DEV Community: Rails Designer

Introducing Hanami Icons

Rails Designer — Fri, 30 Oct 2026 09:35:23 +0000

Most of what can be read here is around and for Ruby on Rails. My main apps still are powered by it. But monopolies are never good. Hanami is the one I get most excited about.

It is why I extracted the core features (pulling SVGs from remote repository, transforming filenames, etc.) of Rails Icons into a Ruby gem, called Icons, so other frameworks can have all the icons in their apps as well.

Hanami Icons is the next wrapper of the Icons gem for… drumroll… Hanami! It gives you the same interface as Rails Icons too, whether that is Heroicons, Lucide, Tabler or even a folder of your own SVGs.

<%== icon "check", class: "text-gray-500" %>

<%== icon "apple", library: "simple_icons", class: "text-black" %>

if you have seen Rails icons, you know Hanami Icons.

One interface, all the libraries

The work is done in Icons. If you do not know it: it syncs SVGs from their source repositories, normalizes names, handles variants and builds sprite markup. Hanami Icons just adds the Hanami-specific parts: a provider, view helpers and CLI generators.

Hanami Icons supports twelve first-party libraries: from good-old Feather to Heroicons to Lucide and Phosphor. That ranges from 49 icons in SidekickIcons to more than 9,000 in Phosphor. You can also add your own library (more on that below).

Install and generate

Hanami loads CLI extensions from the :cli Bundler group, while the view helpers need the gem at runtime. Add it to both:

gem "hanami_icons", group: [:default, :cli]

Then run:

bundle install
hanami generate icons --library=lucide

The generator writes config/providers/icons.rb, registers the icons provider, adds the helpers to app/views/helpers.rb and syncs the library. You can pick several libraries or only the variants you need:

hanami generate icons --libraries=heroicons lucide
hanami generate icons --library=heroicons --variants solid outline

The Hanami configuration can be used to set a default library and variant there:

HanamiIcons.configure do |config|
  config.default_library = "heroicons"
  config.default_variant = "outline"
end

Icons and sprites

The main helper takes the icon name and any attributes you want on the SVG:

icon "check"
icon "check", variant: "solid"
icon "check", library: "heroicons"
icon "check", class: "icon icon-small"
icon "check", aria: { label: "Save" }
icon "check", stroke_width: 2

A page with many icons repeats the same paths over and over. Sprites fix that. Generate one:

hanami generate icons:sprite

Then reference symbols instead of inlining every path:

<%== sprite_icon "check" %>
<%== sprite_icon "search", class: "icon icon-small fill-blue-500" %>

Point config.default_sprite_location at the generated file or at a CDN. Set it to nil to inline the sprite in your layout with icons_sprite. Check the README for more details on sprites. They give you the best of customisability and rendering optimisation.

Custom libraries and syncing

Any directory under app/assets/svg/icons is a “library”. Add a SVG and use it:

app/assets/svg/icons/simple_icons/apple.svg


icon "apple", library: "simple_icons"

Libraries with variants use a variant directory, like heroicons/outline/check.svg. If you want a starting point, hanami generate icons --library=simple_icons creates the folder and adds config.custom_library :simple_icons to the provider.

You can also sync a library from a git repository:

HanamiIcons.configure do |config|
  config.custom_library :my_icons, source: {
    url: "https://github.com/user/icons.git",
    variants: { default: "." }
  }
end

The icons:sync command runs before the app boots, so it does not load providers. It only sees the first-party libraries. Sync a custom library inside the app context instead:

bundle exec hanami run 'Icons::Sync.new(:my_icons).now'

For first-party libraries, syncing is a single command:

hanami generate icons:sync
hanami generate icons:sync --library=heroicons
hanami generate icons:sync --libraries=heroicons lucide

Run it whenever an upstream library adds icons you want. No bundle update hanami_icons needed.

Hanami Icons is at 0.6.0. It is still early. The generators and helpers have tests, but I want to hear from people using it in real apps before calling it 1.0. If something feels awkward, open an issue or send a pull request.

If Hanami Icons saves you an afternoon, a star helps other Hanami developers find it. ⭐

Distort HTML with SVG's feDisplacementMap

Rails Designer — Fri, 23 Oct 2026 07:30:00 +0000

Look at those text effects: wow would you build them?

My first answer, somewhere in the 2000s, would have been Flash. A timeline, some easing curves, spooky rippling text for every band MySpace page. 🤘 After Flash came many other options: jQuery plugins, canvas pixel picking, shaders in a giant WebGL library for one ripple of text.

Luckily today all you need fits in two SVG primitives and one line of CSS.

For a while now, you can apply SVG filters to regular HTML:

filter: url(#my-filter)

The browser then renders your element through that filter. And SVG filters can do wild things. The wildest of all: feDisplacementMap.

How `feDisplacementMap` works

Here’s how feDisplacementMap works. It takes two images. The first one is your content (SourceGraphic, in filter-speak). The second one is a map. For every pixel of your content, the filter reads the color at that position in the map, converts one of its channels to a number between 0 and 1, and shifts the pixel sideways by:

scale * (channelValue - 0.5)

So a red channel value of 0.5 (neutral gray) means “don’t move this pixel”. A value of 1.0 pushes it scale pixels to the right (if you used Photoshop’s distort effect this might be familiar). A value of 0 pulls it back the other way.

This means the real work goes into the map, and you can build that map from anything. The workhorse for organic distortion is feTurbulence: it generates Perlin-style noise, a soft cloud of grays and colors, and that becomes your map. Chain the two primitives and you get distortion for free.

Your first distortion

Enough theory. This is the full setup (and below, real examples!):

<svg width="0" height="0" style="position: absolute">
  <defs>
    <filter id="fd-wavy">
      <feTurbulence type="turbulence" baseFrequency="0.02" numOctaves="2" result="map"/>

      <feDisplacementMap in="SourceGraphic" in2="map" scale="20"/>
    </filter>
  </defs>
</svg>

Look at that! No JavaScript, no canvas, but real text you can select. 🤯 The result="map" names the turbulence output, and in2="map" pipes it into the displacement (you’ll also see xChannelSelector and yChannelSelector on feDisplacementMap; more on those in a second).

Turn the dials

Two attributes control almost everything: baseFrequency on the turbulence and scale on the displacement.

baseFrequency picks the size of the noise blobs. Low values give you big, slow waves. High values give you fine, nervous grain:

scale picks how far pixels travel. Keep it small for a subtle wobble, crank it up and your text melts:

A few more dials worth knowing:

numOctaves layers noise on top of noise. More octaves, more detail.
seed produces a different noise pattern. Great when you want variants that look related (but differ).
xChannelSelector / yChannelSelector set which color channel drives horizontal and vertical shifts. The defaults are R and G. Point yChannelSelector at A and the vertical shift follows the noise’s alpha channel instead. This is how you design maps where X and Y move independently.

