# Component Library

> A collection of built, reusable interface elements — buttons, inputs, modals — that designers and engineers assemble screens from.

- Category: Deliverables & Artifacts
- Canonical: https://www.themasterly.com/glossary/component-library

A component library is a collection of built, reusable interface elements: buttons, inputs, selects, modals, tables, and the states each of them can be in. Designers assemble screens from it and engineers import it, which is what keeps a growing product coherent instead of accumulating a dozen slightly different buttons.

How those parts compose is usually organised with [atomic design](https://www.themasterly.com/glossary/atomic-design). It is the parts, not the system. A [design system](https://www.themasterly.com/glossary/design-system) wraps the same components in [tokens](https://www.themasterly.com/glossary/design-token), usage guidance and governance, and the difference shows up the first time somebody needs a variant that does not exist.

## What a component has to contain

A component is not a picture of a control. Anything short of the following list gets completed by whoever builds the screen, at the end of a sprint, differently each time.

**Every state.** Default, hover, focus, active, disabled, loading, error. Focus in particular is the one most often dropped and the one that strands anybody navigating by keyboard.

**Its variants and its limits.** Sizes, emphasis levels, and a stated position on what it should not be used for.

**Content behaviour.** What happens with a very long label, no label, an icon only, or text in a language that runs 40% longer.

**Accessible behaviour.** Roles, labels, keyboard handling, focus management. This is the hardest part to get right and the strongest argument for starting from an existing headless library rather than writing it yourself.

**Its API.** The props it accepts, what is configurable and what deliberately is not. A component that accepts arbitrary styling is a component that will be used to build something the library was meant to prevent.

## Build, adopt, or wrap

Three routes, and most successful teams take the third.

**Build from scratch.** Full control, and months of work reimplementing keyboard navigation, focus trapping and screen-reader behaviour that other people have already solved. Rarely worth it.

**Adopt a styled library outright.** Fast, and the product ends up looking like the library. Fine for internal tools, awkward for a product with any brand ambition.

**Wrap a headless library.** Take the behaviour and accessibility from an unstyled foundation, apply your own tokens and styling on top. You own the appearance, somebody else maintains the parts that are genuinely hard, and upgrades bring accessibility fixes you did not have to find.

The decision worth making explicitly is which layer you intend to own. Teams that never decide end up owning all of it by accident.

## Keeping design and code in step

The library exists in two places and drifts. This is the failure that quietly ends confidence in the whole enterprise, because once an engineer finds that the design file and the coded component disagree, they stop consulting the design file.

Three things help. Drive both from the same [tokens](https://www.themasterly.com/glossary/design-token), so at least the values cannot diverge independently. Treat the coded component as the source of truth for behaviour, since it is the one users experience. And review them together when either changes, which is a process commitment rather than a tooling one.

Visual regression testing catches the drift that reviews miss, and it is cheap relative to the cost of discovering the divergence six months later across forty screens.

## When to add a component

The bar should be higher than it usually is, because every component is a maintenance commitment.

Add one when the same pattern has appeared a third time and the three uses genuinely match. Two occurrences is a coincidence; a prediction that something will recur is usually wrong.

Do not add one for a single screen, however elaborate that screen is. A one-use component is a piece of that screen filed somewhere inconvenient.

And prefer composition to configuration. A component with eleven boolean props is several components that have been merged for the sake of appearing tidy, and nobody can tell from the outside which combinations are supported.

## Documenting it so people use it

A library nobody can navigate gets rebuilt locally, and the documentation is usually the reason.

**Show the component, not a picture of it.** A live, interactive example with its props exposed answers questions that a screenshot cannot.

**Say when not to use it.** Guidance on the wrong case prevents more misuse than guidance on the right one, and it is the section most often missing.

**Put the code next to the example.** Anything requiring a person to work out the import has already lost to writing something from scratch.

**Keep it beside the work.** Documentation in a separate site that nobody has open is documentation people remember rather than consult, and memory drifts.

## In practice

A team has a library with sixty components and a product that visibly does not use them.

Counting the imports explains it. Fourteen components account for almost all usage. Another dozen are used once each. The rest are used nowhere, having been built in a burst of work before the product needed them.

More usefully, the codebase contains nine local button variants, all created by teams who needed something the library button could not do and who found adding a variant slower than writing one.

The problem was never the components. It was that contributing to the library took two weeks and a review, while working around it took an afternoon, and people behave accordingly.

## Where teams get it wrong

- **Building ahead of need.** Components produced before anything uses them, obscuring the ones that matter.
- **Missing states.** Focus and loading in particular, which get invented during the build.
- **Reimplementing accessibility.** Months of work that a headless library has already done properly.
- **Contribution slower than working around it.** The fast path and the correct path have to be the same one.
- **Configuration sprawl.** Eleven props on one component means the abstraction was wrong.

## Related terms

- [Design System](https://www.themasterly.com/glossary/design-system)
- [Design Token](https://www.themasterly.com/glossary/design-token)
- [UI Design](https://www.themasterly.com/glossary/ui-design)

## Learn more

- [Design system service](https://www.themasterly.com/services/design-system)

## FAQ

**What is a component library?**

A collection of built, reusable interface elements with their states and behaviour, available to be assembled into screens. It usually exists twice, as components in a design tool and as code engineers import, and keeping those two in step is the whole maintenance problem.

**What is the difference between a component library and a design system?**

A component library is the parts. A design system is the parts plus the tokens underneath them, the guidance on when to use each one, and a process for changing them. A library tells you what exists; a system tells you what to use here and who decides when it changes.

**Should we build a component library or use an existing one?**

Start from an existing headless or unstyled library for behaviour and accessibility, and put your own styling on top. Building keyboard handling, focus management and screen-reader behaviour from scratch is months of work that has already been done well several times over.

**How many components should a library have?**

As few as the product genuinely reuses. Ten components used everywhere beat sixty documented and ignored. The signal to add one is the same pattern appearing a third time, not somebody predicting it might.

**How do you stop a component library going stale?**

Give it an owner with time allocated, and make contributing faster than working around it. Libraries decay because a team needs a variant, builds it locally to hit a deadline, and the next team does the same. That is a process failure rather than a technical one.

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