import defer Stage 3 proposal support

[nicolo-ribaudo]

🔍 Search Terms

import defer

✅ 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

We are working towards a PR implementing the https://github.com/tc39/proposal-defer-import-eval/ proposal, to have TypeScript support ready once the proposal reaches Stage 3. For this proposal I think it should behave as follows:

• This features does not affect type checking, as import defer * as n and import * as n have the same type.
• import defer should only be allowed when targeting ESNext modules, as it's not possible to compile it to older ES versions.
• Even though the proposal only supports import defer * as n, we should still parse import defer { ... } and import defer foo and give proper diagnostics, rather than relying on the existing parsing recovery for invalid syntax (that would parse import defer { x } from "y" as import defer = <ident>; { x }; from; "y"

When talking about this with @DanielRosenwasser, he mentioned tsserver support. What would tsserver support look like for a feature like this one? I was thinking maybe a "Convert to deferred namespace import" refactor similarly to how we have a "Convert named imports to namespace import" refactor, but:

• is this something that the TS team prefers to have in the initial implementation or separately?
• am I missing something else that is obvious when it comes to tsserver support?

📃 Motivating Example

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💻 Use Cases

Implementation of a TC39 proposal

[robpalme]

The import defer proposal advanced to Stage 3 in Feb 2025, so the title of this issue should be updated @nicolo-ribaudo

[nicolo-ribaudo]

Question for the TS team: with the go rewrite, how do contributions work? Do I need two PRs, or just one? To which code base?