Move it

A static distortion is nice, but you can use SVG’s own <animate> to, for example, wobble the turbulence’s baseFrequency over time:

<feTurbulence type="turbulence" baseFrequency="0.02" numOctaves="2" result="map">
  <animate attributeName="baseFrequency"
           dur="6s" repeatCount="indefinite"
           values="0.02;0.04;0.02"/>
</feTurbulence>

Animating scale instead gives you a pulse. Animate both baseFrequency and scale and you get heat shimmer off a summer road.

Any image can be a map

My favorite part? The map is just an image. So swap feTurbulence for feImage and feed in any picture. A smooth radial gradient, for example, gives you a lens. One catch: feImage inside filters applied to plain HTML behaves unreliably across browsers, so I render the word inside an inline <svg> instead. Works everywhere:

<svg width="240" height="100">
  <defs>
    <filter id="fd-lens" filterUnits="userSpaceOnUse" x="0" y="0" width="240" height="100">
      <feImage href="data:image/svg+xml,…" x="0" y="0" width="240" height="100" result="m"/>
      <feDisplacementMap in="SourceGraphic" in2="m" scale="40"/>
    </filter>
  </defs>
  <text x="120" y="64" text-anchor="middle" filter="url(#fd-lens)">lens</text>
</svg>

(Grayscale in the middle, white at the edges: pixels near the center stay put, pixels near the edge get pushed outward.)

And once you accept “the map is an image”, you are only limited to your imagination. Feed in a photo, a QR-looking pattern (instant glitch) or a feColorMatrix on top of turbulence. The displacement filter does exactly what the map tells it to. Design the map, design the distortion.

Try it yourself

I’ve wrapped these ideas in a tiny custom element, <displacement-svg>, with seven presets (wavy, glitch, water, heat, lens, liquid, sketch) and attributes to override every dial. As always, the code can be found on GitHub.

Combine feDisplacementMap with feBlend for water-on-glass (to get that iOS’ liquid glass effect 💡). The MDN docs for feDisplacementMap are another great resource.

A reusable tooltip built with a custom element

Rails Designer — Fri, 16 Oct 2026 07:30:00 +0000

Custom elements are amazing. They’re a built-in browser feature, not some library you npm install and then hope it will be maintained. You define a tag once and it just works, anywhere and in any framework (and browser). I have talked about them many times before and think they are a great fit for server-rendered apps like Hotwire is (I even added an extension to Attractive.js to make them even awesomer).

I will use them whenever I need a small, reusable UI behavior. The latest one I used (in my main product) is a tooltip and it’s a good example of why this approach is so clean. 🧼

The usage will show you my point:

<tool-tip label="Search">
  <%= icon "magnifying-glass", aria: {label: "View search"} %>
</tool-tip>

You just wrap any element with tool-tip. That’s it. The label becomes the tooltip text. No Stimulus controller to wire up, no data attributes to memorize or no data-controller noise. Just a tag around the thing you already had.

An important detail (and it actually deserves a separate post!): the element adds display: contents to itself. Why does that matter? Because display: contents removes the custom element from the box tree while keeping its child elements in the normal DOM flow. The extra wrapper will not break your layout, your flexbox or your grid. The <svg> behaves exactly as if the tool-tip weren’t there.

Under the hood, the element leans on the Popover API for the tooltip box. It creates a div, sets popover="hint" and positions it with getBoundingClientRect. The positioning logic is small: it finds the trigger’s rect, centers vertically, places the tooltip to the right and flips left if there’s no room:

let left = triggerRect.right + VIEWPORT_MARGIN
if (left + tooltipRect.width > window.innerWidth - VIEWPORT_MARGIN) {
  left = triggerRect.left - VIEWPORT_MARGIN - tooltipRect.width
}

It also degrades gracefully. If the browser doesn’t support popover, it falls back to the native title attribute.

Now, CSS Anchor Positioning would massively simplify this element. The complete #position method, the scroll listeners, the resize recalculation: it could all collapse into a few lines of CSS if the browser support were there. I skipped it on purpose. Anchor positioning is still too thin on support to rely on (~64% of the level 2 API as of this writing), so the JS stays as-is. When coverage improves, that positioning block is the first thing I’ll delete.

As always, the code can be found on GitHub. It’s a tiny file, and reading it end to end takes less time than this paragraph (okay, maybe not this one, but certainly the previous one…).

Next time you reach for a tooltip gem or package, you could use this custom element instead. Wrap, label, done. 🏁 And keep an eye on anchor positioning, because the day it becomes “baseline wide available” I am going to replace a lot of code. 🧹

Build an accessible paused-GIF using an Custom Element

Rails Designer — Fri, 09 Oct 2026 13:30:00 +0000

A looping GIF is a practical running joke in accessibility (a11y). It moves whether you want it to or not, you can’t stop it, and screen readers have nothing to grab onto. I built a small custom element that fixes all three at once.

The whole thing is paused-gif. Drop it in your markup and it renders a still frame with a button to play the animation:

<paused-gif src="/celebration.gif" alt="A confetti cannon going off at a launch party"></paused-gif>

That’s it. Let me walk through why each accessibility decision in there matters, because that’s the interesting bit.

Pause by default

The “poster frame” (the initial image) is captured the moment the element mounts, so nothing animates until someone clicks “play”. This is the most important a11y win.

Why does it matter? Animated content triggers vertigo and balance disorders for a real chunk of people. WCAG 2.3.3 says anything that moves for more than five seconds needs a pause control. By showing a still image first, I’m compliant before the user even clicks. I also grab the first frame onto a canvas so there’s no flash of a half-loaded GIF:

canvas.getContext("2d").drawImage(frame, 0, 0);

this.#posterUrl = canvas.toDataURL("image/png");

If the canvas read fails (cross-origin issue, mostly), I fall back to the source URL. The poster still shows, the animation stays paused.

A real button, not a div with a click handler (yes, I have seen this)

The toggle is a <button type="button">. This matters more than it looks:

this.#button = document.createElement("button");
this.#button.type = "button";

this.#button.addEventListener("click", () => this.toggle());

Setting type="button" stops it from accidentally submitting a form it happens to live inside. Because it’s a native button, keyboard users get Tab, Enter and Space for free. If you use any other element (e.g. <div>), you’ve just created a problem that already has a solution.

State that screen readers can hear

A play/pause button is a toggle, so it needs aria-pressed. The render method keeps it in sync with the actual state:

this.#button.setAttribute("aria-pressed", String(this.#playing));
this.#button.setAttribute("aria-label", `${this.#playing ? "Pause" : "Play"} animation: ${this.alt}`);

this.#button.textContent = this.#playing ? "Pause" : "Play";

Two things are happening. aria-pressed tells assistive tech this is a toggle and whether it’s on. The dynamic aria-label does something cool: it bakes the image’s alt text right into the control. A screen reader user hears “Play animation: a confetti cannon going off at a launch party” instead of a bare “Play”. The visible text says “Play” or “Pause” for sighted users, while the label carries the context.

