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https://github.com/ittus/refactoring-summary-2nd-javascript
Summary of "Refactoring: Improving the Design of Existing Code (2nd Edition)" by Martin Fowler
https://github.com/ittus/refactoring-summary-2nd-javascript
books refactoring software-engineering
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Summary of "Refactoring: Improving the Design of Existing Code (2nd Edition)" by Martin Fowler
- Host: GitHub
- URL: https://github.com/ittus/refactoring-summary-2nd-javascript
- Owner: ittus
- Created: 2019-11-04T12:14:02.000Z (almost 5 years ago)
- Default Branch: master
- Last Pushed: 2022-07-07T14:15:23.000Z (about 2 years ago)
- Last Synced: 2024-05-02T01:47:22.149Z (5 months ago)
- Topics: books, refactoring, software-engineering
- Homepage:
- Size: 47.9 KB
- Stars: 257
- Watchers: 15
- Forks: 50
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
My personal notes while reading "Refactoring: Improving the Design of Existing Code (2nd Edition)" by Martin Fowler. It only contains some basic concept as my understanding. If you want to learn more, I highly recommend you should buy the book.
If you are the publisher and think this repository should not be public, please write me an email to vuminhthang [dot] cm [at] gmail [dot] com and I will make it private.
Happy reading!
[](https://twitter.com/intent/tweet?text=Refactoring%20Book%20Summary&url=https://github.com/ittus/Refactoring-summary-2nd-javascript&hashtags=refactoring,javascript,software,developers)
## 1. TABLE OF CONTENT
- [1. TABLE OF CONTENT](#1-table-of-content)
- [3. BAD SMELLS IN CODE](#3-bad-smells-in-code)
- [1. Mysterious name](#1-mysterious-name)
- [2. Duplicated code](#2-duplicated-code)
- [3. Long function](#3-long-function)
- [4. Long parameter list](#4-long-parameter-list)
- [5. Global data](#5-global-data)
- [6. Mutable data](#6-mutable-data)
- [7. Divergent change](#7-divergent-change)
- [8. Shotgun surgery](#8-shotgun-surgery)
- [9. Feature envy](#9-feature-envy)
- [10. Data clumps](#10-data-clumps)
- [11. Primittive Obsession](#11-primittive-obsession)
- [12. Repeated switches](#12-repeated-switches)
- [13. Loops](#13-loops)
- [14. Lazy element](#14-lazy-element)
- [15. Speculative generality](#15-speculative-generality)
- [16. Temporary field](#16-temporary-field)
- [17. Message chains](#17-message-chains)
- [18. Middle man](#18-middle-man)
- [19. Insider trading](#19-insider-trading)
- [20. Large class](#20-large-class)
- [21. Alternative Classes with Different Interfaces](#21-alternative-classes-with-different-interfaces)
- [22. Data class](#22-data-class)
- [23. Refused Bequest](#23-refused-bequest)
- [24. Comment](#24-comment)
- [6. MOST COMMON SET OF REFACTORING](#6-most-common-set-of-refactoring)
- [1. Extract Function](#1-extract-function)
- [2. Inline Function](#2-inline-function)
- [3. Extract Variable](#3-extract-variable)
- [4. Inline Variable](#4-inline-variable)
- [5. Change Function Declaration](#5-change-function-declaration)
- [6. Encapsulate Variable](#6-encapsulate-variable)
- [7. Rename Variable](#7-rename-variable)
- [8. Introduce Parameter Object](#8-introduce-parameter-object)
- [9. Combine Functions Into Class](#9-combine-functions-into-class)
- [10. Combine Functions Into Transform](#10-combine-functions-into-transform)
- [11. Split Phase](#11-split-phase)
- [7. ENCAPSULATION](#7-encapsulation)
- [1. Encapsulate Record](#1-encapsulate-record)
- [2. Encapsulate Collection](#2-encapsulate-collection)
- [3. Replace Primitive with Object](#3-replace-primitive-with-object)
- [4. Replace Temp with Query](#4-replace-temp-with-query)
- [5. Extract Class](#5-extract-class)
- [6. Inline Class](#6-inline-class)
- [7. Hide Delegate](#7-hide-delegate)
- [8. Remove Middle Man](#8-remove-middle-man)
- [9. Substitute Algorithm](#9-substitute-algorithm)
- [8. MOVING FEATURES](#8-moving-features)
- [1. Move Function](#1-move-function)
- [2. Move Field](#2-move-field)
- [3. Move Statements into Function](#3-move-statements-into-function)
- [4. Move Statements To Callers](#4-move-statements-to-callers)
- [5. Replace Inline Code with Function Call](#5-replace-inline-code-with-function-call)
- [6. Slide Statements](#6-slide-statements)
- [7. Split Loop](#7-split-loop)
- [8. Replace Loop with Pipeline](#8-replace-loop-with-pipeline)
