`export let` variable doesn't require the variable to be assigned within current file

[lionel-rowe]

🔎 Search Terms

export let, undefined, assignment, assigned

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about export let

⏯ Playground Link

No response

💻 Code

// a.ts
export let x: { foo(): void }

if (!!false) {
    // @ts-expect-error Variable 'x' is used before being assigned.
    x.foo()
}

// b.ts
import { x } from './a.ts'

// TS compiles, throws at runtime
x.foo()

🙁 Actual behavior

TS compiles, only throws at runtime.

🙂 Expected behavior

TS refuses to compile without ! non-null assertion, with error probably being reported in the exporting module a.ts rather than the importing module b.ts.

Additional information about the issue

No response

[MartinJohns]

I believe this is another duplicate of #9998.

[lionel-rowe]

I believe this is another duplicate of #9998.

Possibly? The case I'm interested in is where the exported variable is not even referred to elsewhere within the file. Given that export let variables can't be reassigned in files that import them (including by exporting functions that refer to those variables), that means it could (presumably?) be statically analyzed from a.ts alone that it will be undefined wherever it's imported, in the same way that attempting to access it directly from within a.ts gives a TS error.

[Andarist]

If anything, I'd say this should error in the declaring file. export is "exposing" the variable from a module just like return exposes it from a function:

function test() {
  let foo: { bar: string };

  return { foo }; // Variable 'foo' is used before being assigned.(2454)
}

[kirkwaiblinger]

Related: #60064, #60181, #55887