Show both narrowed type and declared type

Suggestion

🔍 Search Terms

List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily and help provide feedback.

✅ 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

📃 Motivating Example

https://twitter.com/Rich_Harris/status/1437490023763415043

is roughly a confusion on narrowed vs storage type of a given field/binding

// _: any
let _;

// _: any
_ = new Date();

// _: any, but completions from Date
// type doesn't match usability, only assignability
_.getFullYear();

💻 Use Cases

Current UI / typing only shows what can be assigned to a value storage location given a source text location. Need UI to show what can be used from a value storage location given a source text location.

Much of the time these should be the same but showing something about the narrowing would apparently remove some confusions, for example (bikeshed):

let _;
_ = new Date();

// _: any
_;

// becomes...

// _: any as Date
_;

[MartinJohns]

@justingrant I don’t see how a local variable has any relevant runtime footprint, but you can use a Type Assertion here:

return a as string;

That an unknown variable does not narrow on assignment is intentional.

[DanielRosenwasser]

One technical problem (I believe) is that in

   let x;
   x = new Date()
// ^?

by the point of assignment, control flow analysis hasn’t changed the type of x - it’s only after the assignment of x that we’d ever be able to show a different type. As a user, I would expect to see Date, but for all intents and purposes, I think you’d still end up with any.