epiweek

Epidemiological week conversion tool for TypeScript/JavaScript, originally forked from reichlab/mmwr-week. Modernized with a new stack and additional features inspired by dralshehri/epiweeks.

Supports both MMWR (CDC) and ISO week systems.

Features

• Dual week system support: MMWR (CDC) and ISO 8601 week systems
• Date conversion: Convert between epi weeks and JavaScript Date objects
• Week arithmetic: Calculate week differences and apply week offsets
• Comparison operators: Compare epi weeks with equals, isBefore, isAfter
• Date iteration: Iterate over all dates in a week
• Modern stack: TypeScript 5.x, dual ESM/CJS, Vitest, ESLint 9
• Migration from moment.js: Replaced with date-fns for smaller bundle size
• Full test coverage: Results validated against Python dralshehri/epiweeks implementation

Installation

npm install @mu373/epiweek
# or
pnpm install @mu373/epiweek
# or
yarn add @mu373/epiweek

Usage

// ESM
import { EpiWeek } from 'epiweek'

// CommonJS
const { EpiWeek } = require('epiweek')

Basic Usage (MMWR - default)

// Create EpiWeek object
const week = new EpiWeek(2025, 48)
// EpiWeek { year: 2025, week: 48, day: 1, system: 'mmwr' }

// With day specified (legacy API)
new EpiWeek(2025, 48, 3)

// With options object
new EpiWeek(2025, 48, { day: 3 })
new EpiWeek(2025, 48, { system: 'mmwr', day: 3 })

// First day of epi year
week.startDate
// 2024-12-29

// Convert to JS Date
week.toJSDate()
// 2025-11-23

// Set from JS Date
week.fromJSDate(new Date(2025, 11, 27)) // Dec 27, 2025
// mutates week to { year: 2025, week: 52, day: 7 }

// Convert to epiweek format (yyyyww)
week.toEpiweek()
// 202552

// Set from epiweek
week.fromEpiweek(202532)
// mutates week to { year: 2025, week: 32, day: 1 }

// Number of weeks in epi year
week.nWeeks
// 53

// Week difference (this - other)
const other = new EpiWeek(2025, 3)
week.diffWeek(other)
// 29

// Apply week difference
week.applyWeekDiff(10)
// mutates week to { year: 2025, week: 42, day: 1 }

ISO Week System

// Create with ISO system
const isoWeek = new EpiWeek(2025, 10, { system: 'iso' })
const isoWeekWithDay = new EpiWeek(2025, 10, { system: 'iso', day: 5 })

// ISO weeks start on Monday (day 1) and end on Sunday (day 7)
// MMWR weeks start on Sunday (day 1) and end on Saturday (day 7)

Comparison Methods

const a = new EpiWeek(2025, 10)
const b = new EpiWeek(2025, 20)

a.equals(b)   // false
a.isBefore(b) // true
a.isAfter(b)  // false

// Cross-system comparison uses actual calendar dates
const mmwr = new EpiWeek(2024, 1, { system: 'mmwr' })
const iso = new EpiWeek(2024, 1, { system: 'iso' })
mmwr.isBefore(iso) // compares actual JS dates

Date Iteration

const week = new EpiWeek(2025, 10)

// Iterate over all 7 days
for (const date of week.iterDates()) {
  console.log(date) // JS Date objects
}

// Collect as array
const dates = [...week.iterDates()]
// For MMWR: Sun, Mon, Tue, Wed, Thu, Fri, Sat
// For ISO: Mon, Tue, Wed, Thu, Fri, Sat, Sun

MMWR vs ISO Week Systems

Aspect	MMWR (CDC)	ISO
Week starts	Sunday	Monday
Day numbering	Sun=1, Mon=2, ..., Sat=7	Mon=1, Tue=2, ..., Sun=7
Week 1 definition	First week with >=4 days in new year	Week containing Jan 4

API

EpiWeek

Constructor

new EpiWeek(year: number, week?: number, options?: EpiWeekOptions | number)

• year - Epidemiological year
• week - Week number (1-53), defaults to 1
• options - Either a number (legacy day parameter) or options object:
  • system?: 'mmwr' | 'iso' - Week system, defaults to 'mmwr'
  • day?: number - Day of week (1-7), defaults to 1

Properties

• year: number - Epi year
• week: number - Epi week
• day: number - Day of week
• system: WeekSystem - Week system ('mmwr' or 'iso', readonly)
• startDate: Date - First day of epi year (getter)
• nWeeks: number - Number of weeks in this epi year (getter)

Methods

• toJSDate(): Date - Convert to JavaScript Date
• fromJSDate(date?: Date): void - Set from JavaScript Date (defaults to now)
• toEpiweek(): number - Convert to epiweek format (yyyyww)
• fromEpiweek(epiweek: number): void - Set from epiweek
• diffWeek(other: EpiWeek): number - Get week difference (this - other)
• applyWeekDiff(delta: number): void - Add/subtract weeks
• equals(other: EpiWeek): boolean - Check equality (same system, year, week, day)
• isBefore(other: EpiWeek): boolean - Check if before another EpiWeek
• isAfter(other: EpiWeek): boolean - Check if after another EpiWeek
• iterDates(): Generator<Date> - Iterate over all dates in the week

Types

type WeekSystem = 'mmwr' | 'iso'

interface EpiWeekOptions {
  system?: WeekSystem
  day?: number
}

type Epiweek = number // format: yyyyww

License

MIT License