🧠 Understanding Classes vs Interfaces in Angular (TypeScript)

If you’re working with Angular — a powerful framework built with TypeScript — understanding the difference between interfaces and classes is essential.

Knowing when to use each helps you write cleaner, more maintainable code — whether you're defining data models, working with services, or building components.

Let’s explore both with practical examples and real Angular use cases.

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📘 What Is an Interface in TypeScript?

An interface is a contract that defines the shape of an object. It exists only at compile time and doesn’t produce any JavaScript in your final bundle.

🔧 Analogy:

An interface is like a blueprint — it describes what an object should look like but doesn’t actually construct it.

✅ Example: Interface

export interface Expense {

id: number;

category: string;

amount: number;

date: Date;

description: string;

notes?: string; // Optional

project?: string; // Optional

}

This ensures that any object labeled as Expense includes these properties.

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🧱 What Is a Class in TypeScript?

A class includes both the structure and the implementation. Unlike interfaces, classes are compiled into JavaScript and exist at runtime.

🏗️ Analogy:

A class is like the actual building created from the blueprint.

✅ Example: Class

export class Expense {

constructor(

public id: number,

public category: string,

public amount: number,

public date: Date,

public description: string,

public notes?: string,

public project?: string

) {}

getSummary(): string {

return ${this.category}: $${this.amount};

}

}

🛠️ Usage:

const myExpense = new Expense(1, 'Travel', 100, new Date(), 'Flight ticket');

console.log(myExpense.getSummary()); // "Travel: $100"

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⚖️ Interface vs Class: Key Differences

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🧠 When Should You Use Each?

✅ Use Interface when:

• You only need to describe data shape.

• You want zero runtime overhead.

• Typing API responses or data contracts (DTOs).

✅ Use Class when:

• You need logic, methods, or computed properties.

• You plan to instantiate objects.

• You want default values or utility functions.

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📊 Decision Flowchart

📍

E.g., Interface ➜ Simple Shape | Class ➜ Need Logic & Instantiation

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🔁 Classes Implementing Interfaces

Use interfaces for type safety and classes for behavior:

export interface ExpenseShape {

id: number;

category: string;

amount: number;

date: Date;

}

export class Expense implements ExpenseShape {

constructor(

public id: number,

public category: string,

public amount: number,

public date: Date

) {}

getSummary(): string {

return ${this.category}: $${this.amount};

}

}

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🧩 Real Angular Use Cases

💡 Using Interface in a Service

@Injectable({ providedIn: 'root' })

export class ExpenseService {

constructor(private http: HttpClient) {}

getExpenses(): Observable<Expense[]> {

return this.http.get<Expense[]>('/api/expenses');

}

}

💡 Using Class with Behavior

export class Expense {

constructor(

public id: number,

public category: string,

public amount: number,

public date: Date,

public description: string,

public notes?: string

) {}

getSummary(): string {

return ${this.category}: $${this.amount};

}

}

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👨‍👩‍👧 Inheritance with Interfaces & Classes

✅ Interface Inheritance

interface Timestamped {

createdAt: Date;

}

interface Expense extends Timestamped {

id: number;

amount: number;

}

✅ Class Inheritance

class BaseExpense {

constructor(public id: number) {}

getId() {

return this.id;

}

}

class DetailedExpense extends BaseExpense {

constructor(id: number, public amount: number) {

super(id);

}

}

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📌 Summary: Interface vs Class

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🎯 Final Tip

✅ Use interfaces when you need lightweight, type-safe structures.

✅ Use classes when you want to encapsulate logic, instantiate objects, or add behavior inside Angular components or services.

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🙌 Was This Helpful?

If this helped clarify the difference between classes and interfaces in Angular, consider sharing it or dropping a comment with your thoughts!

Let’s make TypeScript more approachable — one concept at a time 💡

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📚 Further Reading

• TypeScript Handbook — Interfaces

• TypeScript Handbook — Classes

• Angular Docs — Dependency Injection