Extract Variable

Complex or repeated expressions are hard to read. Assign them to a descriptive variable that explains intent.

When to Use

• Expression is complex or hard to understand at a glance
• Same expression is used in multiple places within a method
• A sub-expression represents a meaningful concept in the domain
• Boolean conditions with many operators

How to Apply

1. Identify the expression to extract
2. Declare an immutable variable (const/final) and assign the expression
3. Name the variable after the concept it represents, not its type
4. Replace occurrences of the expression with the variable
5. Run tests

Example

// Before
if (order.quantity > 100 && order.itemPrice > 50 && order.employee.seniorityYears > 5) {
  applyDiscount(order);
}

// After
const isLargeOrder = order.quantity > 100;
const isHighValue = order.itemPrice > 50;
const isSeniorEmployee = order.employee.seniorityYears > 5;
if (isLargeOrder && isHighValue && isSeniorEmployee) {
  applyDiscount(order);
}

References

• refactoring.guru/extract-variable