unicorn/numeric-separators-style

1000000 and 10000000 are hard to tell apart. 1_000_000 is easy to read, but only if the groups are all the same size. 10_00_000 is worse than no separators.

By default, decimal numbers of five digits or more are grouped by three, hexadecimal numbers by two, and binary and octal numbers by four. Groups are counted from the right.

--fix rewrites the number.

Configure

Off by default. To turn it on:

[lint.rules]
"unicorn/numeric-separators-style" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"unicorn/numeric-separators-style" = ["error", {
  onlyIfContainsSeparator = false,
  number = { minimumDigits = 5, groupLength = 3 },
  hexadecimal = { minimumDigits = 0, groupLength = 2 },
  binary = { minimumDigits = 0, groupLength = 4 },
  octal = { minimumDigits = 0, groupLength = 4 },
}]

Examples

Reported:

const population = 1000000;
const price = 12_34_567;
const color = 0xFF8800;
const flags = 0b10101010;

Not reported:

const population = 1_000_000;
const price = 1_234_567;
const color = 0xFF_88_00;
const flags = 0b1010_1010;
const year = 2026;

Options

Option	Default	What it does
onlyIfContainsSeparator	false	Only check numbers that already have a _.
number	{ "minimumDigits": 5, "groupLength": 3 }	Settings for decimal numbers.
hexadecimal	{ "minimumDigits": 0, "groupLength": 2 }	Settings for 0x numbers.
binary	{ "minimumDigits": 0, "groupLength": 4 }	Settings for 0b numbers.
octal	{ "minimumDigits": 0, "groupLength": 4 }	Settings for 0o numbers.

Each of the four kinds takes these settings:

Setting	What it does
groupLength	How many digits are in a group.
minimumDigits	Numbers with fewer digits have no separators.
onlyIfContainsSeparator	Overrides the top-level option for this kind of number.
fractionGroupLength	Only for number. How many digits are in a group after the decimal point, counted from the left. By default the fraction has no separators.

With onlyIfContainsSeparator, a number without separators is left alone, and one with separators must have them in the right places:

const population = 1000000;

const population = 10_00000;

To leave hexadecimal numbers such as colors alone unless they have a separator:

const color = 0xFF8800;

With minimumDigits, shorter numbers must not have separators:

const timeout = 30000;
const population = 1_000_000;

const timeout = 30_000;

With fractionGroupLength:

const pi = 3.14159265;

const pi = 3.141_592_65;