financejs

Excel-style time value of money and cash-flow formulas. This is a modern rewrite of tvm-financejs by kgkars.

Why this rewrite

This project keeps the formula behavior and conventions from the original library, but modernizes the implementation and tooling:

• Native ESM
• Named function exports (no class wrapper)
• Native error handling
• Strict type-checking
• JSDoc documentation
• Vitest-based test suite
• 100% test coverage
• ESLint + Prettier setup

Installation

npm install @travishorn/financejs

Usage

import { pmt, rate, irr } from "@travishorn/financejs";

const payment = pmt(0.0525, 5, -10000);
// 2325.733168046526

const periodicRate = rate(60, 500, -25000);
// 0.006183413161254404

const internalRate = irr([-1500, 500, 500, 500, 500]);
// 0.12589832495374934

For full API documentation, visit https://travishorn.github.io/financejs/.

Excel-style conventions

• Outputs are not rounded automatically.
• pv is typically negative for loans/investments (same convention as Excel).
• type means payment timing:
  • 0 = end of period (arrears)
  • 1 = beginning of period (advance)
• rate must match period frequency (e.g., annual rate divided by 12 for monthly periods).

Error behavior

All functions will either return a number, or throw RangeError for invalid inputs or non-convergent iterative solves.

Development

Clone the repository:

git clone https://github.com/travishorn/financejs

Change into the directory:

cd financejs

Run tests:

npm test

Lint with ESLint:

npm run lint

Check types:

npm run lint:types

Format with Prettier:

npm run format

Notes on compatibility

This rewrite is intended to match Excel-style formulas closely (tests validate to 8 decimal places), while using a modern JavaScript module API.

Roadmap

I want to add more Excel financial functions to the project. Since there are over 50 functions, broke them into "tiers." Tiers 1-3 are complete.

• Tier 1: ✓pmt, ✓pv, ✓fv, ✓npv, ✓irr, ✓rate, ✓nper, ✓xnpv, ✓xirr
• Tier 2: ✓ipmt, ✓ppmt, ✓cumipmt, ✓cumprinc, ✓sln, ✓db, ✓ddb, ✓effect, ✓nominal, ✓syd, ✓mirr
• Tier 3: ✓rri, ✓pduration, ✓vdb, ✓fvschedule, ✓dollarde, ✓dollarfr, ✓ispmt
• Tier 4: ✓yield, ✓price, duration, mduration, disc, intrate, received, pricedisc, pricemat, yielddisc, yieldmat
• Tier 5: all others

License

The MIT License

Copyright 2020 kgkars
Copyright 2026 Travis Horn

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.