And the alt isn’t an afterthought. It flows straight through to the underlying <img>:

this.#img.alt = this.alt;

If you forget the alt attribute, it becomes an empty string, which is the correct signal that the image is decorative (the button label already describes it).

decoding = "async" on the image keeps the poster from blocking the main thread. And because src, poster, alt and button-class are all observed attributes, the element re-renders correctly when any of them change after mount.

I keep coming back to web components/elements: the platform already solved accessibility for buttons, images and toggles. Build the control out of native elements, mirror state into ARIA and pause the motion by default. Everything else is detail.

As always, the code can be found on GitHub. If you ship it somewhere, I’d love to see how you theme the button. ❤️

More than click and submit: use any event with Stimulus

Rails Designer — Thu, 01 Oct 2026 17:30:00 +0000

I recently surprised someone by showing you can use any event besides the common click, submit and keydown events. The Stimulus docs list those seven default events and their shorthand equivalents. Enough for most cases. But the data-action descriptor is event->controller#method where you can bind to anything the DOM throws at you: native element events, global document events, even custom dispatched events. All work the same way.

I put together a small demo repo to show a handful of these in action. Lets go over them and maybe you find a way to refactor and simplify one of your Stimulus controllers.

First a few you probably have used before. A <textarea> fires input on every keystroke. The character-counter controller subtracts the current length from maxLength and updates a live counter.

<textarea data-controller="character-counter" data-action="input->character-counter#update" maxlength="200">

Note the difference. input fires on every change. change fires only on blur. For real-time feedback, input is what you want.

A range slider works the same way. The range-display controller grabs event.target.value and writes it into an <output> element. As the user drags, the displayed number updates instantly.

<input type="range" data-action="input->range-display#update">
<output data-range-display-target="value">42</output>

A checkbox fires change when its checked state toggles. The checkbox-reveal controller inspects event.target.checked and toggles a hidden class on a content section.

<input type="checkbox" data-action="change->checkbox-reveal#toggle">
<div data-checkbox-reveal-target="content" class="hidden">…</div>

A <details> element fires toggle every time it opens or closes. The accordion controller uses this to persist state to localStorage.

<details data-controller="accordion" data-action="toggle->accordion#persist">

The controller reads event.target.open to know whether the element was opened or closed and saves a boolean. On connect, it restores the saved state. Native event, no wrapper library.

A code block fires copy when the user selects text and presses Cmd+C. The clipboard controller shows a “Copied!” tooltip for 1.5 seconds, then hides it.

<pre data-controller="clipboard" data-action="copy->clipboard#showTooltip">
  <code>$ rails g controller pages show</code>

  <span data-clipboard-target="tooltip" class="clipboard-tooltip">Copied!</span>
</pre>

The copy event fires before the clipboard is populated, so there is time to show feedback.

A <dialog> fires two distinct events. close fires when the dialog closes via a form submission or button click. cancel fires when the user dismisses it with the Esc key. The modal controller tracks both to display the correct status.

<dialog data-action="close->modal#close cancel->modal#cancel" data-modal-target="dialog">

When Esc is pressed, cancel fires first. The controller sets a flag so close (which fires right after) can distinguish between “closed via button” and “cancelled via Esc”.

A form fires submit as a default event (similar to the Turbo events for form submissions). The form controller shows the :prevent action option to stop the browser from reloading.

<form data-controller="form" data-action="submit->form#validating:prevent">

Stimulus uses @ to scope the event listener to a different element. The full descriptor is event@target->controller#method. Leave off @target and it listens on the controller’s element. Set it to @document or @window and it listens globally.

<section data-controller="visibility" data-action="visibilitychange@document->visibility#toggle">

The visibility controller toggles document.title depending on whether the tab is hidden or visible. The @document suffix is the only way to make this work since visibilitychange fires on the document, not on the controller’s element.

This is also how you hook into Turbo events:

<turbo-frame id="comments" data-action="turbo:frame-render@document->comments#highlight">

And how you listen to custom dispatched events from anywhere. A Stimulus controller, a Turbo Stream response, a WebSocket handler. Call dispatchEvent and bind with the exact same syntax.

element.dispatchEvent(new CustomEvent("cart:updated", { detail: { count: 5 } }))


<span data-controller="cart" data-action="cart:updated@document->cart#updateBadge"></span>

Custom events are just DOM events with a custom name. Stimulus treats them identically to native ones.

One more thing. You can go further with custom action options. I wrote about them here. They let you gate whether an action fires based on custom logic. For example, you could register an open option so the accordion’s persist action only runs when the <details> element opens, not when it closes. All declarative in the HTML, no conditionals in the controller.

The seven default events are convenient, but every DOM event works the same way in Stimulus. Knowing this allows you to write more advanced controllers, simpler. 😊

Lazy loading, optimistic UI and more in Rails with Attractive.js

Rails Designer — Thu, 24 Sep 2026 14:30:00 +0000

This is the last article of a series where I introduce Attractive.js 1.0.0 (in pre-release now).

In this last article I want to highlight how Attractive can be a nice addition and in some cases a complete replacement for Hotwire.

I created a simple message board in Rails. No Stimulus anywhere, nor Turbo Streams. Check it out!

Most of the features of the is build one Attractive extension I intentionaly left out last week; Attract. It serves two purposes:

intercept form request and send fetch instead (we know that technique!), and:
work with attract JSON responses to render HTML based off of template elements

One partial, three uses

The whole thing uses one single partial for a message:

<%# locals: (message:) %>
<li class="message" id="message-<%= message.id %>">
  <small class="author" attract-field="author"><%= message.author %></small>

  <p class="body" attract-field="body"><%= message.body %></p>
</li>

This one partial renders the initial page, drives the optimistic UI and powers the lazy load. Say what?! 🤯 The attract-field attributes tell Attractive’s Attract where to slot data when it renders the template.

Client and server validation

The message body validates on the client with the native Constraint Validation API, via the @validate extensions (mentioned last week). Just required and minlength attributes, plus one attribute on the form:

<form @validate @attract …>
  <input id="author" name="author" type="text" placeholder="Your name">

  <textarea name="body" required minlength="3"></textarea>
</form>

The name field, by contrast, is deliberately left without a client rule. Its validation comes from the server. When Rails rejects it, the errors come back as JSON and get wired into the same native validation UI. Client and server errors render identically.

Optimistic UI

The form has @attract, so submitting it sends a fetch instead of a page reload. A cached <template> renders an optimistic preview instantly, from that same partial, while the server processes the request:

<p><strong @text="message_count">41</strong> messages</p>

<form
  id="new-message"
  @attract
  data-attract-template="message"
  data-attract-target="messages"
  data-attract-position="prepend"
></form>

<ul id="messages">
  <%= render @messages %>
</ul>

<template id="message">
  <%= render "messages/message", message: Message.new %>
</template>

In the example app, the create request sleeps for a second so you can actually see the optimistic message appear before the real response. When it comes back, it holds the actions:

{
  "actions": [
    { "action": "setStore#message_count=42" },
    { "action": "reset", "target": "new-message" }
  ]
}

Multiple actions in one response once the message is saved: set the message count using Reactive’s store and a (bundled) action to reset the form.

