Add a keyword for skipping optional type parameters in generics (to fallback to the default definition)

[parzhitsky]

🔍 Search Terms

unset type parameter default

✅ 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

The suggestion is to add a keyword that says "I don't need to specify this optional type parameter, I am only passing by to a later one, please resolve to whatever the fallbace default type

📃 Motivating Example

Considering the case:

import express from 'express'

interface CreateUserParams {
    name: string
    age: number
}

export type CreateUserRequest = express.Request<
    Record<string, string>, // ❌ unrelated, but there's no way if skipping this
    any,                    // ❌ could've specified it, but decided to not bother
    CreateUserParams        // ✅ actually what is needed and relevant
>

Try it.

All of the type parameters of express.Request are optional. In the case when I only need to specify the third parameter, there is no way of skipping the first two. Instead, I have to go to the definition, see what the fallback type is, and provide the fallback type here explicitly; this gets words if the fallback type isn't exported.

Sometimes this can be mitigated by a better design, sometimes not.

The suggestion is to add a keyword (e.g., CSS's unset or SQL's default) that would do the lookup itself:

export type CreateUserRequest = express.Request<unset, unset, CreateUserParams>

This might be a breaking change for codebases where unset is an existing identifier. If that's a concern, consider ? as the keyword:

export type CreateUserRequest = express.Request<?, ?, CreateUserParams>

💻 Use Cases

1. What do you want to use this for? – Skipping optional parameters in generics defined by upstream libraries.
2. What shortcomings exist with current approaches? – The need to lookup the definition.
3. What workarounds are you using in the meantime? – I look the definition up.

[MartinJohns]

Duplicate of #26242.