrained, yes, but it feels like it's worth embracing this constraint for the time being to see where it leads us :)

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Thanks @geelen for the background, for spending time writing this.

So if you don't have control over the markup being injected then composition doesn't work.
.foo :global(p) will still give you localisation, of course, but if you're building your whole site using composition that's not ideal.

I think this is one of the problems I face right now building a site, not an app or component.

For one, I am trying to avoid using descendent selectors & bare tags as much as possible, except for contextual overrides that don't care about the nature of the things they're overriding

Couldn't agree more. CSS modules is all about simplicity.

I actually faced this exact problem and wrote a monstrous JSPM loader for it. It parses the markdown as YAML (for frontmatter), then markdown then JSX, then returns an ES6 function that accepts a styles object.

Ok that's interesting thanks for sharing. I think a variation of your markdown.js could be used as markdownLoader on webback if you're using CSS modules.

I can never remember, I wanted to tidy it up and release it but it breaks if I touch it...

😂😂😂

The thing I like about this is that we're building our own abstractions on top of the basic CSS mechanisms. I see composes as constrained, yes, but it feels like it's worth embracing this constraint for the time being to see where it leads us :)

I agree! I think is worth to apply this in real projects. Right now I have the feeling that CSS modules is perfect for components and self-contained apps, but for page layout/design I think we are still missing some pieces. I know @markdalgleish made react-themeable to address some of the limitations but this isn't an issue with theming. Maybe we need something else in between 😊.

I'm happy with the answer, please feel free to close this issue.

Cheers 🍺

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@NekR I'm afraid it's simply not possible to use composes with tag selectors. What you want is the @extend CSS spec proposed by Tab Atkins.

But, while I'm finding plenty of uses for composes, by no means is it the only way to use CSS Modules. Not all styles have to be written the same way, not all duplication needs refactoring. One of the benefits of CSS Modules is that it's still just CSS, you can use a few tag selectors if you need to (using :global) and use composes for the rest.

Oh, that and it's all just PostCSS transforms so if you want to go crazy and write a stage that detects if composes is used inside a tag selector and goes and copies the referenced rules (just like a Sass mixin) you could totally do that! ;)

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@NekR I'm afraid it's simply not possible to use composes with tag selectors. What you want is the @extend CSS spec proposed by Tab Atkins.

I do not propose to use compose with tag selectors or even compose action behind the scene. For other selectors than classes we need this:

:global(body) {
  margin: 0;
  / * ... */

  add/merge/whatever: text-color from './colors.css';
}

which would produce:

body {
  margin: 0;
  / * ... */

 color: #333;
}

but not any magic from compose. This probably should be allowed only when selector is marked as :global().

Oh, that and it's all just PostCSS transforms so if you want to go crazy and write a stage that detects if composes is used inside a tag selector and goes and copies the referenced rules (just like a Sass mixin) you could totally do that! ;)

I did not say that it should be still called composes for such case. I understand that it's PostCSS and could be done. I could understand the case when you ask people to contribute to the project, this is fine. But why you are so defensive about adding new features? It would be a bit unfortunate to adding one more PostCSS transformer to load modules, define variables, define all in vars, then in css-module to be able to compose them, etc. merge rule really can help here.

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Hey @NekR, I actually really like this idea. One of the immediate questions people ask about CSS Modules is "how can I do variables?" In fact, someone trying to get a variable-passing syntax into CSS Modules was why we broke out ICSS — so people would be able to write their new ideas in userspace and we'd bring modules into core if we really felt they were globally useful. I'm going through that process with my own stuff, too: css-modules/css-modules-loader-core#18

So we're hoping to tread the line about being defensive about adopting ideas but enthusiastic about supporting them. But we're all new to this, so I apologise if I came across as being defensive.

For me, there are two competing ideas at play. The first is that good design should guide you to good practice. I see using tag selectors and nesting as not good CSS practice, so that makes me reluctant to include anything designed directly for them. The second is that CSS Modules should maintain as much as possible of CSS. For every rule we make, we know there'll be exceptions. If we disable any parts of CSS without giving a better alternative, that's shitty. That's why there's :global, to fall back to native selectors.

Your idea is somewhat positive in both respects. Being able to reuse all your modules in all your styles (regardless if they are modules themselves) is definitely a win, despite wanting to avoid tag selectors where possible. And having compose limited to only modules feels like a deliberate restriction, not because we're disabling any part of CSS, but because this stuff is possible in Sass.

So I could go either way, and I'll invite folks to leave their comments here either for/against this idea. The implementation would be tricky, and doesn't fit well with the way ICSS works, but the question is should we support something like composition from tag selectors?

