no-magic-numbers | Bun Docs

A bare number in the middle of an expression doesn't tell you what it stands for. If the same number is used in several places, you have to find and change each one. A named constant explains the number and keeps it in one place.

The rule doesn't report the number a variable is declared with, the values in an object literal, a number assigned to a property, the second argument of parseInt(), or numbers in JSX. It reports everything else, including 0 and 1.

Configure#

Off by default. To turn it on:

bunfig.toml

[lint.rules]
"no-magic-numbers" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"no-magic-numbers" = ["error", {
  ignore = [], # list of numbers or strings
  ignoreArrayIndexes = false,
  ignoreDefaultValues = false,
  ignoreClassFieldInitialValues = false,
  enforceConst = false,
  detectObjects = false,
  ignoreEnums = false,
  ignoreNumericLiteralTypes = false,
  ignoreReadonlyClassProperties = false,
  ignoreTypeIndexes = false,
}]

Examples#

example.js2 problems

function priceWithTax(price) {
  return price * 1.2;
}

setInterval(refresh, 60000);

example.jsNo problems

const TAX_RATE = 1.2;
const ONE_MINUTE = 60000;

function priceWithTax(price) {
  return price * TAX_RATE;
}

setInterval(refresh, ONE_MINUTE);

example.jsxNo problems

const server = { port: 8080 };
server.timeout = 5000;
const quantity = parseInt(input.value, 10);
const grid = <Grid columns={3} />;

Options#

Option	Default	What it does
`ignore`	`[]`	Numbers that are never reported. Write a `bigint` as a string: `"100n"`.
`ignoreArrayIndexes`	`false`	Allow a number as an index: `items[0]`.
`ignoreDefaultValues`	`false`	Allow a number as a default value for a parameter or in destructuring.
`ignoreClassFieldInitialValues`	`false`	Allow a number as the initial value of a class field.
`enforceConst`	`false`	Report a variable that's declared with a number and isn't a `const`.
`detectObjects`	`false`	Also report numbers in object literals and numbers assigned to a property.
`ignoreEnums`	`false`	TypeScript: allow numbers in `enum` members.
`ignoreNumericLiteralTypes`	`false`	TypeScript: allow numbers in a type alias such as `type Dice = 1 \| 2`.
`ignoreReadonlyClassProperties`	`false`	TypeScript: allow numbers in `readonly` class properties.
`ignoreTypeIndexes`	`false`	TypeScript: allow a number as the index of a type: `Row[0]`.

example.jswith { "ignore": [0, 1] }No problems

const isLast = index === items.length - 1;
const isEmpty = items.length === 0;

Each of these lines is reported by default:

example.js3 problems

const first = items[0];
function retry(times = 3) {}
class Pool { size = 10; }

example.jswith { "ignoreArrayIndexes": true, "ignoreDefaultValues": true, "ignoreClassFieldInitialValues": true }No problems

const first = items[0];
function retry(times = 3) {}
class Pool { size = 10; }

example.jswith { "enforceConst": true }1 problem

let timeout = 5000;

example.jswith { "detectObjects": true }2 problems

const server = { port: 8080 };
server.timeout = 5000;

The same goes for TypeScript. Each of these lines is reported by default:

example.ts5 problems

enum Level { Debug = 10 }
type Dice = 1 | 2;
class Pool { readonly size = 10; }
type First = Row[0];

example.tswith

{ "ignoreEnums": true, "ignoreNumericLiteralTypes": true, "ignoreReadonlyClassProperties": true, "ignoreTypeIndexes": true }

No problems

enum Level { Debug = 10 }
type Dice = 1 | 2;
class Pool { readonly size = 10; }
type First = Row[0];

When not to use it#

Code that's full of numbers with an obvious meaning, such as math, geometry or test data, gets a report on nearly every line.