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no-magic-number

added in: 1.6.0
warning
⚙️
Free+

Warns when number literals are used outside of named constants or variables.

Assigning repeating literals to variables helps avoid inconsistencies when only one of the repeated values is updated.

This rule excludes common constants (by default: -1, 0 and 1) and 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.

⚙️ Config​

Set allowed (default is [-1, 0, 1]) to configure the list of allowed numbers.

Set allow-only-once (default is false) to allow magic numbers that are used one time within a file (example).

Set ignore-instances (default is false) to ignore magic numbers when creating instances (example).

analysis_options.yaml
dart_code_metrics:
  rules:
    - no-magic-number:
        allowed: [3.14, 100, 12]
        allow-only-once: false
        ignore-instances: false

Example​

❌ Bad:

// LINT: Avoid magic numbers. Try extracting them to named constants or variables.
double circleArea(double radius) => 3.14 * pow(radius, 2);

final finalPrice = cart.productCount > 4 // LINT: Avoid magic numbers. Try extracting them to named constants or variables.
  ? cart.totalPrice - (cart.totalPrice * 0.25); // LINT: Avoid magic numbers. Try extracting them to named constants or variables.
  : cart.totalPrice

✅ Good:

const pi = 3.14;
double circleArea(double radius) => pi * pow(radius, 2); // Correct, moved to a named constant

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

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

Example with "allow-only-once"​

Config
analysis_options.yaml
dart_code_metrics:
  rules:
    - no-magic-number:
        allow-only-once: true

✅ Good:

double circleArea(double radius) => 3.14 * pow(radius, 2); // Correct, used only once

Example with "ignore-instances"​

Config
analysis_options.yaml
dart_code_metrics:
  rules:
    - no-magic-number:
        ignore-instances: true

✅ Good:

MyWidget(
  width: 100,
  height: 200, // Correct, instance creations are ignored
);