@obierisk/eslint-plugin-big-number-rules v1.9.3
Note: This has been forked from https://github.com/shuckster/eslint-plugin-big-number-rules. It disables all checks except for certain arithmetic functions to reduce false positives.
โ Enforce ๐ฐ finance-safe ๐งท calculations using bignumber.js (or something similar!) instead of native JavaScript arithmetic and Math functions.
$ pnpm i eslint-plugin-big-number-rules --save-dev
Configuration
After installation, make the plugin available to your eslint
:
// .eslintrc
{
"plugins": ["big-number-rules"]
}
Recommended rules will warn
about everything:
// .eslintrc
{
"plugins": ["big-number-rules"],
"extends": ["plugin:big-number-rules/recommended"]
}
"Everything" means this:
// .eslintrc
{
"plugins": ["big-number-rules"],
"rules": {
"big-number-rules/arithmetic": "warn",
"big-number-rules/assignment": "warn",
"big-number-rules/isNaN": "warn",
"big-number-rules/math": "warn",
"big-number-rules/number": "warn",
"big-number-rules/parseFloat": "warn",
"big-number-rules/rounding": "warn"
},
"settings": {
"big-number-rules": {
// Specify the following if you want rules to
// apply only to files with this declaration:
//
// import ... from 'bignumber.js'
//
"importDeclaration": "bignumber.js",
// Optionally, you can also apply rules only when
// importing the desired specifier from such
// declarations:
//
// import BigNumber from 'bignumber.js'
//
"importSpecifier": "BigNumber"
}
}
}
You can also customise the transformations.
Example transforms:
from | to |
---|---|
0.1 + 0.2 | BigNumber.sum(0.1, 0.2) |
19.99 * 0.1 | BigNumber(19.99).multipliedBy(0.1) |
1 < 2 | BigNumber(1).isLessThan(2) |
2 >>> 4 | BigNumber(2).shiftedBy(4) |
4 << 2 | BigNumber(4).shiftedBy(-2) |
Math.min(1, 2) | BigNumber.minimum(1, 2) |
Math.sign(-6) | BigNumber(-6).comparedTo(0) |
(1).toFixed(2) | BigNumber(1).decimalPlaces(2) |
parseFloat('1.2') | BigNumber('1.2') |
Number.parseFloat('2.1') | BigNumber('2.1') |
Can keep a chain going...
BigNumber.sum(0.1, 0.2) - 0.3
// --> BigNumber.sum(0.1, 0.2).minus(0.3)
3 ** BigNumber(1).plus(2)
// --> BigNumber(3).exponentiatedBy(BigNumber(1).plus(2))
But why?
If you use floating-points for currency (instead of whole-numbers like you probably should) libraries like bignumber.js help keep your code away from the binary floating-point pitfalls of IEEE-754:
const sum = 0.1 + 0.2
sum === 0.3
// false
sum
// 0.30000000000000004
This is the classic example and is often cited, but there are other rare corner-cases that will eventually be caught some time after committing to a currency-unsafe solution.
eslint-plugin-big-number-rules
will translate the example above to:
const sum = BigNumber.sum(0.1, 0.2)
BigNumber(sum).isEqualTo(0.3)
// true
The problem manifests in the first place because in the floating-point number-type of most languages (not just JavaScript!) the mantissa/significand is represented as a power-of-two fraction rather than a power-of-10 decimal:
_ _._____._____._____._____._____._____._____.______.______.__ _ _
_ _| 8 | 4 | 2 | 1 | 1/2 | 1/4 | 1/8 | 1/16 | 1/32 | ... etc
\__________.___________/ \______________________________ _ _ _
Exponent ------^ |
|
Significand ------>-------->----------^
IEEE-754 defines various rules for marshalling these fractions into a decimal, but as you can probably imagine it's not always exact.
Libraries like bignumber.js
helps us work around this. Using them isn't complicated, but it does require a little discipline and vigilance to keep on top of, so an eslint plugin to warn-about the use of JavaScript's native-math methods seemed like a good way to do that.
Credits
eslint-plugin-big-number-rules
was written by Conan Theobald.
He was inspired by the work of these fine Internet folk:
๐
Contributing
To support my efforts with this project, consider checking out the accountancy company I work for: Crunch.
License
MIT licensed: See LICENSE