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No magic number

Configurable

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]