Building Scalable UI Systems in Phoenix LiveView with Reusable HEEx Components

As Phoenix LiveView matures, more developers are using it not just for MVPs or internal tools, but for full-scale production applications with complex user interfaces. And as these apps grow, a new challenge starts to emerge - how do you maintain consistency and quality across dozens or hundreds of views, especially when multiple developers or teams are involved?

The answer lies in building a reusable UI component system. Just as design systems like Tailwind UI, Material Design, or Radix bring structure to frontend development, Phoenix LiveView gives you all the tools to create your own design language - and implement it directly in server-rendered HEEx components. It’s not just about DRY code. It’s about building a foundation where every new feature inherits quality, accessibility, and UX polish by default.

HEEx components are the building blocks of this system. They allow you to encapsulate HTML structure, Tailwind classes, interactivity, and even LiveView state into reusable units. You define them once and use them across your entire app. This gives you the power of React-style componentization - but with the simplicity and transparency of Elixir templates and server-side rendering.

Start small. A Button component is often where design systems begin. Define it as a function component using Phoenix.Component. Add support for different variants like primary, secondary, danger. Include optional icons or loading states. If you want interactivity, wire it up with LiveView events or push_patch links. Your designers care about color and hierarchy. Your developers care about structure and reuse. Your component gives both sides what they want.

From there, you’ll want to build out other base components - inputs, modals, cards, tags, avatars, dropdowns. The trick is to design with slots and assigns in mind. Slots let you define flexible content areas - like the body of a card or the label of a button - while assigns let you configure behavior and style with keyword arguments. This pattern keeps components declarative and composable, which is key to scaling.

For more complex behaviors - like tabs, accordions, or collapsible panels - reach for stateful LiveView components. These can maintain local state like open/closed toggles or selected items, respond to user events with handle_event, and re-render themselves in isolation. This is where LiveView shines. You’re building rich, interactive UI widgets - but they’re server-driven, testable, and share logic with the rest of your app.

A good design system also includes feedback components - alerts, toasts, spinners, skeleton loaders. These are often overlooked but critical for polished UX. Define standard components for them and make sure they handle edge cases like long messages, accessibility roles, and mobile layout. This is what separates a decent UI from a professional one.

Component composition is where the real leverage happens. Once you have your primitives, you can build higher-level patterns - forms with field validation, cards with call-to-action footers, paginated tables with filters. The goal is to reduce duplication and make it easy to follow established patterns. When a new developer joins your team, they should be able to build a polished UI just by combining the building blocks.

Naming and structure matter too. Organize your components under a consistent namespace - like MyAppWeb.Components. Prefix them logically. Use folders for primitives, layouts, interactive widgets, etc. Keep the API surface clean and minimal. Your components are a public interface - treat them like an API, because that’s what they are to your UI developers.

Testing components is surprisingly straightforward. You can write unit tests with Phoenix.ComponentTest, pass different assigns and slots, and assert on the rendered HTML. This gives you confidence that your UI behaves consistently - especially when refactoring. For live components, you can write integration tests using Phoenix.LiveViewTest and simulate user interactions with event pushes.

As your library grows, you’ll want documentation. Add usage examples and expected props. Inline docs are great, but you can also build a style guide or component preview dashboard using a special LiveView route. This becomes invaluable during onboarding and cross-team development. It turns your component system into a source of truth.

The beauty of this approach is that it stays entirely within the Elixir ecosystem. You’re not maintaining a separate JS frontend. You’re not syncing state between clients and servers. Everything is built in LiveView - fast, consistent, and cohesive. You get real-time interactivity, composability, and performance without sacrificing simplicity.

It’s also future-proof. As LiveView continues to evolve, with better diffing, slots, stream support, and JS interop, your component system will continue to pay off. You’re building a UI foundation that grows with the framework and with your product. You’re not locked into a frontend trend - you’re investing in infrastructure that reflects your team’s unique needs.

If you're building a Phoenix LiveView app that needs consistent design, scalable UI patterns, and developer-friendly components, I’ve created a PDF guide to help: Phoenix LiveView: The Pro’s Guide to Scalable Interfaces and UI Patterns. It’s a 20-page deep dive into component architecture, design system best practices, and reusable patterns for building maintainable, production-grade interfaces. Whether you’re just starting to modularize your UI or scaling across teams, this guide will help you do it right.