- [9. Remove Dead Code](#9-remove-dead-code)
- [9. ORGANIZING DATA](#9-organizing-data)
- [1. Split Variable](#1-split-variable)
- [2. Rename Field](#2-rename-field)
- [3. Replace Derived Variable With Query](#3-replace-derived-variable-with-query)
- [4. Change Reference To Value](#4-change-reference-to-value)
- [5. Change Value To Reference](#5-change-value-to-reference)
- [10. SIMPLIFYING CONDITIONAL LOGIC](#10-simplifying-conditional-logic)
- [1. Decompose Conditional](#1-decompose-conditional)
- [2. Consolidate Conditional Expression](#2-consolidate-conditional-expression)
- [3. Replace Nested Conditional with Guard Clauses](#3-replace-nested-conditional-with-guard-clauses)
- [4. Replace Conditional with Polymorphism](#4-replace-conditional-with-polymorphism)
- [5. Introduce Special Case](#5-introduce-special-case)
- [6. Introduce Assertion](#6-introduce-assertion)
- [11. REFACTORING APIS](#11-refactoring-apis)
- [1. Separate Query from Modifier](#1-separate-query-from-modifier)
- [2. Parameterize Function](#2-parameterize-function)
- [3. Remove Flag Argument](#3-remove-flag-argument)
- [4. Preserve Whole Object](#4-preserve-whole-object)
- [5. Replace Parameter with Query](#5-replace-parameter-with-query)
- [6. Replace Query with Parameter](#6-replace-query-with-parameter)
- [7. Remove Setting Method](#7-remove-setting-method)
- [8. Replace Constructor with Factory Function](#8-replace-constructor-with-factory-function)
- [9. Replace Function with Command](#9-replace-function-with-command)
- [10. Replace Command with Function](#10-replace-command-with-function)
- [12. DEALING WITH INHERITANCE](#12-dealing-with-inheritance)
- [1. Pull Up Method](#1-pull-up-method)
- [2. Pull Up Field](#2-pull-up-field)
- [3. Pull Up Constructor Body](#3-pull-up-constructor-body)
- [4. Push Down Method](#4-push-down-method)
- [5. Push Down Field](#5-push-down-field)
- [6. Replace Type Code with Subclasses](#6-replace-type-code-with-subclasses)
- [7. Remove Subclass](#7-remove-subclass)
- [8. Extract Superclass](#8-extract-superclass)
- [9. Collapse Hierarchy](#9-collapse-hierarchy)
- [10. Replace Subclass with Delegate](#10-replace-subclass-with-delegate)
- [11. Replace Superclass with Delegate](#11-replace-superclass-with-delegate)
## 3. BAD SMELLS IN CODE
### 1. Mysterious name
Name should clearly communicate what they do and how to use them
### 2. Duplicated code
Same code structure in more than one place
### 3. Long function
the longer a function is, the more difficult it is to understand
### 4. Long parameter list
Difficult to understand and easily introduce bug
### 5. Global data
Global variable is difficult to track and debug
### 6. Mutable data
Changes to data can often lead to unexpected consequences and tricky bugs
### 7. Divergent change
One module is changed in different ways for different reasons
### 8. Shotgun surgery
When every time you make a change, you have to make a lot of little edits to a lot of different classes
### 9. Feature envy
When a function in one module spends more time communicating with functions or data inside another module than it does within its own module
### 10. Data clumps
Same three or four data items together in lots of places
### 11. Primittive Obsession
Use primitive types instead of custom fundamental types
### 12. Repeated switches
Same conditional switching logic pops up in different places
### 13. Loops
Using loops instead of first-class functions such as filter or map
### 14. Lazy element
A class, struct or function that isn't doing enough to pay for itself should be eliminated.
### 15. Speculative generality
All sorts of hooks and special cases to handle things that aren’t required
### 16. Temporary field
An instance variable that is set only in certain circumstances.
### 17. Message chains
When a client asks one object for another object, which the client then asks for yet another object...
### 18. Middle man
When an object delegates much of its functionality.
### 19. Insider trading
Modules that whisper to each other by the coffee machine need to be separated by using Move Function and Move Field to reduce the need to chat.
### 20. Large class
A class is trying to do too much, it often shows up as too many fields
### 21. Alternative Classes with Different Interfaces
Classes with methods that look to similar.
