TypeScript flashcards

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type vs interface

Both define the shape of an object, but have different capabilities.

Key differences

	`type`	`interface`
Object shape	Yes	Yes
Primitives/unions	Yes	No
Declaration merging	No	Yes
Extends	Via `&`	Via `extends`
Mapped types (`[K in keyof T]`)	Yes	No

Declaration merging (interface only)

ts
interface User { name: string; }
interface User { age: number; }
// result: { name: string; age: number; }

Union types (type only)

ts
type ID = string | number;
type Status = 'active' | 'inactive' | 'banned';

Extending

ts
// interface
interface Animal { name: string; }
interface Dog extends Animal { breed: string; }

// type
type Animal = { name: string; };
type Dog = Animal & { breed: string; };

Rule of thumb: use interface for object shapes that may be extended or merged (e.g. library public APIs). Use type for unions, intersections, and utility types.

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