as const, but for JavaScript

[AlCalzone]

Suggestion

🔍 Search Terms

as const javascript

✅ 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

Several of newer useful TypeScript features are of limited use when writing JavaScript. As an example, the workaround for the recently re-opened #41631 is to use as const, which is not available in JS. Therefore I suggest to add the as const assertion to JS. For this purpose, the @type JSDoc could be reused:

const id1 = `my-tree.0.${someString}`; // inferred as string
/** @type {const} */
const id1 = `my-tree.0.${someString}`; // type: `my-tree.0.${string}`

📃 Motivating Example

See #41631

💻 Use Cases

In iobroker, objects in the database have different types that can be distinguished by how the ID is shaped. The different object types take different properties, so type-checking this would go a long way to reduce errors.

The type declarations already reflect this:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/81e930ed9983d081271ebf1978551b1e4c8b5b16/types/iobroker/iobroker-tests.ts#L649-L707
but this feature is limited to TypeScript devs who can use as const.

While these tests are hard-coded for string literals, in the real world most of our developers use JS and IDs are often built on the fly, e.g.

const id = `my-tree.0.${someString}`; // inferred as string
setObject(id, { /* I could really use a better type for this object */ });

If id was instead typed as my-tree.0.${string}, the object type in setObject could be more specific to the purpose it actually serves.

[jcalz]

#30445 ?

[AlCalzone]

Damn, my search must not be working :(