Goodeggs Money

Reliable money math with BigNumber wrapped inside cents

Usage

npm install goodeggs-money

Cents = require 'goodeggs-money'

new Cents(10).toDollars() # 0.1
new Cents(10).toNumber() # 10
new Cents(10).toString() # "$0.10"

cents = Cents.fromDollars(33.44) # Cents(3344)
cents = cents.plus(1) # safe arithmetic
cents.toDollars()  # 33.45
cents.equals new Cents(3345) # true
cents.greaterThan new Cents(481) # true
cents.lessThan new Cents(8021) # true

_.sumCents([new Cents(1), new Cents(2)]) # Cents(3)

new Cents(new Cents(4)) # Cents(4); safe to re-wrap Cents
new Cents(33.44) # will throw
new Cents(-1) # will throw
new Cents(2).minus(3) # will throw
new Cents(2).minus(3, maxZero: true) # Cents(0)
new Cents(3).times(.5) # will throw
new Cents(3).times(.5, transform: 'round') # Cents(2)
new Cents(3).times(.5, transform: 'floor') # Cents(1)

Warnings

Don't use numerical comparison functions, Cents instances are objects

Built-in numerical comparisons (<, >, ==, <=, >=) don't work properly on objects. Instead use lessThan, greaterThan, equals, lessThanOrEqual, or greaterThanOrEqual.

Why?

Alleviate floating point errors

Floating-point math in computers is notorious for causing strange rounding errors. In JavaScript, this is particularly a problem. Example:

> 0.10 + 0.20
0.30000000000000004

Obviously, when dealing with money, it won't do to have rounding errors like this. They can accumulate and eventually cause weird issues. Instead, better to use whole numbers everywhere, and do math with atomic cents instead of divisible dollars.

No negative money

This module will throw an exception if you do an operation that results in negative money. Our philosophy is to prefer always representing money with positive numbers, and use variable names to express the meaning. (What does it mean for amountOwed to be negative? Not obvious. But having two variables, amountInAccount and amountOwed, is much more meaningful and explicit.)

Make things explicit

Using a module like this, every time you deal with money you'll be manipulating a Cents object instead of just manipulating a generic Number. This means that everyone encountering your code will know what your money variable is supposed to represent.

Additionally, there are operations we might want to take on money that we want to make sure happen the same way every time. This module makes these sorts of operations explicit and simple, at the cost of a bit of extra verbosity.

Support big numbers

This module uses a BigNumber implementation, which supports arbitrarily large integers. No limits on your money!

Contributing

Please follow our Code of Conduct when contributing to this project.

$ git clone https://github.com/goodeggs/goodeggs-money && cd goodeggs-money
$ npm install
$ npm test

Module scaffold generated by generator-goodeggs-npm.