### 22. Data class
Classes that have fields, getting and setting methods for the fields, and nothing else
### 23. Refused Bequest
Subclasses doesn't make uses of parents method
### 24. Comment
The comments are there because the code is bad
## 6. MOST COMMON SET OF REFACTORING
### 1. Extract Function
Extract fragment of code into its own function named after its purpose.
```javascript
function printOwing(invoice) {
printBanner()
let outstanding = calculateOutstanding()
// print details
console.log(`name: ${invoice.customer}`)
console.log(`amount: ${outstanding}`)
}
```
to
```javascript
function printOwing(invoice) {
printBanner()
let outstanding = calculateOutstanding()
printDetails(outstanding)
function printDetails(outstanding) {
console.log(`name: ${invoice.customer}`)
console.log(`amount: ${outstanding}`)
}
}
```
**Motivation**:
- You know what's the code doing without reading the details
- Short function is easier to read
- Reduce comment
### 2. Inline Function
Get rid of the function when the body of the code is just as clear as the name
```javascript
function rating(aDriver) {
return moreThanFiveLateDeliveries(aDriver) ? 2 : 1
}
function moreThanFiveLateDeliveries(aDriver) {
return aDriver.numberOfLateDeliveries > 5
}
```
to
```javascript
function rating(aDriver) {
return aDriver.numberOfLateDeliveries > 5 ? 2 : 1
}
```
**Motivation**
- When Indirection is needless (simple delegation) becomes irritating.
- If group of methods are badly factored and grouping them makes it clearer
### 3. Extract Variable
Add a name to an expression
```javascript
//price is base price quantity discount + shipping
return order.quantity * order.itemPrice -
Math.max(0, order.quantity - 500) * order.itemPrice * 0.05 +
Math.min(order.quantity * order.itemPrice * 0.1, 100)
```
to
```javascript
const basePrice = order.quantity * order.itemPrice
const quantityDiscount = Math.max(0, order.quantity - 500) * order.itemP
const shipping = Math.min(basePrice * 0.1, 100)
return basePrice - quantityDiscount + shipping;
```
**Motivation**
- Break down and name a part of a more complex piece of logic
- Easier for debugging
### 4. Inline Variable
Remove variable which doesn't really communicate more than the expression itself.
```javascript
let basePrice = anOrder.basePrice
return (basePrice > 1000)
```
to
```javascript
return anOrder.basePrice > 1000
```
### 5. Change Function Declaration
Rename a function, change list of parameters
```javascript
function circum(radius) {...}
```
to
```javascript
function circumference(radius) {...}
```
**Motivation**
- Easier to understand
- Easier to reuse, sometime better encapsulation
### 6. Encapsulate Variable
Encapsulate a reference to some data structure
```javascript
let defaultOwner = {firstName: "Martin", lastName: "Fowler"}
```
to
```javascript
// defaultOwner.js
let defaultOwnerData = {firstName: "Martin", lastName: "Fowler"}
export function defaultOwner() { return defaultOwnerData }
export function setDefaultOwner(arg) { defaultOwnerData = arg }
```
**Motivation**
- Provide a clear point to monitor changes and use of the data, like validation.
### 7. Rename Variable
Make shared variable's name can self-explain
```javascript
let a = height * width
```
to
```javascript
let area = height * width
```
### 8. Introduce Parameter Object
Replace groups of data items that regularly travel together with a single data structure
```javascript
function amountInvoiced(startDate, endDate) {}
function amountReceived(startDate, endDate) {}
function amountOverdue(startDate, endDate) {}
```
to
```javascript
function amountInvoiced(aDateRange) {}
function amountReceived(aDateRange) {}
function amountOverdue(aDateRange) {}
```
**Motivation**
- Make explicit the relationship between the data items
- Reduce the size of parameter list
- Make code more consistent
- Enable deeper changes to the code
### 9. Combine Functions Into Class
Form a class base on group of functions that operate closely on a common data
```javascript
function base(aReading) {}
function taxableCharge(aReading) {}
function calculateBaseCharge(aReading) {}
```
to
```javascript
class Reading() {
base() {}
taxableCharge() {}
calculateBaseCharge() {}
}
```
**Motivation**
- Simplify function call by removing many arguments
- Easier to pass object to other parts of the system
### 10. Combine Functions Into Transform
Takes the source data as input and calculates all the derivations, putting each derived value as a field in the output data
```javascript
function base(aReading) {}
function taxableCharge(aReading) {}
```
to
```javascript
function enrichReading(argReading) {
const aReading = _.cloneDeep(argReading)
aReading.baseCharge = base(aReading)
aReading.taxableCharge = taxableCharge(aReading)
return aReading
}
```
**Motivation**
- Avoid duplication of logic
### 11. Split Phase
Split code which do different things into separate modules
```javascript
const orderData = orderString.split(/\s+/)
const productPrice = priceList[orderData[0].split("-")[1]]
