No magic number

Rule id

no-magic-number

Severity

Warning

Description

Warns against using number literals outside of named constants or variables. Exceptions are made for common constants (by default: -1, 0 and 1) and for literals inside DateTime constructor as there is no way to create const DateTime and extracting each int argument to separate named constant is far too inconvenient.

Example

Bad:

double circleArea(double radius) => 3.14 * pow(radius, 2); // warns against 3.14

Good:

const pi = 3.14;
double circleArea(double radius) => pi * pow(radius, 2); // using named constant so no warning

Bad:

...
final finalPrice = cart.productCount > 4 // warns against 4
  ? cart.totalPrice - (cart.totalPrice * 0.25); // warns against 0.25
  : cart.totalPrice
...

Good:

...
const productCountThresholdForDiscount = 4;
const discount = 0.25;
final finalPrice = cart.productCount > productCountThresholdForDiscount
  ? cart.totalPrice - (cart.totalPrice * discount);
  : cart.totalPrice
...

Exception:

final someDay = DateTime(2006, 12, 1); // DateTime has no const constructor

Config example

dart_code_metrics:
  ...
  rules:
    ...
    - no-magic-number:
        allowed: [3.14, 100, 12]