[Saifjafri212213]

Summary

nextInvoiceNumber() now counts on from a number of any length. Before, a number of more than 15 digits lost precision and the next one could come back lower than the one before.

Closes #445

Cause

nextInvoiceNumber() (lib/invoice-types.ts) read the trailing digits with parseInt(match[2], 10) and wrote String(highest.value + 1) back. That is a JavaScript number, which holds a whole number exactly only up to 2^53 (16 digits at most, and 15 to be safe). So:

• 20261031143012345 (a timestamp with milliseconds) gave 20261031143012344, one lower.
• INV-9007199254740993 gave INV-9007199254740992.
• INV-9999999999999999999999 (22 digits) gave INV-000000000000000001e+22.

The "New invoice" and "Duplicate" buttons both call it, so the number field would show one of those.

What changed

• The value is a BigInt: BigInt(match[2]) picks the highest (same rule as before, the later one wins a tie), and String(highest.value + BigInt(1)) is padded to the same width as before. The prefix and zero-padding rules from Invoice generator: the next number adds INV to plain numbers and changes the padding #220 are untouched. I wrote BigInt(1) and not 1n, since tsconfig targets ES2017, the same as lib/time-difference.ts does.
• No other code changed.

How I Tested This

• Added tests to lib/__tests__/invoice-types.test.ts first, and 7 of the new ones failed on the old code: 20261031143012345, INV-9007199254740993, INV-9007199254740992, the 22-digit INV-9999999999999999999999, a 29-digit number, the pick of the highest between numbers a JS number can't tell apart (in both orders, and with three long numbers one of which is zero-padded), and a 40-digit number of nines giving 1 followed by 40 zeros. The other new ones are guards that passed before as well: a 20-digit number whose old result happened to be right, and zero-padded long numbers with a small value (INV-0000000000000000000007, 2026-00000000000000000099).
• Mutated the code in 5 ways (comparing with parseInt, adding with a JS number, no padding, the first number always winning, the lowest winning); a test fails for each.
• npx tsc --noEmit is clean, lint is at develop's 11 warnings, and all 2,534 tests pass (the existing invoice number tests from Invoice generator: the next number adds INV to plain numbers and changes the padding #220 included).
• I ran the function in Node 22 with Vitest, not in a browser, and I did not run next build here, so CI will be the first to build it. BigInt is already used in lib/time-difference.ts, so it is in the supported browsers.

AI assistance

An AI coding assistant helped write the fix and the tests. I ran the checks listed above.