Attract brings 6 more actions to the table. They almost all clone a cached template (like #message above):

append to append to target
prepend to prepend to target
replace to replace target element
before to insert before target
after to clone template, insert after target

And the odd one:

remove to remove target element(s)

The nice thing is, you can use these actions solo without attract as well, like I do for my flash (toast) messages:

<%# locals: (message:, icon: :check, icon_css: …, css: …) %>
<li @action="remove:mounted" data-delay="7500" class="…">
  <%= tag.p safe_join([icon(icon, class: icon_css), message]), aria: {live: :polite}, class: css %>
</li>

The message gets removed from the DOM after a delay of 7500ms.

Lazy loading messages

The message list loads five at a time. At the bottom, a simple form GET request that is triggered when in view (using the bundled whenInView trigger):

<form
  method="get"
  action="/messages?offset=5"
  @action="submit:whenInView"
  @attract
></form>

The server replies with a sequence of actions: append the next batch, when there are more, swap in a fresh “lazy load action” and point it at the next offset or remove it when the list runs out.

render json: {
  actions: [
    { action: "append", target: "messages", template: "message", data: batch },
    { action: "replace", target: "load-more", template: "load-more", data: {} },
    { action: "setAttribute#action=/messages?offset=10", target: "load-more" }
  ]
}

Three actions, one request.

Keyboard shortcuts

The Attractive Element scopes the filter and the keyboard extension wires the shortcuts. @hotkey.meta+k focuses the filter, @keydown.escape clears it, @keydown.meta+enter submits the form.

And that is how Attractive can work alongside Hotwire or replace it; depending on your needs. I am quite excited how this all turned out. It has been multiple months in the making (excluding the time noodling it over in my brain).

I would love for you to give it a try, just the bundled actions for your (Perron-powered 💡) site, along-side Hotwire or even replacing it. Let met know how it goes.

Things I am thinking about adding is support for SSE so you can “broadcast” actions to many and also an attractivejs-rails gem (to remove some of the ceremony you see in the Rails demo app).

Get started with the quickstart and give the repo a star or fix a bug you find! ✨😊

Custom Actions in Attractive.js: one interface, from small to big

Rails Designer — Thu, 17 Sep 2026 14:30:00 +0000

In last week’s announcement, I promised that when the built-in actions aren’t enough, you can write your own. Let me show you how, because this is what was bugging me most: why pull in another library when Attractive.js can do it as well?

Next to that I will also highlight some extra goodies and the bundled extensions that can now be (optionally) added.

Start with a one-liner

The quickest custom action is the js: prefix. It evaluates a JavaScript expression on the element, so you can do things too small for a full action:

<button @click="js:this.textContent=1">0 (I will become 1)</button>

Use it for one-liners only. It goes through the full action pipeline (so all events works as well as data-debounce and data-delay). It runs new Function() under the hood (which needs unsafe-eval in your CSP).

A custom action as a function

The step up from js: is a plain function. Register it with addActions on activate():

import Attractive from "attractivejs";

const sharePage = (element, { dataset }) => {
  const title = dataset.shareTitle || document.title;
  const url = dataset.shareUrl || window.location.href;

  if (navigator.share) {
    navigator.share({ title, url });
  } else {
    // write to clipboard or however you want to handle this
  }
};

Attractive.activate({
  addActions: { sharePage }
});


<button @action="sharePage" data-share-title="Check this out">Share</button>

The function receives the element and a context object with value, target, targets, event and dataset. In a Rails app you’d put this in app/javascript/application.js and you’re done. The markup stays identical to a built-in action.

A custom action as a class

When an action grows beyond a reasonable size, it becomes a class. Any class with a run() method works:

class Reorder {
  run() {
    this.element; // the element with @action
    this.options; // { value, target, targets, event, dataset, … }
  }
}

Attractive.activate({
  addActions: { reorder: Reorder }
});

There’s an optional Action base class that gives you value, dataset and a dispatchEvent helper:

import { Action } from "attractivejs";

export default class Reorder extends Action {
  run() {
    this.value; // the string after # in @action="reorder#…"
    this.dataset;

    this.dispatchEvent("reorder:done", { ids: [1, 2, 3] });
  }
}

Notice what didn’t change: registration is the same as the function version. You started with a one-liner, then a function, then a class and the interface stayed the same the whole way. Pretty cool, right?

Organizing actions

When you have a handful of actions, keep each in its own file and collect them in a barrel (same as how Stimulus is organized):

app/javascript/
  actions/
    reorder.js
    syntax_highlighting.js
    index.js

Each file exports a single function or class as default. The barrel collects them:

// app/javascript/actions/index.js
import syntax_highlighting from "./syntax_highlighting.js";
import reorder from "./reorder.js";

export default { syntax_highlighting, reorder };

Then your application file imports the barrel and activates:

import Attractive from "attractivejs";
import actions from "./actions/index.js";

Attractive.activate({ addActions: actions });

Hooks and error handling

Hooks let you run code around every action and handle errors in one place.

beforeAction runs before each action, return false to cancel it. afterAction runs after a success. onError handles a thrown action:

attractive.onError(({ name, element, options, event, error }) => {
  console.warn(`Action ${name} failed:`, error.message);
});

There are no unhandled rejections, the rest of a chained action still runs. There’s a global Attractive.onError fallback too, handy for wiring up a monitoring service.

The extensions

Attractive ships a few optional extensions you opt into with extendWith.

Keyboard

The keyboard extension gives you @keydown.enter, combos like @keydown.ctrl+k and global @hotkey shortcuts:

import { keyboard } from "attractivejs/keyboard";

Attractive.activate({ extendWith: [keyboard] });


<input @keydown.mod+enter="submit" @target="form">

<a href="/" aria-hidden="true" @hotkey.g.h="/">Homepage</a>

Validate

The validate extension brings client-side form validation using the browser’s native Constraint Validation API. Just add @validate to a form and style the invalid state:

import { validate } from "attractivejs/validate";

Attractive.activate({ extendWith: [validate] });


input:user-invalid {
  border-color: red;
}

Reactive

The reactive extension adds a shared JSON store with @text bindings and a setStore:

<input @input="setStore#name" />

<p>Hello, <span @text="name"></span></p>

Attractive Element

Attractive Element is the last piece I want to highlight today: a custom element base class. It scopes Attractive to a component, so actions and targets work inside it without a connectedCallback, a querySelector or manual teardown.

import { AttractiveElement } from "attractivejs/element";

class Counter extends AttractiveElement {
  connect() {
    this.count = 0;
  }

  increment() {
    this.target("count").textContent = ++this.count;
  }
}

customElements.define("ui-counter", Counter);


<ui-counter>
  <button @click="increment">+</button>

  <output id="count">0</output>
</ui-counter>

connect() and disconnect() replace the lifecycle callbacks. Methods in the HTML become actions. this.target("id") and this.targets(".selector") query within the component, so each component resolves its own targets even when the same id appears twice. It works alongside a document-wide activation without actions firing twice.

