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Interfaces
6 exercises
Interfaces describe the shape of an object:
interface User {
id: number;
name: string;
email?: string; // optional
readonly created: Date; // can't be changed
}You've already used type aliases. Both type and interface can describe object shapes, and TypeScript cares about structural compatibility: if two shapes match, values can be used interchangeably.
Interfaces offer special features:
- Extension –
interface Admin extends User { role: string } - Declaration merging – Multiple interface declarations combine
- Index signatures –
[key: string]: anyfor flexible keys
In this category, you'll:
- Define clear object shapes with optional and readonly properties
- Extend interfaces to build on existing types
- Use index signatures for dictionary-like objects
- Merge interface declarations (useful for augmenting libraries)
- See how interfaces interoperate with type aliases
By the end, you'll know when to use interfaces versus type aliases and how to leverage their unique features.