NPM Ecosystem PK Battle

Moment vs Day.js: The 97% Bundle Size Difference Explained

The Classic Deprecated Titan vs The Lightweight Drop-in

Candidate Amoment→

72.1 kB gzip•MIT

Candidate Bdayjs→

2.7 kB gzip•MIT

WINNER

Direct Telemetry & Spec Comparison

Validated 2026

Metric	moment	dayjs	NPMBot Verdict
Gzipped Bundle Size	72.1 kB	2.7 kB	dayjs wins
Immutability	Mutable (Bug-prone)	Strictly Immutable	dayjs wins
Active Development	Maintenance only	Actively Maintained	dayjs wins
Tree-Shakeable Core	No	Plugin based	dayjs wins
Weekly Downloads	24M	22.5M	moment wins

NPMBot AI Deep Dive & Synthesis

Which Should You Choose in 2026?

Moment.js is officially deprecated for new projects by its maintainers. Its core flaw is mutability: calling `momentInstance.add(1, 'day')` modifies the original date, triggering subtle calculation bugs. Day.js solved this by implementing the exact same familiar method-chaining syntax on top of an immutable core that weighs only 2.7kB gzipped.

NPMBot Bottom Line:

Switch to Day.js immediately. It provides virtually identical syntax while cutting 70kB gzipped from your frontend bundle.

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Architectural Decision Matrix

Scenario: Any web or mobile application in 2026

Usedayjs

Saves ~70kB of gzipped JS, boosts Core Web Vitals, and eliminates in-place mutation bugs.

Code Comparison: Add 7 days and format string

moment Implementation

javascript

// Moment (Mutates original)
import moment from 'moment';
const d = moment('2026-01-01');
const next = d.add(7, 'days'); // d is also mutated!
console.log(next.format('YYYY-MM-DD'));

dayjs Implementation

javascript

// Day.js (Pure immutable)
import dayjs from 'dayjs';
const d = dayjs('2026-01-01');
const next = d.add(7, 'day'); // d remains untouched
console.log(next.format('YYYY-MM-DD'));

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