Class implementing interface implicit types

[asotog]

🔍 Search Terms

Hi all,

is there any plan to allow implicit types when a class is anyways implementing an interface, function types are well defined, but still need to redeclare function types on the class:

thanks in advance

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

It would be cleaner if developer is not forced to redeclare function types when implementing functions coming from an interface

📃 Motivating Example

I'd like to propose an enhancement that could significantly reduce code duplication and improve developer experience.

Currently, when a class implements an interface, we're required to redeclare method types even though they're already defined in the interface:

interface UserService {
getUser(id: string): Promise;
updateUser(user: User): Promise;
}

class UserServiceImpl implements UserService {
// We have to repeat types even though they're defined in the interface
async getUser(id: string): Promise { ... }
async updateUser(user: User): Promise { ... }
}

Since the class explicitly implements the interface, TypeScript could infer these types without us having to redeclare them:

class UserServiceImpl implements UserService {
// Types could be inferred from the interface
async getUser(id) { ... }
async updateUser(user) { ... }
}

This would:

• Follow the DRY principle
• Reduce boilerplate
• Lower maintenance burden when types change
• Decrease chances of inconsistencies
• Support TypeScript's goal of type safety with minimal overhead

Is there any plan to implement this feature? It seems like a natural evolution of TypeScript's powerful type inference capabilities.

💻 Use Cases

1. What do you want to use this for? Write cleaner code
2. What shortcomings exist with current approaches? It would not be a breaking change
3. What workarounds are you using in the meantime? No workarounds, still writing again the parameters types for functions

[whzx5byb]

Duplicate of #32082.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.