Understanding TypeScript: Type vs. Interface - A Detailed Comparison

Type vs Interface in TypeScript: A Complete Guide with Examples

TypeScript, a statically typed superset of JavaScript, enhances code reliability and maintainability by introducing types. Two fundamental constructs for defining types in TypeScript are type aliases and interfaces.

At first glance, they seem similar—they both describe the “shape” of data. But when you dig deeper, there are key differences in features, use cases, and performance.

This blog breaks it all down with clear explanations, practical examples, and guidance on when to use type vs interface.

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What Are Type Aliases and Interfaces?

Type Aliases

A type alias gives a name to any type in TypeScript. It can represent primitives, objects, unions, intersections, tuples, or complex combinations.

type User = {
  name: string;
  age: number;
};

const user: User = { name: "Alice", age: 25 };

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Interfaces

An interface defines the structure of an object or class. It fits naturally into object-oriented programming, supporting inheritance and declaration merging.

interface User {
  name: string;
  age: number;
}

const user: User = { name: "Bob", age: 30 };

So, both look similar—but their differences emerge in specific scenarios.

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Key Differences Between Type and Interface

1. Flexibility in Type Definitions

• Type Aliases are extremely versatile:

  • Can represent primitives, unions, intersections, tuples, mapped types, and conditional types.

type ID = string | number; // Union
type Status = "active" | "inactive"; // Literal
type Point = [number, number]; // Tuple
type User = { name: string } & { age: number }; // Intersection

• Interfaces are limited to describing object shapes or class contracts.

interface User {
  name: string;
  age: number;
}

// ❌ Cannot directly define unions like string | number

✅ Takeaway: Use type for unions, primitives, or advanced compositions. Use interface for object/class shapes.

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2. Extensibility

• Interfaces support declaration merging and extends, making them great for extensibility.

// Declaration merging
interface User {
  name: string;
}
interface User {
  age: number;
}
const user: User = { name: "Alice", age: 25 };

// Extends
interface Admin extends User {
  role: string;
}

• Types don’t merge—you must use intersections.

type User = { name: string };
type Admin = User & { role: string };

const admin: Admin = { name: "Bob", role: "admin" };

✅ Takeaway: Use interface when extensibility and merging matter.

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3. Class Implementation

Both can define contracts for classes, but interfaces are more idiomatic.

interface User {
  name: string;
  greet(): string;
}

class Person implements User {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  greet() {
    return `Hello, ${this.name}!`;
  }
}

✅ Takeaway: Prefer interface for class contracts—it aligns better with OOP.

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4. Performance Considerations

• Interfaces: Faster in large projects due to optimized lookups and caching.
• Types: May add overhead in deeply nested unions or intersections.

✅ Takeaway: For large-scale apps, prefer interface for object shapes.

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Practical Examples

Example 1: Union Types with Type Aliases

type ID = string | number;
type Status = "active" | "inactive";

function getUser(id: ID, status: Status) {
  return { id, status };
}

👉 Only type works here—interfaces can’t represent unions.

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Example 2: Declaration Merging with Interfaces

interface Window {
  customProperty: string;
}
interface Window {
  customMethod(): void;
}

window.customProperty = "Hello";
window.customMethod = () => console.log("Custom method");

👉 Only interface supports merging—great for extending global types.

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Example 3: Combining Types with Intersection

type Name = { name: string };
type Age = { age: number };
type User = Name & Age;

const user: User = { name: "Alice", age: 25 };

👉 Types make ad-hoc combinations easy.

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Example 4: Complex Types with Type Aliases

type Optional<T> = {
  [K in keyof T]?: T[K];
};

type User = { name: string; age: number };
type OptionalUser = Optional<User>;

const user: OptionalUser = { name: "Alice" }; // ✅ Valid

👉 Advanced features like mapped types are only possible with type.

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When to Use Type vs Interface

Use interface when:

• Defining object shapes or class contracts.
• You need declaration merging (e.g., extending Window).
• Inheritance with extends is required.
• Large projects where compilation performance matters.

Use type when:

• Defining unions, primitives, or tuples.
• Creating complex type compositions with intersections.
• Using mapped or conditional types.
• You want lightweight, one-off type definitions.

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Common Misconceptions

• “Interfaces are only for objects” → Mostly true, but they can be extended indirectly for flexibility.
• “Types are less powerful” → Not at all—types are more flexible for advanced scenarios.
• “You must pick one” → Wrong! You can (and should) use both where appropriate.

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Conclusion

Both type and interface are powerful tools in TypeScript.

• Interfaces excel in OOP, inheritance, and declaration merging.
• Types shine for unions, intersections, and advanced type logic.

👉 A good rule of thumb:

• Use interface for objects and classes.
• Use type for everything else.

By understanding their strengths, you’ll write cleaner, more maintainable TypeScript code that scales with your project.

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