Introduction to Day.js

Day.js is a minimalist JavaScript date library that parses, validates, manipulates, and displays dates and times for modern browsers with a largely Moment.js-compatible API. If you use Moment.js, you already know how to use Day.js.

Why Day.js?

• Tiny size: Only ~2KB gzipped (dayjs.min.js), max 2.99 KB. Compare to Moment.js at ~67KB.
• Immutable: All API operations that change the Day.js object return a new instance instead of mutating the original.
• Chainable: Methods can be chained naturally: dayjs().startOf('month').add(1, 'day').format('YYYY-MM-DD').
• Plugin architecture: Core is minimal; extend with plugins only when you need them.
• i18n: Internationalization support with locale files loaded on demand.
• TypeScript support: Type definitions included via index.d.ts.
• Moment.js-compatible API: Familiar API patterns for anyone who has used Moment.js.

Key Concepts

Immutability

Once a Day.js object is created, it cannot be changed. Any operation that modifies the date returns a new Day.js instance:

const d1 = dayjs();
const d2 = d1.add(1, 'day');
// d1 is unchanged, d2 is a new Day.js object

Chaining

Day.js methods can be chained together:

dayjs()
  .startOf('month')
  .add(7, 'day')
  .set('hour', 8)
  .format('YYYY-MM-DD HH:mm:ss');
// e.g. "2024-06-08 08:00:00"

Plugin System

The core library is intentionally minimal. Additional features are loaded via plugins:

import relativeTime from 'dayjs/plugin/relativeTime';
dayjs.extend(relativeTime);

dayjs('2020-01-01').fromNow(); // "4 years ago"

Comparison with Moment.js