const orderPrice = parseInt(orderData[1]) * productPrice
```
to
```javascript
const orderRecord = parseOrder(orderString)
const orderPrice = price(orderRecord, priceList)
function parseOrder(aString) {
const values = aString.split(/\s+/)
return {
productID: values[0].split("-")[1],
quantity: parseInt(values[1])
}
}
function price(order, priceList) {
return order.quantity * priceList[order.productID]
}
```
**Motivation**
- Make the different explicit, revealing the different in the code
- Be able to deal with each module separately
## 7. ENCAPSULATION
### 1. Encapsulate Record
Create record (class) from object
```javascript
organization = {name: "Acme gooseberries", country: "GB"}
```
to
```javascript
class Organization {
constructor(data) {
this._name = data.name
this._country = data.country
}
get name() { return this._name }
set name(arg) { this._name = arg }
get country() { retrun this._country }
set country(arg) { this._country = arg }
}
```
**Motivation**
- Hide what's stored and provide methods to get value
- Easier to refactoring, for example: rename
### 2. Encapsulate Collection
A method returns a collection. Make it return a read-only view and provide add/remove methods
```javascript
class Person {
get courses() { return this._courses }
set courses(aList) { this._courses = aList }
}
```
to
```javascript
class Person {
get courses() { return this._courses.slice() }
addCourse(aCourse) {}
removeCourse(aCourse)
}
```
**Motivation**
- Change to the collection should go through the owning class to prevent unexpected changes.
- Prevent modification of the underlying collection for example: return a copy or read-only proxy instead of collection value
### 3. Replace Primitive with Object
Create class for data
```javascript
orders.filter(o => "high" === o.priority || "rush" === o.priority)
```
to
```javascript
orders.filter(o => o.priority.higherThan(new Priority("normal")))
```
**Motivation**
- Encapsulate behaviour with data
### 4. Replace Temp with Query
Extract the assignment of the variable into a function
```javascript
const basePrice = this._quantity * this._itemPrice
if (basePrice > 1000) {
return basePrice * 0.95
} else {
return basePrice * 0.98
}
```
to
```javascript
get basePrice() { return this._quantity * this._itemPrice }
//...
if (this.basePrice > 1000) {
return this.basePrice * 0.95
} else {
return this.basePrice * 0.98
}
```
**Motivation**
- Avoid duplicating the calculation logic in similar functions
### 5. Extract Class
Extract class base on a subset of data and a subset of methods
```javascript
class Person {
get officeAreaCode() { return this._officeAreaCode }
get officeNumber() { return this._officeNumber }
}
```
to
```javascript
class Person {
get officeAreaCode() { return this._telephoneNumber.areaCode }
get officeNumber() { return this._telephoneNumber.number }
}
class TelephoneNumber {
get areaCode() { return this._areaCode }
get number() { return this._number }
}
```
**Motivation**
- Smaller class is easier to understand
- Separate class's responsibility
### 6. Inline Class
Merge class if class isn't doing very much. Move its feature to another class then delete it.
```javascript
class Person {
get officeAreaCode() { return this._telephoneNumber.areaCode }
get officeNumber() { return this._telephoneNumber.number }
}
class TelephoneNumber {
get areaCode() { return this._areaCode }
get number() { return this._number }
}
```
to
```javascript
class Person {
get officeAreaCode() { return this._officeAreaCode }
get officeNumber() { return this._officeNumber }
}
```
**Motivation**
- Class is no longer pulling its weight and shouldn’t be around any more
- When want to refactor pair of classes. First Inline Class -> Extract Class to make new separation
### 7. Hide Delegate
A client is calling a delegate class of an object, create methods on the server to hide the delegate.
```javascript
manager = aPerson.department.manager
```
to
```javascript
manager = aPerson.manager
class Person {
get manager() {
return this.department.manager
}
}
```
**Motivation**
- Client doesn't need to know and response to delegation's change
- Better encapsulation
### 8. Remove Middle Man
Client call the delegate directly
```javascript
manager = aPerson.manager
class Person {
get manager() {
return this.department.manager
}
}
```
to
```javascript
manager = aPerson.department.manager
```
**Motivation**
- When there are too many delegating methods
### 9. Substitute Algorithm
Replace complicated algorithm with simpler algorithm
```javascript
function foundPerson(people) {
for (let i = 0; i < people.lenth; i++) {
if (people[i] === "Don") {
return "Don"
}
if (people[i] === "John") {
return "John"
}
if (people[i] === "Kent") {
return "Kent"
}
}
return ""
}
```
to
```javascript
function foundPerson(people) {
const candidates = ["Don", "John", "Kent"]
return people.find(p => candidates.includes(p)) || ""
}
```
**Motivation**
- Change to algorithm which make changes easier
- The clearer algorithm is, the better.
