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.

---

Combine Functions into Transform

Before	After
function base(aReading) { /* ... */ } function taxableCharge(aReading) { /* ... */ }	function enrichReading(argReading) { const aReading = _.cloneDeep(argReading); aReading.baseCharge = base(aReading); aReading.taxableCharge = taxableCharge(aReading); return aReading; }

Sometimes we see groups of functions repeatedly operating together over a chunk of data. These functions may be independent and well-defined, but their responsibilities are tightly related to some other, bigger aspect of a piece of computation. In these cases, we can create a higher-order transform function to express this bigger aspect and keep all clients consistent.

Working example

The working example for this refactoring is a small system that calculates the taxable charge for... tea (?). It contains helper functions to calculate the baseRate and the taxThreshold, but there is a lot of duplication when it comes to calculating the base charge inside the clients of this code. Our goal is to combine these different aspects into a transform function that takes care of all the details and returns an enhanced object.

The starting code for this example contains a tax-utils module with the utility functions mentioned above (baseRate and taxThreshold), it also contains the code for the clients themselves:

function baseRate(month, year) {
  return RATES_BY_MONTH_AND_YEAR[`${month}/${year}`];
}

function taxThreshold(year) {
  return THRESHOLDS_BY_YEAR[year];
}

Clients one and two have the logic for calculating the base charge duplicated:

// client 1:
const aReading = acquireReading();
const baseCharge = baseRate(aReading.month, aReading.year) * aReading.quantity;

// client 2:
const aReading = acquireReading();
const base = baseRate(aReading.month, aReading.year) * aReading.quantity;

While client three has a method already created to do this job:

// client 3
const aReading = acquireReading();
const basicChargeAmount = calculateBaseCharge(aReading);

console.log(`basic charge amount is ${basicChargeAmount}`);

function calculateBaseCharge(aReading) {
  return baseRate(aReading.month, aReading.year) * aReading.quantity;
}

After finishing this refactoring, the code will look quite different, but the core part is the new enrichReading.

Test suite

Writing a test suite for this example was hard, mainly because this domain doesn't quite make sense but also because the baseRate and taxThreshold functions were not well explored in the book (of course because they were not the focus). So a dummy logic was put in place, using lookup objects with fixed values.

For the enrichReading function, though, we have enough tests to make sure it behaves the way we expect. The test suite looks like this:

describe('enrichReading', () => {
  const inputReading = { customer: 'Ivan', quantity: 10, month: 5, year: 2017 };

  it('should return a copy of the original object', () => {
    const resultingReading = enrichReading(inputReading);
    expect(resultingReading.customer).toEqual(inputReading.customer);
    expect(resultingReading.month).toEqual(inputReading.month);
    expect(resultingReading.year).toEqual(inputReading.year);
    expect(resultingReading.quantity).toEqual(inputReading.quantity);
  });

  it('should enrich the reading with the base charge', () => {
    const resultingReading = enrichReading(inputReading);
    expect(typeof resultingReading.baseCharge).toBe('number');
  });

  it('should enrich the reading with the taxable charge', () => {
    const resultingReading = enrichReading(inputReading);
    expect(typeof resultingReading.taxableCharge).toBe('number');
  });
});

Steps

To perform this refactoring, we first introduce the enrichReading function, which will be responsible for adding the new fields we want to the reading object. To start with, this function does nothing but create and return a clone of the original reading input object:

diff --git a/src/reading/index.js b/src/reading/index.js
@@ -1,7 +1,14 @@
+const { cloneDeep } = require('lodash');
+
 const reading = { customer: 'Ivan', quantity: 10, month: 5, year: 2017 };

 function acquireReading() {
   return reading;
 }

-module.exports = { acquireReading };
+function enrichReading(aReading) {
+  const result = cloneDeep(aReading);
+  return result;
+}
+
+module.exports = { acquireReading, enrichReading };

diff --git a/src/reading/index.test.js b/src/reading/index.test.js
@@ -0,0 +1,14 @@
+const { enrichReading } = require('.');
+
+describe('enrichReading', () => {
+  it('should return a copy of the original object', () => {
+    const inputReading = { customer: 'Ivan', quantity: 10, month: 5, year: 2017 };
+
+    const resultingReading = enrichReading(inputReading);
+
+    expect(resultingReading.customer).toEqual(inputReading.customer);
+    expect(resultingReading.month).toEqual(inputReading.month);
+    expect(resultingReading.year).toEqual(inputReading.year);
+    expect(resultingReading.quantity).toEqual(inputReading.quantity);
+  });
+});

