Replace Magic Number with Symbolic Constant

Bare literals (numbers, strings) scattered through code are "magic" — their meaning is unclear and they must be updated in every occurrence if they change.

When to Use

• A literal value has domain meaning (e.g., 3.14159, 0.2, "ACTIVE")
• The same literal appears in more than one place
• The literal is part of a formula or business rule
• Code reviewers cannot understand the value without context

How to Apply

1. Declare a constant with a descriptive ALL_CAPS (or language-idiomatic) name
2. Replace all occurrences of the literal with the constant
3. Run tests

Example

// Before
function annualSalary(monthlySalary: number): number {
  return monthlySalary * 12;
}
function applyTax(amount: number): number {
  return amount * 1.2;
}

// After
const MONTHS_PER_YEAR = 12;
const VAT_MULTIPLIER = 1.2;

function annualSalary(monthlySalary: number): number {
  return monthlySalary * MONTHS_PER_YEAR;
}
function applyTax(amount: number): number {
  return amount * VAT_MULTIPLIER;
}

References

• refactoring.guru/replace-magic-number-with-symbolic-constant