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.

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Replace Derived Variable With Query

Before	After
class Clz { get discountedTotal() { return this._discountedTotal; } set discount(aNumber) { const old = this._discount; this._discount = aNumber; this._discountedTotal += old - aNumber; } }	class Clz { get discountedTotal() { return this._baseTotal - this._discount; } set discount(aNumber) { this._discount = aNumber; } }

In exact sciences, we build new things on top of truth, there's the only way we can be sure our work is correct. Therefore, the source of the truth itself is paramount for all programs, and it's not difficult to understand why having multiple sources of truth brings additional confusion and complexity. This refactoring helps remove these multiple sources.

Working example

We have two working examples, both extracted from the book, representing the same scenario: a program that holds a production plan. The first example looks into the case where there's only one source for the derived variable, while the second looks into the case where there are multiple sources.

Single source

For this scenario, the ProductionPlan class looks like this:

export class ProductionPlan {
  constructor() {
    this._adjustments = [];
    this._production = 0;
  }

  get production() {
    return this._production;
  }

  applyAdjustment(anAdjustment) {
    this._adjustments.push(anAdjustment);
    this._production += anAdjustment.amount;
  }
}

We want to get rid of the private _production variable, transforming it into a getter that's dynamically calculated based on the adjustments.

Test suite

The test suite for the ProductionPlan class covers the edge case where no adjustments were made, as well as a regular case where an adjustment was made, making sure that production reflects this fact.

import { ProductionPlan } from './index';

describe('ProductionPlan', () => {
  it('should return product as zero if no adjustments were made', () => {
    const productionPlan = new ProductionPlan();
    expect(productionPlan.production).toBe(0);
  });

  it('should return product as 10 if an adjusment of 10 monetary units were made', () => {
    const productionPlan = new ProductionPlan();
    productionPlan.applyAdjustment({ amount: 10 });
    expect(productionPlan.production).toBe(10);
  });
});

With these tests in place, we're safe to proceed.

Steps

We don't want to break the code that's currently working, so we start by introducing a calculatedProduction getter. This getter will be used as a bridge between the current behavior and our envisioned behavior (where the derived _production field no longer exists):

+++ b/src/single-source/index.js
@@ -8,6 +8,10 @@ export class ProductionPlan {
     return this._production;
   }
+  get calculatedProduction() {
+    return this._adjustments.reduce((sum, a) => sum + a.amount, 0);
+  }
+
   applyAdjustment(anAdjustment) {
     this._adjustments.push(anAdjustment);
     this._production += anAdjustment.amount;

Then, as a sanity check, we introduce an assertion to make sure production has the same value as calculatedProduction:

+++ b/src/single-source/index.js
@@ -1,3 +1,5 @@
+import assert from 'node:assert';
+
 export class ProductionPlan {
   constructor() {
     this._adjustments = [];
@@ -5,6 +7,7 @@ export class ProductionPlan {
   }
   get production() {
+    assert(this._production === this.calculatedProduction);
     return this._production;
   }

Our tests are still passing, so we can move forward with confidence. We can now return the value of calculatedProduction at the production getter:

+++ b/src/single-source/index.js
@@ -8,7 +8,7 @@ export class ProductionPlan {
   get production() {
     assert(this._production === this.calculatedProduction);
-    return this._production;
+    return this.calculatedProduction;
   }
   get calculatedProduction() {

The assertion is no longer needed, so we remove it:

+++ b/src/single-source/index.js
@@ -1,5 +1,3 @@
-import assert from 'node:assert';
-
 export class ProductionPlan {
   constructor() {
     this._adjustments = [];
@@ -7,7 +5,6 @@ export class ProductionPlan {
   }
   get production() {
-    assert(this._production === this.calculatedProduction);
     return this.calculatedProduction;
   }

Finally, we can inline calculatedProudction into production getter:

+++ b/src/single-source/index.js
@@ -5,10 +5,6 @@ export class ProductionPlan {
   }
   get production() {
-    return this.calculatedProduction;
-  }
-
-  get calculatedProduction() {
     return this._adjustments.reduce((sum, a) => sum + a.amount, 0);
   }

And then, we can remove the _production field:

+++ b/src/single-source/index.js
@@ -1,7 +1,6 @@
 export class ProductionPlan {
   constructor() {
     this._adjustments = [];
-    this._production = 0;
   }
   get production() {
@@ -10,6 +9,5 @@ export class ProductionPlan {
   applyAdjustment(anAdjustment) {
     this._adjustments.push(anAdjustment);
-    this._production += anAdjustment.amount;
   }
 }

And that's it!