One interface for everything, from a one-liner to a function to a class to a custom element. That’s why I am so excited about Attractive: it works neatly alongside your bigger frameworks but can also stand on its own feet.

Everything you need is in the docs and the repo is always open for a star and improvement and bugfixes. ☺️

Next week, it comes all together: all of this inside a real Rails app (optimistic UI included 🤫).

Attractive.js 1.0.0: interactive HTML without one line of JavaScript

Rails Designer — Thu, 10 Sep 2026 14:30:00 +0000

I wrote a JavaScript library. Again (or rather I rewrote the JavaScript library I write some time ago).

I had the idea for Attractive for quite some time, based on the following thought: why do I reach for a framework to toggle a class, add an attribute or copy a bit of text?

So I made Attractive.js: a humble set of declarative HTML actions. You put the behavior on the element itself. No virtual DOM, no state management, no component lifecycle to learn or more of things like that. Reading the markup tells you what the page does. In that regard it is much like Stimulus, but with less ceremony/setup (and Attractive gives you actions out of the gate).

This is an overview of what 1.0.0 looks like (currently available as pre-release).

In a nutshell:

complete new syntax
extensibility (create your own custom actions)
addons like keyboard, reactive, validate and attract (to make Attractive.js a worthy competitor for Hotwire!)
A base class to write custom elements, named Attractive Element

This week I cover the basics of Attractive.js, next week I go over how to write custom actions and the week following it all comes together in a real Rails app where I show how it can replace Hotwire. But first…

What’s new in 1.0.0

The biggest change is the syntax. Actions used to live in data-action and data-target. Now they’re just @action and @target.

<button @action="toggleClass#active" @target="panel">Toggle</button>

<div id="panel">Content</div>

That’s the whole interaction: an attribute that says what happens and one that says where.

For existing users, it is just a rename:

-<button data-action="mouseenter->addClass#hovered">
+<button @mouseenter="addClass#hovered">

The event-type attributes read like sentences, they’re easier to find in a grep and the old -> bulky syntax is gone. The @ shorthand means the same as @action. And simple setups now work from a single script tag with the activate attribute, no manual Attractive.activate() call:

<script src="//unpkg.com/attractivejs" type="module" activate></script>

The actions

Everyday interactions are covered out of the box. Class and attribute actions (toggleClass, addClass, removeClass, cycleClass, the same for attributes and data attributes), style actions, copy to clipboard, confirm, dialogs, focus, form submit and reset, scroll to and more.

There’s also a js: prefix for the occasional one-liner:

<button @action="js:this.textContent=1">0</button>

Handy when a custom action is a bit too much. For anything bigger, you can write your own action (which is next week’s post).

Triggers and gates

Beyond actions, you can control when they run with triggers and gates, appended to the value with a colon.

Triggers run an action without user input. :mounted runs immediately when the element appears. :whenVisible fires when it scrolls into view:

<img
  @action="addClass#loaded:whenVisible"
  data-src="/image.jpg"
  loading="lazy"
/>

Gates guard an action. :once runs it a single time. :preventDefault calls event.preventDefault(). :whenOutside only fires when the click target is outside the element, which is how the dropdown example from below closes itself:

<button @action="toggleClass#open:once">Click once</button>

<div @action="removeClass#open:whenOutside" @target="menu">…</div>

You can register your own triggers and gates too, with addTriggers and addGates.

What you can build with zero JavaScript

To show what this means in practice, here are a few components you can write without a line of your own JavaScript.

A copy button is one attribute pointing at the target to copy an API key , code example and so on:

<button @action="copy" @target="card-example">Copy</button>

<code id="card-example">…</code>

A dialog (with a newsletter sign up) opens at a certain point within your article:

<div @action="openModal:whenInView" @target="newsletter"></div>

<dialog id="newletter">…</dialog>

Auto-save form data with a debounce:

<form action="POST" id="post">
  <textarea name="body" @action="submit" @target="post" data-debounce="400"></textarea>
</form>

Tabs and accordions are toggleAttribute against the right target. Toasts remove themselves with a :mounted trigger and a data-delay=8000. Even a carousel is just cycleClass on a custom property.

The default event per element type means a button just works with @action alone.

You can see all actions in action in the docs.

How small is it?

The full build is about ~7 KB gzipped (no external dependencies). If you’re feeling fancy, the core engine is less than 5 KB and you can import only the actions you use. That only applies to a bundler setup, though. With importmap, you pin one minified file and you’re done. Either way, it’s a fraction of what you’d drag in for a framework.

Where I’d use this

Attractive.js works great on static sites for those small bits of interactivity, but above examples also show how it works great for server-rendered apps or to replace those bulky toggle_class_controller.js 😅

The core has been solid for a while. Try it with the script tag from the quickstart and if you want to make me happy (besides giving Attractive a try), star the repo. 🌟

Next week: writing your own actions. After that: putting all of it to work in a Rails app.

Re-Organized configuration in Rails

Rails Designer — Fri, 04 Sep 2026 09:30:00 +0000

A while back I wrote about organizing configuration in Rails. The idea was simple: drop YAML files into config/configurations/ and get namespaced constants like Config::Bot.api_key instead of the clunky Rails.application.config.bot.api_key.

It worked well. But every YAML file needed manual wiring: <%= ENV.fetch("BOT_API_KEY", Rails.application.credentials.dig(:bot, :api_key)) %>. For every key. Across every file. Ugh!

So I rebuilt it. Same clean Config::Namespace.key API, but now it chains through all three sources automatically.

Before (old module’s YAML):

# config/bot.yml
shared:
  api_key: <%= ENV.fetch("BOT_API_KEY", Rails.application.credentials.dig(:bot, :api_key)) %>
  user_agent: "MyAwesomeBot/1.0"
  timeout: 10

After (new module’s YAML):

# config/bot.yml
shared:
  # Config::Bot.api_key is still available and will check environment variables and then check credentials
  user_agent: "MyAwesomeBot/1.0"
  timeout: 10

One API. Three sources. No more guessing where a value lives.

You can find the full code on GitHub. What follows are the parts I find most interesting.

Lazy namespaces with const_missing

The old version scanned a directory at boot and called const_set for every YAML file. That works, but it means every namespace is loaded whether you use it or not.

