Ember.js Template Imports: Part 5

What about styles? (A bonus post!)

Assumed Audience: Other members of the Ember.js and Glimmer.js communities. (Others are welcome to read and chime in, of course, but this is absolutely an in-house” discussion!)

Previously in this series, I have covered Teaching and Understanding, Tooling, and Testing. I had planned for this fifth post in the series to be the final post in the series, summarizing the tradeoffs and concluding my case for <template>. However, as a few folks have mentioned styles over the course of the last many months, I thought it would be helpful to add one extra post discussing CSS in particular.

Fundamentally, there are two basic approaches available for styling in the modern front-end ecosystem — everything out there is a variation on one of these themes:

  1. Standalone style definitions. This is the classic approach: defining styles in a completely separate context from the JavaScript and HTML. This can be accomplished any number of ways, from a single monolithic CSS file to a bunch of files integrated together via a build step with Sass or PostCSS to atomic styles like Tailwind or Tachyons.

  2. Integrated style definitions. This is the approach which became much more popular in the late 2010s: style definitions which are directly related in some way to UI components. This includes CSS Modules, styled components, Emotion, vanilla-extract, and no doubt more. Notably, many of these can also work with some of the preprocessing tools from the classic build approach. The big upsides to these are usually tooling integration and built-in per-component scoping.

All of these work perfectly well with any of the template imports proposals. This is the reason I originally left this aside, and it’s also the reason this is the shortest of the posts. The templating layer is a fundamental part of a front-end framework: it is the thing a front-end framework must do. By contrast, a framework can be styling-method-agnostic. Frameworks certainly can provide first-class primitives for it — as Vue and Svelte do — but it is not a necessary constraint for the integration of state change with DOM updates, unlike templating.

This is by no means to diminish the importance of styling, which I take to be an under-appreciated element of truly great web applications. It is rather to make clear and explicit what constraints it does and doesn’t face compared to templates.

There are certainly interesting moves we could make with styles in any of the proposals. It’s easy to imagine introducing a <style> block alongside the template and <script> tags in an SFC design, much as Svelte has done:

<script>
  const isChristmas = date =>
    date.getMonth() === 11 &&
    date.getDate() === 25;
</script>

<style>
  .christmas {
    color: green;
    background: red;
  }
</style>

{{#if (isChristmas @today)}}
  <p class='christmas'>Merry Christmas!</p>
{{/if}}

Notice, however, that even here with an SFC design we do not need to use a <style> tag, because the semantics of template imports make JavaScript values available to the template. If we wanted to use vanilla-extract instead, we could do that:

<script>
  import { style } from '@vanilla-extract/css';
  
  const christmas = style({
    color: 'green',
    background: 'red',
  });

  const isChristmas = date =>
    date.getMonth() === 11 &&
    date.getDate() === 25;
</script>

{{#if (isChristmas @today)}}
  <p class={{christmas}}>Merry Christmas!</p>
{{/if}}

Exactly the same holds for the template literals and <template> designs: as long as we provide a value in a way that can be integrated into the precompileTemplate invocation which all of these formats compile to, it will just work.”1 And indeed, that means that these even work with imports-only, though with the usual constraints:

import { style } from '@vanilla-extract/css';
  
export const christmas = style({
  color: 'green',
  background: 'red',
});

export const isChristmas = date =>
  date.getMonth() === 11 &&
  date.getDate() === 25;
---
import { isChristmas, christmas } from './christmas.js';
---

{{#if (isChristmas @today)}}
  <p class={{christmas}}>Merry Christmas!</p>
{{/if}}

In the case of the <template> design in particular, I can imagine that community members might want to implement a <style> element which compiles a CSS language declaration to a scoped value:

const Style = <style>
  .christmas {
    color: 'green';
    background: 'red';
  }
</style>

const isChristmas = date =>
  date.getMonth() === 11 &&
  date.getDate() === 25;

<template>
  {{#if (isChristmas @today)}}
    <p class={{Style.christmas}}>Merry Christmas!</p>
  {{/if}}
</template>

This is quite nice! But again, the key point is that this does not need to be built into the framework: there is no fundamental coupling between the way we generate our styles and the rendering or state management layers. The upside to the template imports design is that any and all such solutions just work.” It may even be that in the future, we as a community find one that we particularly prefer and therefore write an RFC to ship out-of-the-box support for it. But it simply isn’t a constraint in any way for the design of template imports. Next up: the conclusion of the series, with a summary analysis of the tradeoffs!


  1. If you’d like to prove this to yourself, create a new app on Ember 3.28.4+, and make a component JavaScript file with this body and render it:

    import { precompileTemplate } from '@ember/template-compilation';
    import { setComponentTemplate } from '@ember/component';
    import templateOnly from '@ember/component/template-only';
    import { helper } from '@ember/component/helper';
    
    const Style = {
      christmas: 'christmas_1234abcd',
      normal: 'normal_9876beef',
    };
    
    const isChristmas = helper(
      ([date]) =>
        date.getMonth() === 11 &&
        date.getDate() === 25
    );
    
    const now = helper(() => new Date());
    
    const template = precompileTemplate(
      `
      {{#if (isChristmas (now))}}
        <p class={{Style.christmas}}>Happy Christmas!</p>
      {{else}}
        <p class={{Style.normal}}>Have a nice day!</p>
      {{/if}}
      `,
      {
        scope: { Style, isChristmas, now },
      }
    );
    
    export default setComponentTemplate(template, templateOnly());
    

    Then you can check that it applies the correct class in the inspector. ↩︎