## 8. MOVING FEATURES
### 1. Move Function
Move a function when it references elements in other contexts more than the one it currently resides in
```javascript
class Account {
get overdraftCharge() {}
}
class AccountType {}
```
to
```javascript
class Account {}
class AccountType {
get overdraftCharge() {}
}
```
**Motivation**
- Improve encapsulation, loose coupling
### 2. Move Field
Move field from once class to another
```javascript
class Customer {
get plan() { return this._plan }
get discountRate() { return this._discountRate }
}
```
to
```javascript
class Customer {
get plan() { return this._plan }
get discountRate() { return this.plan.discountRate }
}
```
**Motivation**
- Pieces of data that are always passed to functions together are usually best put in a single record
- If a change in one record causes a field in another record to change too, that’s a sign of a field in the wrong place
### 3. Move Statements into Function
When statement is a part of called functions (always go togeter), move it inside the function
```javascript
result.push(`
title: ${person.photo.title}
`)
result.concat(photoData(person.photo))
function photoData(aPhoto) {
return [
`
location: ${aPhoto.location}
`,
`date: ${aPhoto.data.toDateString()}
`
]
}
```
to
```javascript
result.concat(photoData(person.photo))
function photoData(aPhoto) {
return [
`
title: ${aPhoto.title}
`,
`location: ${aPhoto.location}
`,
`date: ${aPhoto.data.toDateString()}
`
]
}
```
**Motivation**
- Remove duplicated code
### 4. Move Statements To Callers
```javascript
emitPhotoData(outStream, person.photo)
function emitPhotoData(outStream, photo) {
outStream.write(`
title: ${photo.title}
\n`)
outStream.write(`location: ${photo.location}
\n`)
}
```
to
```javascript
emitPhotoData(outStream, person.photo)
outStream.write(`
location: ${photo.location}
\n`)
function emitPhotoData(outStream, photo) {
outStream.write(`
title: ${photo.title}
\n`)
}
```
**Motivation**
- When common behavior used in several places needs to vary in some of its call
### 5. Replace Inline Code with Function Call
Replace the inline code with a call to the existing function
```javascript
let appliesToMass = false
for (const s of states) {
if (s === "MA") appliesToMass = true
}
```
```javascript
appliesToMass = states.includes("MA")
```
**Motivation**
- Remove duplication
- Meaningful function name is easier to understand
### 6. Slide Statements
Move related code to near each other
```javascript
const pricingPlan = retrievePricingPlan()
const order = retreiveOrder()
let charge
const chargePerUnit = pricingPlan.unit
```
to
```javascript
const pricingPlan = retrievePricingPlan()
const chargePerUnit = pricingPlan.unit
const order = retreiveOrder()
let charge
```
**Motivation**
- It makes code easier to understand and easier to extract function
### 7. Split Loop
Split the loop which does two different things
```javascript
let averageAge = 0
let totalSalary = 0
for (const p of people) {
averageAge += p.age
totalSalary += p.salary
}
averageAage = averageAge / people.length
```
to
```javascript
let totalSalary = 0
for (const p of people) {
totalSalary += p.salary
}
let averageAge = 0
for (const p of people) {
averageAge += p.age
}
averageAge = averageAge / people.length
```
**Motivation**
- Easier to use
- Easier to understand because each loop will do only 1 thing
### 8. Replace Loop with Pipeline
Replace loop with collection pipeline, like `map` or `filter`
```javascript
const names = []
for (const i of input) {
if (i.job === "programmer") {
names.push(i.name)
}
}
```
to
```javascript
const names = input.filter(i => i.job === "programmer").
