classnames vs clsx: A Deep Dive for Frontend Engineers

When building dynamic user interfaces, we often need to toggle CSS classes based on component state, props, or application context. Writing inline conditionals like ${isActive ? 'active' : ''} ${isLoading ? 'loading' : ''} quickly becomes messy and error-prone. That’s where classnames and clsx come in — both provide clean, declarative APIs to compose class strings safely. But under the hood, they differ in philosophy, scope, and performance characteristics.

🧩 Core Behavior: What Gets Included?

Both libraries accept the same basic input types:

• Strings: always included
• Objects: keys included if value is truthy
• Arrays: recursively flattened

// Shared behavior example
const isActive = true;
const isLoading = false;

// Both return "btn primary active"
classnames('btn', 'primary', { active: isActive, loading: isLoading });
clsx('btn', 'primary', { active: isActive, loading: isLoading });

However, their treatment of edge cases diverges.

Handling Falsy Values

classnames explicitly filters out all falsy values (false, null, undefined, 0, NaN, empty string):

// Returns "visible" — skips 0 and empty string
classnames('visible', 0, '', null, undefined);

clsx behaves identically here — both treat these as non-renderable and omit them.

Nested Arrays and Deep Structures

classnames fully flattens arbitrarily deep arrays:

// Returns "a b c d"
classnames(['a', ['b', ['c']], 'd']);

clsx only flattens one level deep by design:

// Returns "a b,c d" — inner array becomes "b,c" via .join(' ')
clsx(['a', ['b', 'c'], 'd']);

In practice, this rarely matters because most developers avoid deeply nested class arrays. But if your codebase uses recursive component composition that generates nested structures, classnames is more forgiving.

⚡ Performance Characteristics

clsx is optimized for the 95% case: flat inputs with strings and simple objects. It avoids recursion and uses tight loops, making it consistently faster in benchmarks involving typical React prop patterns.

classnames, while still fast, carries slight overhead from its robust flattening logic and broader type handling. This difference is negligible in most apps but can add up in:

• Large lists with frequent re-renders
• Server-rendered pages generating thousands of elements
• Animation-heavy components updating classes every frame

If you’re building a dashboard with 10k+ dynamic table rows, clsx might shave off meaningful milliseconds.

📦 Bundle Impact and Tree-Shaking

Both packages are tiny, but clsx has a simpler internal structure that plays better with modern bundlers. Its ESM build exports a single function with no side effects, enabling perfect tree-shaking even in complex dependency graphs.

classnames also supports ESM, but its historical UMD roots mean some older tooling configurations might pull in slightly more dead code. In contemporary setups (Vite, modern Webpack/Rollup), this gap has mostly closed.

💬 Developer Experience and Debugging

Both offer identical DX for standard usage. However, when things go wrong:

• classnames’s exhaustive flattening can mask bugs caused by accidentally passing nested arrays
• clsx’s stricter approach surfaces structural issues earlier (e.g., seeing "b,c" instead of "b c" reveals unintended nesting)

Consider this debugging scenario:

// Accidentally double-wrapped array
const extraClasses = [['highlight']]; 

// classnames: silently works → "btn highlight"
// clsx: exposes mistake → "btn highlight" (wait, why isn't it working?)
// Actually returns "btn highlight" in both? Let's correct:

// clsx(['btn', extraClasses]) → "btn highlight" (same as classnames)
// Correction: The real difference appears with deeper nesting:

const badInput = ['btn', [['error']]];
// classnames → "btn error"
// clsx → "btn error" (still same? Actually...)

Upon closer inspection, the practical debugging difference is minimal because both handle two-level nesting correctly. The divergence only appears at three+ levels, which is uncommon. So this point is often overstated in discussions.

🔄 Migration and Interoperability

The APIs are so similar that switching between them usually requires only a find-and-replace of the import statement:

- import classNames from 'classnames';
+ import clsx from 'clsx';

- const className = classNames('foo', { bar: true });
+ const className = clsx('foo', { bar: true });

This near-perfect compatibility means you can evaluate either without major refactoring risk.

🛠️ When to Prefer Which

Choose clsx when:

• You’re starting a new project and want the leanest possible dependency
• Your app has performance-critical rendering paths
• You follow strict conventions that avoid deeply nested class arrays
• You use modern tooling (Vite, esbuild) that benefits from its clean ESM export

Choose classnames when:

• Maintaining a legacy codebase that already uses it
• Your team values maximum compatibility with unpredictable input shapes
• You integrate third-party components that assume classnames-style deep flattening
• You prefer a library with two decades of real-world validation

🔁 Final Recommendation

For new greenfield projects, clsx is generally the better choice — it’s faster, smaller, and sufficient for virtually all real-world use cases. Its constraints encourage cleaner data structures.

For existing applications, stick with whichever you already use unless you have specific performance bottlenecks tied to class composition. The migration effort rarely justifies the marginal gains.

Most importantly: don’t overthink it. Both solve the core problem elegantly, and your team’s consistency matters more than micro-optimizations. Pick one, enforce it via lint rules, and move on to harder problems.