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Thanks for the response @geelen. I understand your position and that you do not want to bloat CSS Modules with every feature requested or made it another Sass/Less/Stylus/etc. I actually did not mean allowing cascade selectors (hah) like this .my-elem p, I mostly care about the ability to use .css (compose/merge/...) files which are modules for external world, e.g. having main.css which is not a module or having "critical path css" directly in index.html, but with ability to reuse defined rules from CSS Modules.

So it indeed might be a good idea to allow new keyword merge still to be applied only to class selectors, but since it's not composing, it will allow this case to work:

:global(.my-class-for-body) {
  margin: 0;
  / * ... */

  merge: text-color from './colors.css';
}

which will produce:

.my-class-for-body {
  margin: 0;
  / * ... */

 color: #333;
}

So in this case we still have restriction to compose/merge only classes/into classes, but will ability to export merged rules to global scope.

So I could go either way, and I'll invite folks to leave their comments here either for/against this idea.

Yep, feedback and discussion would be good here. Maybe other people have better ideas :-)

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Isn't it something, that less/sass/etc already do? I thought if I want
styles merging, I would use less for that and then pipe it to css-modules.

That' exactly what I would propose too.

CSS Modules is designed is a way, that allows to use less/sass/stylus/postcss on top of it. So you get all the fancy features of css preprocessors while still working on a modulized CSS file. This way you can import your variables/mixins. composes is just a "normal" CSS rule and don't have a "special" syntax so every css preprocessor should be able to emit it. It's also common to use autoprefixer on top of CSS Modules.

I. e. with webpack "css-loader?modules!autoprefixer-loader!sass-loader".

(less.js is currently a bit buggy, because it rewrites url(...)s is a bad way)

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This looks like to have "modules" I need to have too much files, isn't?

// vars.css; postcss vars

$text_color = #333;
$brand_clor = red;

// colors.css; css module
@import './vars.css'

.brand {
  color: $brand_color;
}

.text {
  color: $text_color;
}

// main.css, simple file
@import './vars.css'

body {
  /* ... */
  color: $text_color;
}

// my-component.css, css module
.button {
  compose: brand from './colors.css'
}

This is really confusing. As per your blog posts you are positioning CSS Modules as a replace for less/sass/whatever preprocessor, and now you are saying that it's not replacement but a some piece like any other Postcss plugin. If so, then I believe CSS Modules is already done at this point since it cannot bring anything new to the world here what Postcss can't.

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I need to have too much files, isn't?

I wouldn't say so. I would say you now have it nicely separated.

As per your blog posts you are positioning CSS Modules as a replace for less/sass/whatever preprocessor, and now you are saying that it's not replacement but a some piece like any other Postcss plugin.

less/sass/whatever are tools that make it easier to write CSS.

CSS Modules is a technology which make it possible to write modulized CSS.

CSS Modules may make less/sass/whatever unnecessary, because your CSS files are just so simple that you don't need tools that make writing of them easier...

But there is no need to style the body tag in your case. Just use a <Application> component and set you base styles in there.

/* text.css */
.normal {
  font-family: Arial;
  color: #333;
}

.brand {
  composes: normal;
  color: red;
  font-weight: bold;  
}

/* Application.css */
.context {
  composes: normalText from "./text.css";
}

/* BrandButton.css */
.button {
  composes: button from "./Button.css";
  composes: brand from "./text.css";
}

/* Application.js */
import styles from "./Application.css";
class Application extends React.Component {
  render() {
    return <div className={styles.context}>
      {this.props.children}
    </div>;
  }
}

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@NekR CSS-modules is a panacea from CSS inheritance/specificity/globality decease. It gives you a tool to write small, clean, isolated styled modules. It is completely up to you if you want to use variables, imports and other workarounds from old css world - you still are able to use sass for that. And css-modules give you a great opportunity to mix and match tools.

It is never meant to be a replacement for sass. It is just a completely new way of styling things. It is up to you if you want to replace your styling techniques with this new way, or not. Many people do not replace but add.

We replaced completely in our projects and can't be happier with new way of styling components for past 2 months.

PS: please, try to be a little more humble and lower your tone, it starts to feel sort of harsh.

Cheers

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PS: please, try to be a little more humble and lower your tone, it starts to feel sort of harsh.

You know, my tone it exactly how it wanted it to because I am extremely disappointed with your answers here.
I already got your about about CSS Modules independency many times. I see that you do not wanted it to be a replacement for pre-processors, but this is exactly how you pointed and promoted this out for people. And this is only reason why they came here complaining and blaming about variables. This seems to me like your fault, not people's. Even more, after people came here all that you did (across many issues) is saying: "do it yourself as a PostCSS", "Use another tool", etc. Seems extremely disappointed for people which went here for panacea, right?

CSS-modules is a panacea from CSS inheritance/specificity/globality decease. It gives you a tool to write small, clean, isolated styled modules

I understand what CSS Modules is great tool and how it can help.

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