Then, we update client3.js to use it:

diff --git a/src/client3.js b/src/client3.js
@@ -1,7 +1,8 @@
-const { acquireReading } = require('./reading');
+const { acquireReading, enrichReading } = require('./reading');
 const { baseRate } = require('./tax-utils');

-const aReading = acquireReading();
+const rawReading = acquireReading();
+const aReading = enrichReading(rawReading);
 const basicChargeAmount = calculateBaseCharge(aReading);

 console.log(`basic charge amount is ${basicChargeAmount}`);

With this in place, we can now enrich the reading with the baseCharge calculation in the enrichReading function:

diff --git a/src/reading/index.js b/src/reading/index.js
@@ -1,4 +1,5 @@
 const { cloneDeep } = require('lodash');
+const { baseRate } = require('../tax-utils');

 const reading = { customer: 'Ivan', quantity: 10, month: 5, year: 2017 };

@@ -8,7 +9,12 @@ function acquireReading() {

 function enrichReading(aReading) {
   const result = cloneDeep(aReading);
+  result.baseCharge = calculateBaseCharge(result);
   return result;
 }

+function calculateBaseCharge(aReading) {
+  return baseRate(aReading.month, aReading.year) * aReading.quantity;
+}
+
 module.exports = { acquireReading, enrichReading };

diff --git a/src/reading/index.test.js b/src/reading/index.test.js
@@ -1,14 +1,18 @@
 const { enrichReading } = require('.');

 describe('enrichReading', () => {
-  it('should return a copy of the original object', () => {
-    const inputReading = { customer: 'Ivan', quantity: 10, month: 5, year: 2017 };
+  const inputReading = { customer: 'Ivan', quantity: 10, month: 5, year: 2017 };

+  it('should return a copy of the original object', () => {
     const resultingReading = enrichReading(inputReading);
-
     expect(resultingReading.customer).toEqual(inputReading.customer);
     expect(resultingReading.month).toEqual(inputReading.month);
     expect(resultingReading.year).toEqual(inputReading.year);
     expect(resultingReading.quantity).toEqual(inputReading.quantity);
   });
+
+  it('should enrich the reading with the base charge', () => {
+    const resultingReading = enrichReading(inputReading);
+    expect(typeof resultingReading.baseCharge).toBe('number');
+  });
 });

And update client3.js to use the baseCharge field from the enriched reading object:

diff --git a/src/client3.js b/src/client3.js
@@ -3,7 +3,7 @@ const { baseRate } = require('./tax-utils');

 const rawReading = acquireReading();
 const aReading = enrichReading(rawReading);
-const basicChargeAmount = calculateBaseCharge(aReading);
+const basicChargeAmount = aReading.baseCharge;

 console.log(`basic charge amount is ${basicChargeAmount}`);

With this done, we can remove the now unused calculateBaseCharge function:

diff --git a/src/client3.js b/src/client3.js
@@ -1,12 +1,7 @@
 const { acquireReading, enrichReading } = require('./reading');
-const { baseRate } = require('./tax-utils');

 const rawReading = acquireReading();
 const aReading = enrichReading(rawReading);
 const basicChargeAmount = aReading.baseCharge;

 console.log(`basic charge amount is ${basicChargeAmount}`);
-
-function calculateBaseCharge(aReading) {
-  return baseRate(aReading.month, aReading.year) * aReading.quantity;
-}