map(i => i.name)
```
**Motivation**
- Easier to understand the flow of data
### 9. Remove Dead Code
```javascript
if (false) {
doSomethingThatUsedToMatter()
}
```
to
```javascript
```
**Motivation**
- Easier and quicker for developer to understand the codebase
## 9. ORGANIZING DATA
### 1. Split Variable
Any variable with more than one responsibility should be replaced with multiple variables, one for each responsibility
```javascript
let temp = 2 * (height + width)
console.log(temp)
temp = height * width
console.log(temp)
```
to
```javascript
const perimeter = 2 * (height + width)
console.log(perimeter)
const area = height * width
console.log(area)
```
**Motivation**
- Easier to understand
### 2. Rename Field
```javascript
class Organization {
get name() {}
}
```
to
```javascript
class Organization {
get title() {}
}
```
### 3. Replace Derived Variable With Query
Remove anny variables which cloud be easily calculate
```javascript
get discountedTotal() { return this._discountedTotal }
set discount(aNumber) {
const old = this._discount
this._discount = aNumber
this._discountedTotal += old - aNumber
}
```
to
```javascript
get discountedTotal() { return this._baseTotal - this._discount }
set discount() { this._discount = aNumber }
```
**Motivation**
- Minimize scope of mutable data
- A calculate makes it clearer what the meaning of data is
### 4. Change Reference To Value
Treat data as value. When update, replace entire inner object with a new one
```javascript
class Product {
applyDiscount(arg) {
this._price.amount -= arg
}
}
```
to
```javascript
class Product {
applyDiscount(arg) {
this._price = new Money(this._price.amount - arg, this._price.currency)
}
}
```
**Motivation**
- Immutable data is easier to deal with
### 5. Change Value To Reference
When need to share an object in different place, or have duplicated objects
```javascript
let customer = new Customer(customerData)
```
to
```javascript
let customer = customerRepository.get(customerData.id)
```
**Motivation**
- Update one reference is easier and more consistent than update multiple copies
## 10. SIMPLIFYING CONDITIONAL LOGIC
### 1. Decompose Conditional
Decomposing condition and replacing each chunk of code with a function call
```javascript
if (!aDate.isBefore(plan.summerStart) && !aData.isAfter(plan.summerEnd)) {
charge = quantity * plan.summerRate
} else {
charge = quantity * plan.regularRate + plan.regularServiceCharge
}
```
to
```javascript
if (summer()) {
charge = summerCharge()
} else {
charge = regularCharge()
}
```
**Motivation**
- Clearer intention of what we're branching on
### 2. Consolidate Conditional Expression
Consolidate different condition check which the result action is same to a single condition check with single result
```javascript
if (anEmployee.seniority < 2) return 0
if (anEmployee.monthsDisabled > 12) return 0;
if (anEmployee.isPartTime) return 0
```
to
```javascript
if (isNotEligableForDisability()) return 0
function isNotEligableForDisability() {
return ((anEmployee.seniority < 2)
|| (anEmployee.monthsDisabled > 12)
|| (anEmployee.isPartTime))
}
```
**Motivation**
- Often lead to Extract Function, which reveal intead of the code by function name
- If conditions are not related, don't consolidate them
### 3. Replace Nested Conditional with Guard Clauses
If condition is unusual condition, early return (Guard Clauses) and exist the function
```javascript
function getPayAmount() {
let result
if (isDead) {
result = deadAmount()
} else {
if (isSeparated) {
result = separatedAmount()
} else {
if (isRetired) {
result = retiredAmount()
} else {
result = normalPayAmount()
}
}
}
return result
}
```
to
```javascript
function getPayAmount() {
if (isDead) return deadAmount()
if (isSeparated) return separatedAmount()
if (isRetired) return retiredAmount()
return normalPayAmount()
}
```
**Motivation**
- It shows conditional branch are normal or unusual
### 4. Replace Conditional with Polymorphism
Using object oriented class instead of complex condition
```javascript
switch (bird.type) {
case 'EuropeanSwallow':
return 'average'
case 'AfricanSwallow':
return bird.numberOfCoconuts > 2 ? 'tired' : 'average'
case 'NorwegianBlueParrot':
return bird.voltage > 100 ? 'scorched' : 'beautiful'
default:
return 'unknow'
}
```
to
```javascript
class EuropeanSwallow {
get plumage() {
return 'average'
}
}
class AfricanSwallow {
get plumage() {
return this.numberOfCoconuts > 2 ? 'tired' : 'average'
}
}
class NorwegianBlueParrot {
get plumage() {
return this.voltage > 100 ? 'scorched' : 'beautiful'
}
}
```
**Motivation**
- Make the separation more explicit
### 5. Introduce Special Case
Bring special check case to a single place
```javascript
if (aCustomer === "unknown") {
customerName = "occupant"
}
```
to
```javascript
class UnknownCustomer {
get name() {
return "occupant"
}
}
```
**Motivation**
- Remove duplicate code
- Easy to add additional behavior to special object
### 6. Introduce Assertion
Make the assumption explicit by writing an assertion
```javascript
if (this.discountRate) {
base = base - (this.discountRate * base)
}
```
to
```javascript
assert(this.discountRate >= 0)
if (this.discountRate) {
base = base - (this.discountRate * base)
}
```
**Motivation**
- Reader can understand the assumption easily
- Help in debugging
## 11. REFACTORING APIS
### 1. Separate Query from Modifier
Separate function that returns a value (query only) and function with side effects (example: modify data)