This version uses const_missing instead. Reference Config::Bot for the first time and a Namespace object is created lazily:

def self.const_missing(name)
  MUTEX.synchronize do
    @namespaces ||= {}
    @namespaces[name] ||= Namespace.new(name)
  end
end

The Mutex isn’t there by accident. In threaded environments (Puma, Solid Queue), two threads could hit const_missing simultaneously. Mutex makes sure only one namespace object gets created.

The three source chain

Each Namespace uses method_missing to resolve a key:

def method_missing(method, ...)
  key = method.to_s.delete_suffix("!")
  bang = method.to_s.end_with?("!")

  environment_key = "#{@prefix}_#{key.upcase}"
  return @environment_cache[environment_key] if @environment_cache.key?(environment_key)

  value = Rails.application.credentials.dig(@credentials_key.to_sym, key.to_sym)
  return value unless value.nil?

  value = from_yaml(key)
  return value unless value.nil?

  raise(NotFoundError, "Config::#{@name}.#{key} not found") if bang

  nil
end

ENV is checked first. It caches a snapshot at boot via ENV.to_h, so there’s no hash lookup penalty on every access. Then credentials. Then the YAML file via Rails.application.config_for. Bang methods raise instead of returning nil.

The convention for ENV keys follows the namespace: Config::Bot.timeout checks ENV["BOT_TIMEOUT"]. Config::Stripe.api_key checks ENV["STRIPE_API_KEY"]. Predictable and consistent (change to your likings though).

Bang methods and discovery

I added two small quality of life features. Config::Bot.api_key! raises if nothing is found (useful in initializers where a missing secret should fail fast). And Config::Bot.keys lists everything available for that namespace, merging credential keys and YAML keys:

def keys
  credential_keys = Rails.application.credentials.dig(@credentials_key.to_sym)

  [credential_keys, yaml_config].compact.flat_map { it.keys }.uniq
end

There’s also a reload method that clears the ENV cache and YAML cache (for when you’re changing values in the console during development).

What about Rails 8.2’s creds?

Rails 8.2 (currently on main) is introducing Rails.app.creds, which provides Rails.app.creds.require(:stripe, :api_key) and Rails.app.creds.option(:stripe, :api_key). It checks ENV first, then encrypted credentials. Same layered idea, same bang and regular pattern.

The difference is scope. Rails.app.creds focuses on secrets (things that belong in encrypted credentials or ENV). My Config module adds YAML as a third tier, which is where I keep public configuration like URLs, timeouts, API versions and feature flags.

Both solve the same fundamental problem: stop guessing where a value lives and let a unified lookup handle it. Rails.app.creds is the Rails core answer (less setup, more opinionated). My Config module is the DIY answer (flexible, more tiers, yours to tweak).

I’ve been using this version of the Config module in production for some time now. The bang methods alone have caught two misconfigured deployments before they reached customers (the app refuses to boot if a required key is missing). The triple lookup means I can move a value from YAML to credentials to ENV without touching application code. Each tier handles what it’s best at.

The code is on GitHub if you want to drop it into your own app or adapt it.

SVG sprites: improve performance (and keep configurability)

Rails Designer — Thu, 27 Aug 2026 11:30:00 +0000

A table with 200 rows and 5 action icons per row. That’s 1,000 inline SVGs dropped into your HTML. When does that start to impact your performance?

By default, Rails Icons renders each icon as a full <svg> element. Fine for a handful of icons. But at scale, it adds up.

I built a simple benchmark to measure exactly how much (feel free to fork and improve ❤️). Two pages, identical layout, 1,000 icons each. One uses inline SVGs you use and love. The other uses an SVG sprite you will surely start to love after reading this. 😬

The benchmark is on GitHub.

How SVG sprites work

A SVG sprite is a single file (but can also be embedded) with multiple icon definitions wrapped in <symbol> tags:

<svg xmlns="http://www.w3.org/2000/svg">
  <symbol id="user" viewBox="0 0 24 24">
    <!-- path -->
  </symbol>
  <symbol id="pencil" viewBox="0 0 24 24">
    <!-- path -->
  </symbol>
</svg>

To render an icon, reference it with <use>:

<svg class="icon"><use href="/sprite.svg#user"/></svg>

The browser downloads the sprite file once and caches it. Every <use> is a lightweight DOM reference, not a full SVG subtree. Compare that to inline SVGs where every single icon is a complete <svg> element with all its paths. 1,000 icons means 1,000 unique SVG subtrees parsed, styled and laid out by the browser.

Why not font icons or image tags?

Before SVGs became the standard, font icons were the go-to (if you been around for some time, you surely have used them; I thought they were really cool!). They have problems: limited to one color (unless you stack multiple fonts), inconsistent rendering across browsers and operating systems and poor accessibility. And you’re loading an entire font file for a handful of icons.

Image tags (<img src="icon.svg">) solve the accessibility issue but you lose all styling control. You can’t change the color, stroke width or size via CSS. Every variant needs a separate file. Ouch!

SVG sprites give you the best of both. The <use>-element inherits CSS from the parent <svg>, so you can control color, size and stroke width the same way you do with inline SVGs.

Fully configurable, single HTTP request, accessible by default.

The benchmark

Two pages, identical layout, 1,000 icons each:

200 users in a table, 5 icons per row
Same ERB template, same CSS, same layout
Only difference: one page uses icon, the other uses sprite_icon (both helper methods from Rails Icons)

Inline:

<%= icon "user" %>
<%= icon "pencil" %>
<%= icon "arrow-path" %>
<%= icon "lock-closed" %>
<%= icon "trash" %>

Sprite:

<%= sprite_icon "user" %>
<%= sprite_icon "pencil" %>
<%= sprite_icon "arrow-path" %>
<%= sprite_icon "lock-closed" %>
<%= sprite_icon "trash" %>

The numbers

Metric	Inline SVGs	SVG Sprite	Difference
Single icon markup	~1.7 KB	~170 bytes	90% smaller
Total icon HTML (1,000x)	~1.7 MB	~170 KB	90% smaller
Rendered page size	~1.8 MB	~250 KB	86% smaller
DOM nodes	~13,000	~7,000	46% fewer
DOM loaded	~320ms	~210ms	34% faster

The sprite page loads faster, transfers less data and creates a simpler DOM tree.

Using sprites with Rails Icons

Rails Icons 1.9+ ships with built-in sprite support. Configure which icons to include in your initializer:

# config/initializers/rails_icons.rb
RailsIcons.configure do |config|
  config.default_library = "heroicons"

  config.sprite = {
    heroicons: {
      outline: %w[user pencil arrow-path lock-closed trash]
    }
  }
end

Then use sprite_icon in your views:

<%= sprite_icon "user" %>
<%= sprite_icon "pencil" %>

That’s it. The sprite file is served at /rails_icons/sprite.svg. This means you can:

Cache it ; it changes only when you add or remove icons
Put it behind a CDN ; it’s a static asset

When to reach for sprites

Inline SVGs are fine when you have a couple dozen of icons per page. Landing pages, simple forms, blog posts. The extra bytes don’t matter.

