Applying Clean Code Principles: The Law of Demeter in TypeScript

Lesson 3

Introduction

Welcome to the third lesson of the "Applying Clean Code Principles" course. In our journey so far, we've discussed the importance of the DRY (Don't Repeat Yourself) principle to eliminate redundancy in code. We followed that with the KISS (Keep It Simple, Stupid) principle, which highlights the value of simplicity in software development. Today, our spotlight is on the Law of Demeter — a key guideline in object-oriented programming. By limiting the knowledge that an object has about other objects, this lesson will guide you in crafting more maintainable and modular code. 🤓

Understanding the Law of Demeter

The Law of Demeter was introduced by Karl J. Lieberherr and suggests that an object should only communicate with its immediate collaborators, avoiding the entire system. By reducing dependency between parts, you'll find your code easier to maintain and scale. In simple terms, a method X of the class C should only call methods of:

Class C itself
An object created by X
An object passed as an argument to X
An object held in an instance variable of C
A static field

With these principles, you control how parts of your application interact, leading to a more organized structure. Let's explore how this works with examples. 🚀

First Rule Example

For the first point, a method should only access its class's methods:

TypeScript
1class Car {
2    public start() {
3        this.checkFuel();
4        this.ignite();
5    }
6    
7    private checkFuel() {
8        console.log("Checking fuel level...");
9    }
10    
11    private ignite() {
12        console.log("Igniting the engine...");
13    }
14}

In this example, the start method interacts solely with methods within the Car class itself. This shows how you maintain clear boundaries adhering to the Law of Demeter.

Second Rule Example

Next, a method can interact with the objects it creates:

TypeScript
1class Library {
2    public borrowBook(title: string): Book {
3        const book = new Book(title);
4        book.issue();
5        return book;
6    }
7}
8
9class Book {
10    private title: string;
11    
12    constructor(title: string) {
13        this.title = title;
14    }
15    
16    public issue() {
17        console.log("Book issued: " + this.title);
18    }
19}

Here, the Library class creates a Book and calls the issue method on it. This usage pattern complies with the Law of Demeter, where Library interacts with the newly created Book. 📚

Third Rule Example

Continuing, let's look at interacting with objects passed as arguments:

TypeScript
1class Printer {
2    public print(document: Document) {
3        document.sendToPrinter();
4    }
5}
6
7class Document {
8    public sendToPrinter() {
9        console.log("Document is being printed...");
10    }
11}

The Printer class method print communicates with the Document object passed as an argument, aligning with the Law of Demeter by limiting communication to direct method parameters. 🖨️

Fourth Rule Example

Objects held in instance variables of a class can also be accessed:

TypeScript
1class House {
2    private door = new Door();
3    
4    public lockHouse() {
5        this.door.close();
6    }
7}
8
9class Door {
10    public close() {
11        console.log("Door is closed.");
12    }
13}

In this example, the House class interacts with its door through the lockHouse method, showcasing compliance by interacting with an object it holds in an instance variable. 🏠

Fifth Rule Example

Finally, let's see a method interacting with static fields. While static fields are convenient, they should generally be used cautiously since they can lead to shared state issues in larger applications:

TypeScript
1class TemperatureConverter {
2    private static readonly conversionFactor = 9.0 / 5.0;
3    
4    public celsiusToFahrenheit(celsius: number): number {
5        return Math.round(celsius * TemperatureConverter.conversionFactor + 32);
6    }
7}

Here, conversionFactor is defined as a readonly variable to indicate that it's a constant, and to ensure correct calculations, the division is a double. Accessing static fields like this complies with the Law of Demeter. 🌡️

Violation Example

Here's an example that violates the Law of Demeter:

TypeScript
1class Person {
2    private address: Address;
3    
4    constructor(address: Address) {
5        this.address = address;
6    }
7    
8    public getAddressDetails(): string {
9        return "Address: " + this.address.getFirstName() + " " + this.address.getLastName() +
10               ", " + this.address.getStreet() + 
11               ", " + this.address.getCity() + 
12               ", " + this.address.getCountry() + 
13               ", ZipCode: " + this.address.getZipCode();
14    }
15}
16
17class Address {
18    private firstName: string;
19    private lastName: string;
20    private street: string;
21    private city: string;
22    private country: string;
23    private zipCode: string;
24    
25    constructor(firstName: string, lastName: string, street: string, city: string, country: string, zipCode: string) {
26        this.firstName = firstName;
27        this.lastName = lastName;
28        this.street = street;
29        this.city = city;
30        this.country = country;
31        this.zipCode = zipCode;
32    }
33    
34    public getFirstName(): string {
35        return this.firstName;
36    }
37    
38    public getLastName(): string {
39        return this.lastName;
40    }
41    
42    public getStreet(): string {
43        return this.street;
44    }
45    
46    public getCity(): string {
47        return this.city;
48    }
49    
50    public getCountry(): string {
51        return this.country;
52    }
53    
54    public getZipCode(): string {
55        return this.zipCode;
56    }
57}

In this case, Person is directly accessing multiple fields through Address, leading to tight coupling. Person relies on the internal structure of Address, which might result in fragile code.

Refactored Example

Let's refactor the previous code to adhere to the Law of Demeter:

TypeScript
1class Person {
2    private address: Address;
3    
4    constructor(address: Address) {
5        this.address = address;
6    }
7    
8    public getAddressDetails(): string {
9        return this.address.getAddressLine();
10    }
11}
12
13class Address {
14    private firstName: string;
15    private lastName: string;
16    private street: string;
17    private city: string;
18    private country: string;
19    private zipCode: string;
20    
21    constructor(firstName: string, lastName: string, street: string, city: string, country: string, zipCode: string) {
22        this.firstName = firstName;
23        this.lastName = lastName;
24        this.street = street;
25        this.city = city;
26        this.country = country;
27        this.zipCode = zipCode;
28    }
29    
30    public getAddressLine(): string {
31        return this.firstName + " " + this.lastName +
32               ", " + this.street +
33               ", " + this.city + 
34               ", " + this.country +
35               ", ZipCode: " + this.zipCode;
36    }
37}

By encapsulating all the address details within the getAddressLine method in the Address class, the dependency is minimized, and Person no longer accesses Address's internals directly.

Summary and Next Steps

The Law of Demeter plays a vital role in writing clean, modular code by ensuring objects only interact with their closest dependencies. By understanding and implementing these guidelines, you enhance the modularity and maintainability of your code. As you move on to the practice exercises, challenge yourself to apply these principles and evaluate your code's interactions. Keep these lessons in mind as essential steps toward mastering clean code! 🌟

Enjoy this lesson? Now it's time to practice with Cosmo!

Practice is how you turn knowledge into actual skills.