date-fns vs dayjs

utility

date-fns

dayjs

Date-fns and dayjs are both modern JavaScript date utility libraries designed as lightweight alternatives to Moment.js. Date-fns takes a functional programming approach with over 200 pure functions that operate directly on native Date objects, each importable individually for optimal tree-shaking. Dayjs offers a chainable, object-oriented API that closely mimics Moment.js syntax while maintaining a tiny 2KB core footprint, extended through an optional plugin system.

This comparison matters for developers migrating from Moment.js or starting new projects where date manipulation is essential. Date-fns appeals to teams prioritizing functional programming patterns, aggressive bundle optimization, and modular architectures. Dayjs targets developers seeking familiar Moment.js-style chaining with minimal configuration, particularly those working on smaller applications or prototypes where setup speed and API familiarity outweigh granular optimization needs.

Head-to-Head

Bundle Size & Tree-Shakingtie

date-fns

Full package is 22MB unpacked, but modular design (one function per file) enables exceptional tree-shaking. Importing only what you need typically results in 6KB gzipped or less in production builds. Bundlers like Webpack and Rollup efficiently eliminate unused functions.

dayjs

Core library is just 2KB gzipped (670KB unpacked) with minimal plugins adding ~1KB each. Without aggressive tree-shaking, the base footprint is smaller, but importing multiple plugins can approach date-fns' optimized size. Less granular tree-shaking due to object-oriented structure.

API Design & Ergonomics→ dayjs wins

date-fns

Purely functional with dates passed as first arguments: `format(new Date(), 'MM/dd/yyyy')`. Immutable by design—functions never mutate inputs. No method chaining means more verbose multi-step operations. Predictable and testable but less fluent for complex transformations.

dayjs

Chainable Moment.js-compatible syntax: `dayjs().add(7, 'day').format('MM/DD/YYYY')`. Immutable objects returned at each step prevent side effects while maintaining fluent readability. Significantly shorter code for sequential operations. Easier transition for Moment.js users.

Performance Characteristics→ date-fns wins

date-fns

Operates directly on native Date objects without wrapper classes, reducing object creation overhead. Functional approach allows V8 optimizations. Benchmarks show strong performance, especially when tree-shaken functions are inlined by bundlers. No prototype chain lookups.

dayjs

Creates wrapper objects for chaining, adding minimal overhead. Generally performs comparably to date-fns in real-world scenarios. Slightly faster in some isolated benchmarks due to optimized plugin architecture. Performance difference is negligible for most applications.

Internationalization & Locales→ date-fns wins

date-fns

Supports 70+ locales through individual imports (e.g., `import { es } from 'date-fns/locale'`). Each locale adds ~2-10KB. Comprehensive formatting options with full control over locale-specific behavior. Requires explicit locale passing to functions.

dayjs

18+ locales loaded via plugins: `dayjs.locale('es')`. Global locale setting simplifies usage but reduces granularity. Smaller locale files (~1-3KB each). Adequate for common use cases but less comprehensive than date-fns for edge cases or custom formatting.

TypeScript Integrationtie

date-fns

First-class TypeScript support with types included in the package. Strong type inference for all 200+ functions. Functional signatures make types straightforward and predictable. Date parameter types enforce compile-time safety without casting.

dayjs

Full TypeScript definitions included. Plugin types require separate imports but integrate seamlessly. Chainable methods have excellent type inference. Slightly more complex types due to plugin system, but well-documented and intuitive in practice.

Ecosystem & Moment.js Migration→ dayjs wins

date-fns

Requires API translation from Moment.js—no direct compatibility layer. 200+ functions cover extensive use cases but demand learning new naming conventions. Strong adoption in modern frameworks (used in shadcn/ui, popular React libraries). Well-documented with clear migration guides.

dayjs

Explicitly designed as a Moment.js drop-in replacement with nearly identical API. Migration often requires only changing imports. Plugin system mirrors Moment.js extensions. Lower friction for teams with existing Moment.js expertise, but smaller function library than date-fns.

Verdict

Choose date-fns if you're building production applications where bundle size optimization is critical and your team embraces functional programming patterns. Its superior tree-shaking capabilities make it ideal for large-scale applications using modern bundlers, especially when you need only a subset of date operations. The functional API integrates naturally with React hooks, Redux reducers, and other FP-oriented codebases. Date-fns is the stronger choice for teams that value explicit, testable code over syntactic sugar, and when comprehensive locale support or extensive date manipulation functions are requirements.

Choose dayjs for rapid prototyping, smaller projects, or when migrating from Moment.js with minimal code changes. If your team is already familiar with Moment.js patterns, dayjs offers the fastest path to modernization without retraining. The chainable API significantly improves readability for complex date transformations, making it excellent for applications with heavy date formatting in UI layers. Dayjs excels when developer experience and minimal configuration outweigh the need for aggressive bundle optimization—particularly in projects where the 2KB base cost is acceptable and tree-shaking infrastructure isn't mature.

When to Use Each

If you are building a React component library with strict bundle size requirements → use date-fns because tree-shaking ensures consumers only pay for functions they actually import, and the functional API composes well with hooks and pure components.

If you are migrating a legacy Moment.js codebase under tight deadlines → use dayjs because the API compatibility allows near-drop-in replacement with minimal code changes, reducing migration risk and QA overhead.

If you are developing a multi-tenant SaaS platform with extensive internationalization needs → use date-fns because it provides 70+ locales with granular control over formatting, and you can import only the locales each tenant requires.

If you are building an internal dashboard or admin panel where developer productivity matters more than 5KB bundle differences → use dayjs because the chainable syntax makes complex date transformations more readable and maintainable without sacrificing immutability.

Learn More

date-fns →dayjs →

Head-to-Head

Bundle Size & Tree-Shakingtie

date-fns

dayjs

API Design & Ergonomics→ dayjs wins

date-fns

dayjs

Performance Characteristics→ date-fns wins

date-fns

dayjs

Internationalization & Locales→ date-fns wins

date-fns

dayjs

TypeScript Integrationtie

date-fns

dayjs

Ecosystem & Moment.js Migration→ dayjs wins

date-fns

dayjs

Verdict

When to Use Each