```javascript
function alertForMiscreant (people) {
for (const p of people) {
if (p === "Don") {
setOffAlarms()
return "Don"
}
if (p === "John") {
setOffAlarms()
return "John"
}
}
return ""
}
```
to
```javascript
function findMiscreant (people) {
for (const p of people) {
if (p === "Don") {
return "Don"
}
if (p === "John") {
return "John"
}
}
return ""
}
function alertForMiscreant (people) {
if (findMiscreant(people) !== "") setOffAlarms();
}
```
**Motivation**
- Immutable function (query only) is easy to test and reuse
### 2. Parameterize Function
Combine function with similar logic and different literal value
```javascript
function tenPercentRaise(aPerson) {
aPerson.salary = aPerson.salary.multiply(1.1)
}
function fivePercentRaise(aPerson) {
aPerson.salary = aPerson.salary.multiply(1.05)
}
```
to
```javascript
function raise(aPerson, factor) {
aPerson.salary = aPerson.salary.multiply(1 + factor)
}
```
**Motivation**
- Increase usefulness of the function
### 3. Remove Flag Argument
Remove _literal_ flag argument by clear name functions
```javascript
function setDimension(name, value) {
if (name === 'height') {
this._height = value
return
}
if (name === 'width') {
this._width = value
return
}
}
```
to
```javascript
function setHeight(value) { this._height = value }
function setWidth(value) { this._width = value }
```
**Motivation**
- Easy to read and understand code
- Be careful if flag argument appears more than 1 time in the function, or is passed to further function
### 4. Preserve Whole Object
Passing whole object instead of multiple parameters
```javascript
const low = aRoom.daysTempRange.low
const high = aRoom.daysTempRange.high
if (aPlan.withinRange(low, high)) {}
```
to
```javascript
if (aPlan.withInRange(aRoom.daysTempRange)) {}
```
**Motivation**
- Shorter parameter list
- Don't need to add additional parameter if function needs more data in the future
- Be careful if function and object are in different modules, which make tight coupling if we apply this refactor
### 5. Replace Parameter with Query
```javascript
availableVacation(anEmployee, anEmployee.grade)
function availableVacation(anEmployee, grade) {
// calculate vacation...
}
```
to
```javascript
availableVacation(anEmployee)
function availableVacation(anEmployee) {
const grade = anEmployee.grade
// calculate vacation...
}
```
**Motivation**
- Shorter list of parameters
- Simpler work for the caller (because fewer parameters)
- Be careful because it can increase function's dependency
### 6. Replace Query with Parameter
Replace internal reference with a parameter
```javascript
targetTemperature(aPlan)
function targetTemperature(aPlan) {
currentTemperature = thermostat.currentTemperature
// rest of function...
}
```
to
```javascript
targetTemperature(aPlan, thermostat.currentTemperature)
function targetTemperature(aPlan, currentTemperature) {
// rest of function...
}
```
**Motivation**
- Reduce function's dependency
- Create more pure functions
### 7. Remove Setting Method
Make a field immutable by removing setting method
```javascript
class Person {
get name() {...}
set name(aString) {...}
}
```
to
```javascript
class Person {
get name() {...}
}
```
### 8. Replace Constructor with Factory Function
```javascript
leadEngineer = new Employee(document.leadEngineer, 'E')
```
to
```javascript
leadEngineer = createEngineer(document.leadEngineer)
```
**Motivation**
- You want to do more than simple construction when you create an object.
### 9. Replace Function with Command
Encapsulate function into its own object
```javascript
function score(candidate, medicalExam, scoringGuide) {
let result = 0
let healthLevel = 0
// long body code
}
```
to
```javascript
class Scorer {
constructor(candidate, medicalExam, scoringGuide) {
this._candidate = candidate
this._medicalExam = medicalExam
this._scoringGuide = scoringGuide
}
execute() {
this._result = 0
this._healthLevel = 0
// long body code
}
}
```
**Motivation**
- You want to add complimentary operation, such as undo
- Can have richer lifecycle
- Can build customization such as inheritance and hooks
- Easily break down a complex function to simpler steps
### 10. Replace Command with Function
```javascript
class ChargeCalculator {
constructor (customer, usage){
this._customer = customer
this._usage = usage
}
execute() {
return this._customer.rate * this._usage
}
}
```
to
```javascript
function charge(customer, usage) {
return customer.rate * usage
}
```
**Motivation**
- Function call is simpler than command object
## 12. DEALING WITH INHERITANCE
### 1. Pull Up Method
Move similar methods in subclass to superclass
```java
class Employee {...}
class Salesman extends Employee {
get name() {...}
}
class Engineer extends Employee {
get name() {...}
}
```
to
```java
class Employee {
get name() {...}
}
class Salesman extends Employee {...}
class Engineer extends Employee {...}
```
**Motivation**
- Eliminate duplicate code in subclass
- If two methods has similar workflow, consider using Template Method Pattern
### 2. Pull Up Field
Pull up similar field to superclass to remove duplication
```java
class Employee {...}
class Salesman extends Employee {
private String name;
}
class Engineer extends Employee {
private String name;
}
```
to
```java
class Employee {
protected String name;
}
class Salesman extends Employee {...}
class Engineer extends Employee {...}
```
### 3. Pull Up Constructor Body
You have constructors on subclasses with mostly identical bodies.