Commit history

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

Commit SHA	Message
fb1df73	introduce calculatedProduction getter
be30da4	assert production has the same value as calculatedProduction
e873ca2	return calculatedProduction at production getter
53d995a	remove assertion for production === calculatedProduction
c14966c	inline calculatedProudction into production getter
af4e704	remove _production field

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

Multiple sources

This scenario has the same ProductionPlan class but is a bit more involved since the class' constructor now also accepts an initial value for production. The class looks like this:

export class ProductionPlan {
  constructor(production) {
    this._adjustments = [];
    this._production = production;
  }

  get production() {
    return this._production;
  }

  applyAdjustment(anAdjustment) {
    this._adjustments.push(anAdjustment);
    this._production += anAdjustment.amount;
  }
}

Test suite

The test suite for ProductionPlan is fairly similar to the one in the previous example, with the only difference being a test that covers the case where we provide an initial value to production. It looks like this:

import { ProductionPlan } from './index';

describe('multiple sources', () => {
  describe('ProductionPlan', () => {
    it('should return the predefined production if no adjustments were made', () => {
      const productionPlan = new ProductionPlan(10);
      expect(productionPlan.production).toBe(10);
    });

    it('should return the pre-defined proudction plus the adjustments made', () => {
      const productionPlan = new ProductionPlan(10);
      productionPlan.applyAdjustment({ amount: 10 });
      expect(productionPlan.production).toBe(20);
    });
  });
});

With these tests in place, we can move forward.

Steps

We start by splitting the production variable:

+++ b/src/multiple-sources/index.js
@@ -1,15 +1,16 @@
 export class ProductionPlan {
   constructor(production) {
+    this._initialProduction = production;
+    this._productionAccumulator = 0;
     this._adjustments = [];
-    this._production = production;
   }
   get production() {
-    return this._production;
+    return this._initialProduction + this._productionAccumulator;
   }
   applyAdjustment(anAdjustment) {
     this._adjustments.push(anAdjustment);
-    this._production += anAdjustment.amount;
+    this._productionAccumulator += anAdjustment.amount;
   }
 }

Then we can proceed with the same approach used in the previous example. We introduce a calculatedProduction getter:

+++ b/src/multiple-sources/index.js
@@ -9,6 +9,10 @@ export class ProductionPlan {
     return this._initialProduction + this._productionAccumulator;
   }
+  get calculatedProduction() {
+    return this._adjustments.reduce((sum, a) => sum + a.amount, 0);
+  }
+
   applyAdjustment(anAdjustment) {
     this._adjustments.push(anAdjustment);
     this._productionAccumulator += anAdjustment.amount;

Then, we add an assertion to make sure productionAccumulator equals calculatedProduction:

+++ b/src/multiple-sources/index.js
@@ -1,3 +1,5 @@
+import assert from 'node:assert';
+
 export class ProductionPlan {
   constructor(production) {
     this._initialProduction = production;
@@ -6,6 +8,7 @@ export class ProductionPlan {
   }
   get production() {
+    assert(this._productionAccumulator === this.calculatedProduction);
     return this._initialProduction + this._productionAccumulator;
   }

Then we start using calculatedProduction at the production getter:

+++ b/src/multiple-sources/index.js
@@ -9,7 +9,7 @@ export class ProductionPlan {
   get production() {
     assert(this._productionAccumulator === this.calculatedProduction);
-    return this._initialProduction + this._productionAccumulator;
+    return this._initialProduction + this.calculatedProduction;
   }
   get calculatedProduction() {

And we can now safely remove the assertion:

+++ b/src/multiple-sources/index.js
@@ -1,5 +1,3 @@
-import assert from 'node:assert';
-
 export class ProductionPlan {
   constructor(production) {
     this._initialProduction = production;
@@ -8,7 +6,6 @@ export class ProductionPlan {
   }
   get production() {
-    assert(this._productionAccumulator === this.calculatedProduction);
     return this._initialProduction + this.calculatedProduction;
   }

Finally, we can inline calculatedProduction:

+++ b/src/multiple-sources/index.js
@@ -6,11 +6,7 @@ export class ProductionPlan {
   }
   get production() {
-    return this._initialProduction + this.calculatedProduction;
-  }
-
-  get calculatedProduction() {
-    return this._adjustments.reduce((sum, a) => sum + a.amount, 0);
+    return this._initialProduction + this._adjustments.reduce((sum, a) => sum + a.amount, 0);
   }
   applyAdjustment(anAdjustment) {

And that's it!

Commit history

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

Commit SHA	Message
0f0eae1	split production variable
5c2466b	introduce calculatedProduction getter
744a4a1	assert productionAccumulator equals calculatedProduction
8c5d077	return calculatedProduction at production getter
20129d6	remove assertion for calculatedProduction
a31ea87	inline calculatedProduction
4a362f1	remove _productionAccumulator field

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