no-magic-number

added in: 1.6.0

warning

⚙️

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]
        allow-only-once: true