Sprites are a good choice when icons multiply. Like with data tables, kanban boards and activity feeds.

The trade-off is one extra HTTP request for the sprite file. On an icon-heavy page, that request pays for itself many times over. 💪

SVG sprites are available in Rails Icons 1.9. Add them to your next icon-heavy view and see the difference.

Minimal JS, modern CSS anchored product tours

Rails Designer — Thu, 20 Aug 2026 18:30:00 +0000

Ever wondered how to build those interactive product tours that spotlight elements and walk users through your app? Ever wondered how you could with just roughly 100 lines of JS and modern CSS? Wonder now more, I will show and it will look like this:

Yes, just around 100 LoC 🤯

I built one using CSS Anchor Positioning and a Stimulus controller. The approach uses two techniques worth talking about: a clip-path spotlight effect (useful in all sorts of contexts) and CSS anchor positioning for the dialog (still very new as of mid-2026).

The full code, as always, is on GitHub.

The clip-path trick

The overlay covers the viewport with a semi-transparent background. But instead of sitting on top of everything, it uses clip-path: polygon() to cut a hole around the target element, creating a spotlight effect.

#clipPath(target) {
  const { top, right, bottom, left } = target.getBoundingClientRect()
  const gap = 8

  this.#overlay.style.clipPath =
    `polygon(0% 0%, 100% 0%, 100% 100%, 0% 100%, ` +
    `0% ${top - gap}px, ${left - gap}px ${top - gap}px, ` +
    `${left - gap}px ${bottom + gap}px, ${right + gap}px ${bottom + gap}px, ` +
    `${right + gap}px ${top - gap}px, 0% ${top - gap}px)`
}

The polygon traces the full viewport boundary, then cuts inward around the target’s bounding box with an 8px gap. A requestAnimationFrame loop keeps the hole positioned correctly during scroll and resize.

This technique works for any overlay-with-a-hole scenario: tooltips, walkthroughs, masking, focus modes.

Anchoring the dialog

The dialog card attaches itself to the target element using CSS Anchor Positioning. No JS position calculation, no ResizeObserver.

[data-controller="onboarding"] > .card {
  position: fixed;
  position-anchor: --onboarding-target;
  left: anchor(--onboarding-target right);
  top: anchor(--onboarding-target top);
  position-try-fallbacks: flip-inline, flip-block, flip-start;
}

The JS sets anchor-name: --onboarding-target on whichever element the current step targets. The card’s CSS declares position-anchor: --onboarding-target, and anchor() functions read the target’s position. The card places itself at the target’s right edge, top-aligned.

If there isn’t room to the right, position-try-fallbacks lets the browser try alternatives: flip to the left, flip above, flip both axes. Each fallback is tested in order until one fits the viewport. No media queries, no JS overflow detection.

As of mid-2026, Anchor Positioning ships in most browsers. Without support, the card renders without anchoring (the anchor() values fall back to auto). The tour still works, the spotlight still works, the dialog just won’t track the target’s position. That’s a reasonable degradation for an enhancement like this.

[!TIP]

Read this article for a great primer on anchor positioning

One small Stimulus controller

One Stimulus controller to get it all together. On connect it creates the overlay and card elements. Each step is a JSON object with an element selector, title, description, and optional image. Moving through steps updates the anchor-name target, repopulates the dialog content, and the requestAnimationFrame loop keeps the clip-path in sync.

It is the kind of Stimulus controller you can drop into any project, update the step data and you have a working tour. The kind of Stimulus controller I like.

The two techniques are independently useful. The clip-path polygon approach works for any spotlight or masking use case. Anchor positioning removes an entire category of JS positioning code. Combined, they make a feature that usually requires a library into about 100 lines of Stimulus.

position-try-fallbacks is a CSS property that I love to see available! ❤️ Overflow handling has always been a JS problem. Now the browser handles it natively. Look for opportunities to use it anywhere you’re positioning elements! 🛝

Choosing colors (for developers)

Rails Designer — Fri, 14 Aug 2026 07:30:00 +0000

The eleven-shade color scale from Tailwind CSS is one of those ideas I keep stealing. You know the one: 50 through 950 gives you enough stops for backgrounds, borders, hover states, text, everything. I use it even when I’m not using Tailwind.

But I almost never use the defaults (your site/app ends up looking like every other site built by LLMs).

The fix is not complicated. You don’t need a color theory degree or a five-color palette generator (you know the ones 🥹). You only need two or three color scales, and you can derive them all from a single value.

How to choose your brand hue

Rule of thumb: pick a color that fits the problem, not your personal taste (I like pink but not use it all the time).

Finance, B2B and enterprise sites use blues (hue 200-250).
Trust, stability, conservative. Health and wellness lean green or teal (120-200). Calm, natural.
Creative and entertainment go purple, pink, orange (270-360). Energy, fun.
Food and hospitality use reds, oranges, warm yellows (0-60). Appetite, warmth.

These are guidelines, not rules. A creative agency can use blue. A bank can use orange. But the defaults exist for a reason. They match user expectations.

Still stuck? Steal from a site you like. Find an app or landing page whose color feels right for your domain. Open DevTools, grab the hue from any OKLCH color on the page. That is your starting point. Adjust from there.

If you have an image (hero photo, logo, brand asset), sample a prominent color from it. Drop it into a color picker, get the OKLCH hue. Images that already look good together contain colors that already work together.

You do not need to nail it on the first try. Pick something reasonable, build the palette, see it on a real page. If it feels wrong, shift the hue by 30 and check again. Five minutes of tweaking beats an hour of analysis. 💪

Pick a neutral, name it `--gray`

First, a naming trick. Whatever neutral you settle on, alias it to --gray-*. This way your templates reference --gray-500 regardless of which tone you picked.

:root {
  --gray-50: #f8fafc;
  --gray-100: #f1f5f9;
  --gray-200: #e2e8f0;
  /* … */
}

The point is the name --gray stays the same. Want a warmer tone next month? Change the values, not every template.

If I use Tailwind, I use the same trick (but map them with the --color- prefix so bg-gray-500 works):

@theme {
  --color-gray-50: var(--gray-50);
  --color-gray-100: var(--gray-100);
}

One hue to rule them

OKLCH makes this easy. Instead of hardcoding eleven hex values for your brand, derive them from a single hue variable.

I covered the technique before in Building user-customizable themes with Tailwind CSS. The idea: OKLCH separates color into lightness, chroma and hue. Keep lightness and chroma constant per shade level, vary only the hue.

