Comparing
2KB immutable date time library alternative to Moment.js with the same modern API
Next generation testing framework powered by Vite
| Metric | dayjs | vitest |
|---|---|---|
| Ecosystem | npm | npm |
| Difficulty | Intermediate ✓ | Advanced |
| Weekly downloads | 43.9M | 44.5M↑ |
| Latest version | v1.11.20 | v4.1.2 |
| Recipes | 1 | 1 |
dayjs only
Shared
vitest only
Use Day.js when you need date parsing, formatting, arithmetic (add/subtract days/months), or comparisons in browser code where bundle size matters. Its plugin system covers timezones, relative time ('3 hours ago'), duration, and ISO weeks.
Use Vitest for any Vite-based project (React, Vue, Svelte) that needs unit or integration tests. It's also a good Jest replacement in non-Vite Node projects that want faster test execution and native ES module support.
For complex timezone-aware calendar applications, consider Temporal (the upcoming TC39 proposal) or date-fns with explicit timezone handling. Day.js's plugin system can grow large if you need many features.
For Next.js projects, Jest is still the recommended and better-documented choice. Vitest's browser mode (for running tests in a real browser) is still maturing — for end-to-end browser tests, use Playwright or Cypress.
dayjs
vitest