Day.js

Overview

Day.js is a JavaScript library designed for efficient date and time operations, offering a familiar API structure often compared to Moment.js but with a focus on minimalism and performance. It has gained traction as a preferred alternative for developers seeking to reduce their application's bundle size without sacrificing comprehensive date manipulation capabilities. The library is entirely free and open-source, promoting broad adoption across various JavaScript environments, including web browsers and Node.js applications.

With a core library size of approximately 2KB minified and gzipped, Day.js significantly reduces the overhead associated with date utilities in web projects. This small footprint is particularly beneficial for single-page applications and mobile web experiences where every kilobyte impacts load times and user experience. Its API design mirrors many of Moment.js's conventions, which lowers the learning curve for developers transitioning from that ecosystem. For instance, creating a Day.js object, formatting dates, or performing arithmetic operations like adding days or months follows a similar, intuitive pattern.

Key strengths of Day.js include its immutable design, meaning all operations return a new Day.js object rather than modifying the original. This characteristic helps prevent unintended side effects and promotes more predictable code. Furthermore, Day.js provides robust internationalization (i18n) support through locale plugins, allowing developers to format dates and times according to global standards. Its plugin system is also instrumental for tree-shaking, enabling developers to include only the specific functionalities required, such as timezone support or relative time formatting, further optimizing bundle size. This modularity makes Day.js suitable for a wide range of applications, from simple date displays to complex scheduling systems, across both front-end and server-side JavaScript stacks.

Day.js positions itself as an excellent choice for modern web applications and server-side JavaScript environments that prioritize performance and maintainability. Its architectural design, which emphasizes a small core and extensible plugins, makes it adaptable to evolving project requirements without incurring excessive dependencies. While Moment.js is no longer under active development for new features, Day.js continues to be maintained, offering a forward-looking solution for date and time handling in JavaScript. Developers looking for a fast, reliable, and lean date utility will find Day.js to be a compelling option.

Key features

• Moment.js-compatible API: Offers a familiar interface for parsing, validating, manipulating, and formatting dates, easing migration for developers transitioning from Moment.js.
• Lightweight core: The library's core is approximately 2KB minified + gzipped, contributing to smaller JavaScript bundles and faster page loads.
• Immutability: All operations return a new Day.js object, ensuring that original date objects are not modified, which enhances predictability and reduces side effects.
• Plugin system: Extends functionality without increasing the core library size. Plugins provide features such as timezone support, advanced formatting, and relative time calculations.
• Internationalization (i18n): Supports multiple locales through dedicated plugins, allowing dates and times to be displayed according to regional conventions.
• Tree-shaking friendly: The modular architecture ensures that only the necessary code is included in the final bundle, optimizing application performance.
• Browser and Node.js support: Compatible with both client-side web applications and server-side JavaScript environments.

Pricing

Day.js is an open-source project distributed under the MIT License, making it entirely free to use for both personal and commercial projects. There are no paid tiers, subscriptions, or feature limitations based on cost.

Common integrations

Day.js is a standalone JavaScript library that can be integrated into virtually any JavaScript project, regardless of the framework or environment. Its common integration points involve:

• Modern JavaScript frameworks: Easily integrated into applications built with React, Vue, Angular, Svelte, and other frameworks for handling date and time logic within components. Consult the React state management documentation for how state can hold Day.js objects.
• Node.js applications: Used in server-side environments for date parsing, validation, and manipulation, such as handling timestamps in API endpoints or database interactions.
• Build tools (Webpack, Rollup, Parcel): Its modular design and tree-shaking compatibility work well with modern bundlers to optimize the final application size. Refer to the Parcel JavaScript recipes for module bundling.
• TypeScript projects: Day.js provides TypeScript declaration files, ensuring type safety and enhanced developer experience in TypeScript-based projects.

Alternatives

• date-fns: A comprehensive, modular JavaScript date utility library that emphasizes pure functions and immutability, allowing developers to pick only the functions they need.
• Luxon: A modern, immutable, and more robust alternative to Moment.js, built with a focus on internationalization and time zone handling, by the Moment.js team.
• Moment.js: A widely used JavaScript library for parsing, validating, manipulating, and formatting dates. While popular, it is no longer under active development for new features and has a larger bundle size compared to alternatives.
• Native JavaScript Date objects: The built-in Date object offers fundamental date and time operations, though it often requires more verbose code for complex manipulations and lacks robust internationalization features.

Getting started

To begin using Day.js in your project, install it via npm or yarn. This example demonstrates how to install Day.js and use it to get the current date, add 7 days, and format the output.

# Using npm
npm install dayjs

# Using yarn
yarn add dayjs

Once installed, you can import Day.js and use its API. Here's a basic example:

import dayjs from 'dayjs';

// Get the current date and time
const now = dayjs();
console.log('Current date and time:', now.format('YYYY-MM-DD HH:mm:ss'));

// Add 7 days to the current date
const sevenDaysLater = now.add(7, 'day');
console.log('Date 7 days from now:', sevenDaysLater.format('MMMM D, YYYY'));

// Check if a date is before another date
const somePastDate = dayjs('2023-01-15');
console.log('Is now after 2023-01-15?', now.isAfter(somePastDate));

// Use a plugin for locale specific formatting (example: Spanish locale)
import 'dayjs/locale/es'; // Import Spanish locale
import advancedFormat from 'dayjs/plugin/advancedFormat'; // Import a plugin
dayjs.extend(advancedFormat); // Extend Day.js with the plugin

const formattedInSpanish = now.locale('es').format('dddd, D [de] MMMM [de] YYYY');
console.log('Current date in Spanish:', formattedInSpanish);

// Calculate difference in days
const date1 = dayjs('2024-01-01');
const date2 = dayjs('2024-01-31');
const diffInDays = date2.diff(date1, 'day');
console.log('Difference between 2024-01-01 and 2024-01-31 in days:', diffInDays);