ℹ️ This repository is part of my Refactoring catalog based on Fowler's book with the same title. Please see kaiosilveira/refactoring for more details.

---

Decompose Conditional

Before	After
let charge; if (!aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd)) { charge = quantity * plan.summerRate; } else { charge = quantity * plan.regularRate + plan.regularServiceCharge; }	if (summer()) charge = summerCharge(); else charge = regularCharge();

Conditionals can get tricky fast, be it because of the length of each conditional leg, because of the rules applied to each branch, or because of the intrinsic nature of the conditions. This refactoring helps bring clarity to all these cases.

Working example

Our working example, extracted from the book, is a program that calculates the charge for a plan, applying special rules if it is summertime. The code looks like this:

export function calculateCharge(aDate, plan, quantity) {
  let charge;

  if (!aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd)) {
    charge = quantity * plan.summerRate;
  } else {
    charge = quantity * plan.regularRate + plan.regularServiceCharge;
  }

  return charge;
}

Test suite

The test suite covers the regular case and the special case:

describe('calculateCharge', () => {
  it('should calculate the charge for the summer', () => {
    const quantity = 10;
    const aDate = new ManagedDate('2021-07-01');
    const plan = {
      summerStart: new Date('2021-06-01'),
      summerEnd: new Date('2021-09-01'),
      summerRate: 1,
      regularRate: 2,
      regularServiceCharge: 3,
    };

    expect(calculateCharge(aDate, plan, quantity)).toEqual(10);
  });

  it('should calculate the charge for the other seasons', () => {
    const quantity = 10;
    const aDate = new ManagedDate(new Date('2021-01-01'));
    const plan = {
      summerStart: new Date('2021-06-01'),
      summerEnd: new Date('2021-09-01'),
      summerRate: 1,
      regularRate: 2,
      regularServiceCharge: 3,
    };

    expect(calculateCharge(aDate, plan, quantity)).toEqual(23);
  });
});

And with these tests in place, we're safe to move forward.

Steps

We start by extracting the conditions that define whether or not it's summertime into its own function:

+++ b/src/index.js
@@ -15,7 +15,11 @@export class ManagedDate {
 export function calculateCharge(aDate, plan, quantity) {
   let charge;
-  if (!aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd)) {
+  function summer() {
+    return !aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd);
+  }
+
+  if (summer()) {
     charge = quantity * plan.summerRate;
   } else {
     charge = quantity * plan.regularRate + plan.regularServiceCharge;

Then, we start extracting each branch of the conditional. We start with the then case, i.e., the charge calculation applied to summertime:

+++ b/src/index.js
@@ -19,8 +19,12 @@ export function calculateCharge(aDate, plan, quantity) {
     return !aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd);
   }
+  function summerCharge() {
+    return quantity * plan.summerRate;
+  }
+
   if (summer()) {
-    charge = quantity * plan.summerRate;
+    charge = summerCharge();
   } else {
     charge = quantity * plan.regularRate + plan.regularServiceCharge;
   }

And then we do the same for the else clause:

+++ b/src/index.js
@@ -23,10 +23,14 @@ export function calculateCharge(aDate, plan, quantity) {
     return quantity * plan.summerRate;
   }
+  function regularCharge() {
+    return quantity * plan.regularRate + plan.regularServiceCharge;
+  }
+
   if (summer()) {
     charge = summerCharge();
   } else {
-    charge = quantity * plan.regularRate + plan.regularServiceCharge;
+    charge = regularCharge();
   }
   return charge;

As a final touch, we can simplify the code by replacing the if statement with a ternary operator:

+++ b/src/index.js
@@ -13,8 +13,6 @@ export class ManagedDate {
 }
 export function calculateCharge(aDate, plan, quantity) {
-  let charge;
-
   function summer() {
     return !aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd);
   }
@@ -27,11 +25,6 @@ export function calculateCharge(aDate, plan, quantity) {
     return quantity * plan.regularRate + plan.regularServiceCharge;
   }
-  if (summer()) {
-    charge = summerCharge();
-  } else {
-    charge = regularCharge();
-  }
-
+  const charge = summer() ? summerCharge() : regularCharge();
   return charge;
 }

And that's it!

Commit history

Below there's the commit history for the steps detailed above.

Commit SHA	Message
3faf4bb	extract summer conditions to its own function
5542a9d	extract summer charge calculation to its own function
4859092	extract regular charge calculation to its own function
9e8fd5e	replace if statement with ternary operator

For the full commit history for this project, check the Commit History tab.