:root {
  --brand-hue: 250;

  --brand-50: oklch(.99 .01 var(--brand-hue));
  --brand-100: oklch(.98 .02 var(--brand-hue));
  --brand-200: oklch(.94 .04 var(--brand-hue));
  --brand-300: oklch(.86 .08 var(--brand-hue));
  --brand-400: oklch(.74 .14 var(--brand-hue));
  --brand-500: oklch(.6 .18 var(--brand-hue));
  --brand-600: oklch(.52 .16 var(--brand-hue));
  --brand-700: oklch(.44 .14 var(--brand-hue));
  --brand-800: oklch(.36 .12 var(--brand-hue));
  --brand-900: oklch(.26 .08 var(--brand-hue));
  --brand-950: oklch(.16 .04 var(--brand-hue));
}

Change --brand-hue from 250 to 10 and every shade updates. The palette stays harmonious because lightness and chroma never shift.

One thing to watch: OKLCH math gets you 90% of the way there. Different hues at the same lightness and chroma can feel slightly different to the eye — a purple at .6 .18 reads differently than a yellow at .6 .18. You might need to bump a stop or two for balance. That is normal. Tweak and move on.

Gray from your brand tint

Create a neutral palette the same way. Drop the chroma near zero:

:root {
  --gray-50: oklch(.99 .005 var(--brand-hue));
  --gray-100: oklch(.97 .005 var(--brand-hue));
  --gray-200: oklch(.93 .005 var(--brand-hue));
  --gray-300: oklch(.86 .008 var(--brand-hue));
  --gray-400: oklch(.76 .01 var(--brand-hue));
  --gray-500: oklch(.64 .01 var(--brand-hue));
  --gray-600: oklch(.54 .01 var(--brand-hue));
  --gray-700: oklch(.44 .01 var(--brand-hue));
  --gray-800: oklch(.34 .008 var(--brand-hue));
  --gray-900: oklch(.24 .005 var(--brand-hue));
  --gray-950: oklch(.14 .005 var(--brand-hue));
}

These read as neutral grays. But they carry a faint undertone of your brand color. The result is cohesive without being matchy-matchy.

Color theory with math

Complementary colors sit opposite each other on the color wheel. In OKLCH that is + 180 degrees.

:root {
  --accent-50: oklch(.99 .01 calc(var(--brand-hue) + 180));
  --accent-100: oklch(.98 .02 calc(var(--brand-hue) + 180));
  --accent-200: oklch(.94 .04 calc(var(--brand-hue) + 180));
  --accent-300: oklch(.86 .08 calc(var(--brand-hue) + 180));
  --accent-400: oklch(.74 .14 calc(var(--brand-hue) + 180));
  --accent-500: oklch(.6 .18 calc(var(--brand-hue) + 180));
  --accent-600: oklch(.52 .16 calc(var(--brand-hue) + 180));
  --accent-700: oklch(.44 .14 calc(var(--brand-hue) + 180));
  --accent-800: oklch(.36 .12 calc(var(--brand-hue) + 180));
  --accent-900: oklch(.26 .08 calc(var(--brand-hue) + 180));
  --accent-950: oklch(.16 .04 calc(var(--brand-hue) + 180));
}

That is the simplest color scheme: complementary. High contrast.

But you have more options. Here is how color schemes translate to OKLCH math:

Analogous are hues next to your brand (± 30). Low contrast, harmonious. Good for backgrounds and subtle variety.
Complementary have an opposite hue (+ 180). High contrast, pops. Good for CTAs and accents.
Split complementary when the two hues on either side of the complement (+ 150 and - 150). Same contrast as complementary but safer. Less jarring.
Triadic are three hues evenly spaced (+ 120 and + 240). Balanced. Gives you a third color for free.

--analogous-light: calc(var(--brand-hue) + 30);
--analogous-dark: calc(var(--brand-hue) - 30);
--split-a: calc(var(--brand-hue) + 150);
--split-b: calc(var(--brand-hue) - 150);
--triad-a: calc(var(--brand-hue) + 120);
--triad-b: calc(var(--brand-hue) + 240);

Pick one scheme and stick with it. The math guarantees the relationship. You just pick the starting hue.

All together now

:root {
  --brand-hue: 250;

  /* Gray (low chroma, brand undertone) */
  --gray-50: oklch(.99 .005 var(--brand-hue));
  --gray-100: oklch(.97 .005 var(--brand-hue));
  --gray-200: oklch(.93 .005 var(--brand-hue));
  --gray-300: oklch(.86 .008 var(--brand-hue));
  --gray-400: oklch(.76 .01 var(--brand-hue));
  --gray-500: oklch(.64 .01 var(--brand-hue));
  --gray-600: oklch(.54 .01 var(--brand-hue));
  --gray-700: oklch(.44 .01 var(--brand-hue));
  --gray-800: oklch(.34 .008 var(--brand-hue));
  --gray-900: oklch(.24 .005 var(--brand-hue));
  --gray-950: oklch(.14 .005 var(--brand-hue));

  /* Brand */
  --brand-50: oklch(.99 .01 var(--brand-hue));
  --brand-100: oklch(.98 .02 var(--brand-hue));
  --brand-200: oklch(.94 .04 var(--brand-hue));
  --brand-300: oklch(.86 .08 var(--brand-hue));
  --brand-400: oklch(.74 .14 var(--brand-hue));
  --brand-500: oklch(.6 .18 var(--brand-hue));
  --brand-600: oklch(.52 .16 var(--brand-hue));
  --brand-700: oklch(.44 .14 var(--brand-hue));
  --brand-800: oklch(.36 .12 var(--brand-hue));
  --brand-900: oklch(.26 .08 var(--brand-hue));
  --brand-950: oklch(.16 .04 var(--brand-hue));

  /* Accent (complementary) */
  --accent-50: oklch(.99 .01 calc(var(--brand-hue) + 180));
  --accent-100: oklch(.98 .02 calc(var(--brand-hue) + 180));
  --accent-200: oklch(.94 .04 calc(var(--brand-hue) + 180));
  --accent-300: oklch(.86 .08 calc(var(--brand-hue) + 180));
  --accent-400: oklch(.74 .14 calc(var(--brand-hue) + 180));
  --accent-500: oklch(.6 .18 calc(var(--brand-hue) + 180));
  --accent-600: oklch(.52 .16 calc(var(--brand-hue) + 180));
  --accent-700: oklch(.44 .14 calc(var(--brand-hue) + 180));
  --accent-800: oklch(.36 .12 calc(var(--brand-hue) + 180));
  --accent-900: oklch(.26 .08 calc(var(--brand-hue) + 180));
  --accent-950: oklch(.16 .04 calc(var(--brand-hue) + 180));
}

Use them anywhere:

.btn-primary {
  background: var(--brand-500);

  &:hover {
    background: var(--brand-600);
  }
}

.card {
  background: var(--gray-50);
  border: 1px solid var(--gray-100);
}

.badge-new {
  color: var(--accent-700);
  background: var(--accent-100);
}

Three palettes. One variable to change them all. No hex codes to manage. Winning! 🏆

Above :root{} content is easily copied from one app or site to your next one. Making sure you can return building quickly.