Create a superclass constructor; call this from the subclass methods
```javascript
class Party {...}
class Employee extends Party {
constructor(name, id, monthlyCost) {
super()
this._id = id
this._name = name
this._monthlyCost = monthlyCost
}
}
```
to
```javascript
class Party {
constructor(name){
this._name = name
}
}
class Employee extends Party {
constructor(name, id, monthlyCost) {
super(name)
this._id = id
this._monthlyCost = monthlyCost
}
}
```
### 4. Push Down Method
If method is only relevant to one subclass, moving it from superclass to subclass
```javascript
class Employee {
get quota {...}
}
class Engineer extends Employee {...}
class Salesman extends Employee {...}
```
to
```javascript
class Employee {...}
class Engineer extends Employee {...}
class Salesman extends Employee {
get quota {...}
}
```
### 5. Push Down Field
If field is only used in one subclass, move it to those subclasses
```java
class Employee {
private String quota;
}
class Engineer extends Employee {...}
class Salesman extends Employee {...}
```
to
```java
class Employee {...}
class Engineer extends Employee {...}
class Salesman extends Employee {
protected String quota;
}
```
### 6. Replace Type Code with Subclasses
```javascript
function createEmployee(name, type) {
return new Employee(name, type)
}
```
to
```javascript
function createEmployee(name, type) {
switch (type) {
case "engineer": return new Engineer(name)
case "salesman": return new Salesman(name)
case "manager": return new Manager (name)
}
}
```
**Motivation**
- Easily to apply Replace Conditional with Polymorphism later
- Execute different code depending on the value of a type
### 7. Remove Subclass
You have subclasses do to little. Replace the subclass with a field in superclass.
```javascript
class Person {
get genderCode() {return "X"}
}
class Male extends Person {
get genderCode() {return "M"}
}
class Female extends Person {
get genderCode() {return "F"}
}
```
to
```javascript
class Person {
get genderCode() {return this._genderCode}
}
```
### 8. Extract Superclass
If 2 classes have similar behaviors, create superclass and move these behaviors to superclass
```javascript
class Department {
get totalAnnualCost() {...}
get name() {...}
get headCount() {...}
}
class Employee {
get annualCost() {...}
get name() {...}
get id() {...}
}
```
to
```javascript
class Party {
get name() {...}
get annualCost() {...}
}
class Department extends Party {
get annualCost() {...}
get headCount() {...}
}
class Employee extends Party {
get annualCost() {...}
get id() {...}
}
```
**Motivation**
- Remove duplication
- Prepare for Replace Superclass with Delegate refactor
### 9. Collapse Hierarchy
Merge superclass and subclass when there are no longer different enough to keep them separate
```javascript
class Employee {...}
class Salesman extends Employee {...}
```
to
```javascript
class Employee {...}
```
### 10. Replace Subclass with Delegate
"Favor object composition over class inheritance" (where composition is effectively the same as delegation
```javascript
class Order {
get daysToShip() {
return this._warehouse.daysToShip
}
}
class PriorityOrder extends Order {
get daysToShip() {
return this._priorityPlan.daysToShip
}
}
```
to
```javascript
class Order {
get daysToShip() {
return (this._priorityDelegate)
? this._priorityDelegate.daysToShip
: this._warehouse.daysToShip
}
}
class PriorityOrderDelegate {
get daysToShip() {
return this._priorityPlan.daysToShip
}
}
```
**Motivation**
- If there are more than 1 reason to vary something, inheritance is not enough
- Inheritance introduce very close relationship
### 11. Replace Superclass with Delegate
If functions of the superclass don’t make sense on the subclass, replace with with delegate
```javascript
class List {...}
class Stack extends List {...}
```
to
```javascript
class Stack {
constructor() {
this._storage = new List();
}
}
class List {...}
```
**Motivation**
- Easier to maintain code