Inner types

[FilipeBeck]

Suggestion

🔍 Search Terms

• inner types
• nested types

✅ Viability Checklist

My suggestion meets these guidelines:

• 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 feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A kind of "syntax sugar" to merge declarations. It is equivalent to using namespace. Some possibilities:

/**
 * Classes and interfaces 
 */

class Listing <T> {
    inner interface Pagination {
        page: number
        totalPages: number
        records: T[]
    }
    // `Pagination` is implicitly `Pagination<T>` 
    constructor(private pagination: Pagination) {}
}

const aPagination: Listing<string>.Pagination = {
    // ...
}

/**
 * Functions.
 */

function fetch(method: fetch.Method) { // Maybe, just `fetch(method: Method)` is enough
    inner type Method = 'GET' | 'PUT' | 'POST'

    const request = // ...
}

const aMethod: fetch.Method = 'GET'


/**
 * Types.
 */
type Payload = Payload.User | Payload.Address { // Maybe, just `Payload = User | Address` is enough
    inner interface User {
        // ...
    }

    inner interface Address {
        // ...
    }
}

const aPayload: Payload = Payload.Address

• Maybe inner is not the better keyword name.
• Maybe a access level for classes (public, protected, private).

[jcalz]

Seems like #7061

[tjjfvi]

Also #17588

[MartinJohns]

Also somewhat related: #41470.

---

I find the idea of "local" types declared within the implementation body to leak outside extremely unpleasant and disturbing. This would also require additional new syntax for declaration files, which was not considered at all in this issue. I hardly see any benefit and a lot of complexity.

And unfortunately the issue template for feature requests was not filled out.

[typescript-bot]

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