TypeScript Differences Between Type Aliases and Interfaces

TypeScript: Differences Between Type Aliases and Interfaces

TypeScript, a superset of JavaScript, offers developers the ability to write strongly-typed code, improving readability and reducing runtime errors. Among its many features, type aliases and interfaces are commonly used to define complex types. While they share similarities, they also have distinct differences that make each better suited to specific scenarios. This blog post delves into the key differences and best use cases for type aliases and interfaces in TypeScript.

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What Are Type Aliases and Interfaces?

• Type Aliases: A type alias is a way to create a custom name for any type, including primitive types, object types, or even union and intersection types. It’s declared using the type keyword.

  type Point = {
    x: number;
    y: number;
  };
  
  type StringOrNumber = string | number;
  

• Interfaces: An interface is a way to define the structure of an object. It’s used primarily for defining object types and is declared using the interface keyword.

  interface Point {
    x: number;
    y: number;
  }
  

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Key Differences

1. Extensibility

• Interfaces: Interfaces are inherently extendable. You can use the extends keyword to create a new interface that builds upon an existing one.

  interface Shape {
    color: string;
  }
  
  interface Circle extends Shape {
    radius: number;
  }
  

  Additionally, interfaces can be merged. If you declare an interface with the same name multiple times, TypeScript automatically merges them:

  interface Shape {
    color: string;
  }
  
  interface Shape {
    borderWidth: number;
  }
  
  const square: Shape = {
    color: "blue",
    borderWidth: 2,
  };
  

• Type Aliases: While type aliases cannot be merged, they can extend other types using intersection types.

  type Shape = {
    color: string;
  };
  
  type Circle = Shape & {
    radius: number;
  };
  
  const circle: Circle = {
    color: "red",
    radius: 10,
  };
  

2. Usage Scope

• Type Aliases: Type aliases can define a wider range of types, such as primitive types, union types, and tuple types.

  type ID = string | number;
  type Coordinates = [number, number];
  

• Interfaces: Interfaces are primarily used for defining object shapes. They cannot represent union or tuple types directly.

  // Invalid with interfaces:
  interface ID {
    // Cannot represent string | number
  }
  

3. Declaration Context

• Interfaces: Interfaces can only be used in the context of object types, making them more specialized for defining object structures.

• Type Aliases: Type aliases offer broader usage, allowing developers to define unions, intersections, tuples, and primitive aliases in addition to objects.

4. Implementation in Classes

Both interfaces and type aliases can be implemented by classes. However, interfaces are the preferred choice for this purpose due to their object-oriented nature.

interface Printable {
  print(): void;
}

class Document implements Printable {
  print() {
    console.log("Printing document...");
  }
}

Type aliases, while possible, are less intuitive when used in this context.

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Best Practices and When to Use Each

Use Interfaces When:

• Defining object shapes, especially if extensibility and reusability are important.
• You need to take advantage of interface merging.
• Working with object-oriented patterns and class implementations.

Use Type Aliases When:

• Defining non-object types such as unions, intersections, or tuples.
• You need to represent more complex types beyond object shapes.
• Simplicity and readability are a priority for defining reusable types.

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Conclusion

Type aliases and interfaces are both powerful tools in TypeScript, each suited to specific use cases. Interfaces shine in scenarios where object-oriented programming and extensibility are key, while type aliases excel in defining diverse, non-object types. By understanding their differences and strengths, developers can make informed decisions to write cleaner, more efficient TypeScript code.