Objects & Interfaces
Interface vs Type Alias — When to Use Which
Master comparing TypeScript interfaces vs type aliases, declaration merging, union types, performance differences, and architectural selection guidelines.
TypeScript provides two core tools to name custom shapes and types: interface and type alias. While they overlap in 80% of use cases, key differences determine which tool is best suited for your architectural needs.
1. Syntax Comparison for Object Shapes
Both interface and type can define object structures:
2. Key Differences
1. Declaration Merging (Interfaces Only)
Interfaces with the same name in the same scope automatically merge their property declarations. Type aliases throw a duplicate identifier error.2. Unions & Primitives (Type Aliases Only)
Type aliases can name primitive types, union types, tuple types, and mapped types directly. Interfaces cannot.3. Extending & Combining
extends.&).3. Interactive Code Playground
Compare interface vs type in practice:
4. Selection Decision Matrix
| Capability / Requirement | interface | type Alias | |
|---|---|---|---|
| Object Shapes & Class Contracts | Recommended | Supported | |
| Declaration Merging | Yes (Built-in) | No | |
| Primitive / Union / Tuple Types | No | Yes (type ID = string \ | number) |
| Extending / Inheritance Syntax | extends | Intersections (&) | |
| Performance (Compiler Caching) | Slightly Faster for flat objects | Flexible | |
| Public API / Library Exports | Recommended for consumer extensibility | Recommended for complex utility types |
5. Best Practice Guidelines
1. Use interface for Object Shapes & Public APIs: When defining object contracts, React component props, or library interfaces that external consumers might need to extend or augment via declaration merging.
2. Use type for Unions, Primitives & Mapped Types: When defining union types (string | number), tuples, function signatures, or mapped/conditional utility types.