Type-check Import Attributes in static imports

[mkubilayk]

🔍 Search Terms

import attributes, import assertions

✅ 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 feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

TypeScript 5.3 Beta supports the latest updates to the import attributes proposal, which is great to see!

It's possible to augment the global ImportAttributes type with host-specific attributes. However, auto-complete and type-checking of import attributes seems to only work with dynamic import(). It would be nice if this could also work for attributes in static declarations.

📃 Motivating Example

https://www.typescriptlang.org/play?target=99&ts=5.3.0-dev.20231002#code/CYUwxgNghgTiAEBzCB7ARlC8DeBYAUPPAJYB2ALiDAGZRgLwCSAtgA4ozkCC55MxaAK6UAzjgJEi5AJ6sQALngAiAFYiUpJRPgBfAnvwFibDuXgAqeFDGkx1GCmbLqKFEvgB3YuQAWOeDJyikqkKOQAtGoa7joA3AQAbijEwPC28Yb4xuycABRKLm4ANOKEnt4+inhlUrIKyqERUZraBjoAlBkEQA

declare global {
    interface ImportAttributes {
        type: "json";
    }
}

import * as ns from "foo" with { type: "not-json" }; // no error
void ns;

import("foo", {
    with: {
        type: "not-json", // Type '"not-json"' is not assignable to type '"json"'. (2322)
    },
});

💻 Use Cases

1. What do you want to use this for?

https://github.com/tc39/proposal-import-attributes#motivation

In addition, we have an internal use case in Bloomberg to use Import Attributes as an annotation to achieve lazy-loading by deferring module evaluation. This is an early implementation of the TC39 proposal for Deferring Module Evaluation that annotates the static import.

It would be a better developer experience if auto-complete was available for Import Attributes in the static form.

2. What shortcomings exist with current approaches?

It is possible to write a static import declaration using unknown/invalid attributes.

3. What workarounds are you using in the meantime?

None.

[RyanCavanaugh]

Consensus from the design meeting was that we just "forgot" to do this.

Additional point for type authors - use caution when defining type like this. The correct way to make an "open-ended union", as should be used here, is to make a dummy interface and use keyof so that other declarations can introduce new items to the union:

  interface ImportAttributes {
    type: keyof AllowedTypes;
  }
  // In my .d.ts
  interface AllowedTypes {
    "json": unknown;
  }
  // In other .d.ts
  interface AllowedTypes {
    "png": unknown;
  }