Hybrid Types (Callable Interfaces)

Because JavaScript is dynamic, an object can simultaneously act as both a callable function AND an object with attached properties. In TypeScript, an interface that describes a callable object with properties is called a Hybrid Type or Callable Interface.

1. Syntax for Hybrid Types

A hybrid type combines a Call Signature ((param: Type): ReturnType) with standard object property declarations inside an interface definition.

2. Real-World Use Case: Express Middleware & Axios Instance

Popular JavaScript libraries rely heavily on hybrid types:

Axios: axios('/api/user') is a callable function, but also has methods like axios.get(), axios.post(), and property axios.defaults.

Express: app(req, res) is a request listener function, but also has methods like app.use() and app.listen().

Example: Typing an Axios-like Fetch Utility

3. Construct Signatures (new Operator)

Interfaces can also define Construct Signatures using the new keyword to type functions that act as constructor functions instantiated via new:

4. Interactive Code Playground

Test hybrid types below:

5. Common Pitfalls & Edge Cases

Manual Type Assertion Requirement: Because JavaScript object instantiation cannot create a function and attach properties in a single literal, implementing a hybrid function usually requires a type assertion (as Counter).

6. Summary Table

| Property Signature | propName: Type; | Types object property access fn.propName |