@zenuinebit/invoice-number

Sequential invoice numbers that restart each financial year, plus a check for the Indian GST format rule. No dependencies. Dates are "YYYY-MM-DD" strings.

import { next, checkGstFormat, financialYear } from "@zenuinebit/invoice-number";

const format = { pattern: "INV/{FY}/{SEQ:4}" };       // reset defaults to "financial-year"

// First invoice: no counter yet
let r = next(null, "2025-04-01", format);              // { number: "INV/2025-26/0001", counter: { period: "2025-26", seq: 1 } }
r = next(r.counter, "2025-09-10", format);             // "INV/2025-26/0002"
r = next(r.counter, "2026-04-01", format);             // "INV/2026-27/0001" - new financial year

checkGstFormat("INV/2025-26/0042");                    // { valid: true, problems: [] }
financialYear("2025-03-31");                           // "2024-25"

Pattern tokens: {FY} 2025-26, {FY2} 2526, {YYYY}, {YY}, {MM}, {SEQ:n} (sequence padded to n digits) and {SEQ}. Reset: "financial-year" (default), "calendar-year", "month", "never". fyStartMonth defaults to 4 (April).

Use it safely

• The package keeps no state. Store the counter in your database and call next inside the same transaction (or with a row lock) that saves the invoice. Two requests reading the same counter would otherwise issue the same number.
• A back-dated invoice is fine inside the current period but refused if it falls in an earlier one, so it cannot reuse a number.
• checkGstFormat tests Rule 46: at most 16 characters, only letters, digits, / and -. INV/2025-26/00042 is 17 characters and fails, so size your pattern for it.

Not included: credit-note or debit-note series, multiple series per branch (use one counter per series), and gap detection.