Moving forward, we can update client1.js to use the baseCharge field from the enriched reading object:

diff --git a/src/client1.js b/src/client1.js
@@ -1,7 +1,7 @@
-const { acquireReading } = require('./reading');
-const { baseRate } = require('./tax-utils');
+const { acquireReading, enrichReading } = require('./reading');

-const aReading = acquireReading();
-const baseCharge = baseRate(aReading.month, aReading.year) * aReading.quantity;
+const rawReading = acquireReading();
+const aReading = enrichReading(rawReading);
+const baseCharge = aReading.baseCharge;
 console.log(`base charge is ${baseCharge}`);

And also update client2.js to do the same:

diff --git a/src/client2.js b/src/client2.js
@@ -1,8 +1,9 @@
-const { acquireReading } = require('./reading');
-const { baseRate, taxThreshold } = require('./tax-utils');
+const { acquireReading, enrichReading } = require('./reading');
+const { taxThreshold } = require('./tax-utils');

-const aReading = acquireReading();
-const base = baseRate(aReading.month, aReading.year) * aReading.quantity;
+const rawReading = acquireReading();
+const aReading = enrichReading(rawReading);
+const base = aReading.baseCharge;
 const taxableCharge = Math.max(0, base - taxThreshold(aReading.year));

 console.log(`base charge is ${base}`);

Now we're just left with the calculation of the taxable charge to deal with. We can start by enriching the reading object with the taxableCharge calculation info as part of the enrichReading function:

diff --git a/src/reading/index.js b/src/reading/index.js
@@ -1,5 +1,5 @@
 const { cloneDeep } = require('lodash');
-const { baseRate } = require('../tax-utils');
+const { baseRate, taxThreshold } = require('../tax-utils');

 const reading = { customer: 'Ivan', quantity: 10, month: 5, year: 2017 };

@@ -10,6 +10,7 @@ function acquireReading() {
 function enrichReading(aReading) {
   const result = cloneDeep(aReading);
   result.baseCharge = calculateBaseCharge(result);
+  result.taxableCharge = Math.max(0, result.baseCharge - taxThreshold(aReading.year));
   return result;
 }

diff --git a/src/reading/index.test.js b/src/reading/index.test.js
@@ -15,4 +15,9 @@ describe('enrichReading', () => {
     const resultingReading = enrichReading(inputReading);
     expect(typeof resultingReading.baseCharge).toBe('number');
   });
+
+  it('should enrich the reading with the taxable charge', () => {
+    const resultingReading = enrichReading(inputReading);
+    expect(typeof resultingReading.taxableCharge).toBe('number');
+  });
 });

And then we can update client2.js to use the taxableCharge field from the enriched reading object:

diff --git a/src/client2.js b/src/client2.js
@@ -1,10 +1,9 @@
 const { acquireReading, enrichReading } = require('./reading');
-const { taxThreshold } = require('./tax-utils');

 const rawReading = acquireReading();
 const aReading = enrichReading(rawReading);
 const base = aReading.baseCharge;
-const taxableCharge = Math.max(0, base - taxThreshold(aReading.year));
+const taxableCharge = aReading.taxableCharge;

 console.log(`base charge is ${base}`);
 console.log(`taxable charge is ${taxableCharge}`);

And that's it! Now we have a transform function that enriches the reading object with the fields we need. This function can be reused anywhere in the system without worrying about specific formulas or details.

Commit history

The commit history for the steps detailed above is listed below:

Commit SHA	Message
9699b10	introduce enrichReading function
87c373f	update client3 to use enrichReading
45c1c8e	enrich reading with baseCharge in the enrichReading function
724f88a	update client3 to use baseCharge field from enriched reading obj
d152883	remove unused calculateBaseCharge function
d3a7250	update client1 to use baseCharge from the enriched reading
0871d35	update client2 to use the baseCharge field from the enriched reading obj
7a8b88f	enrich reading with taxableCharge info at enrichReading fn
5ee217b	update client2 to use the taxableCharge field from the enriched reading obj

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