dynamically imported module without import / export is treated as script by tsc

[kt3k]

(Note: This probably doesn't affect any meaningful use cases, but I create issue for record.)

When we have the following scripts:

a.ts

const a = 1;
await import("./b.ts");

b.ts

const a = 2;

Then execute a.ts and it causes the following error.

$ deno run --allow-read a.ts
error: TS2451 [ERROR]: Cannot redeclare block-scoped variable 'a'.
const a = 2;
      ^
    at file:///Users/kt3k/0/di-test/b.ts:1:7

    'a' was also declared here.
    const a = 1;
          ^
        at file:///Users/kt3k/0/di-test/a.ts:1:7

This doesn't happen when:

• b.ts has any import or export statement
• import of b.ts is not dynamic import

---

$ deno --version
deno 1.7.5 (release, x86_64-apple-darwin)
v8 9.0.123
typescript 4.1.4

[bartlomieju]

For the record this is TSC error and not error coming from V8.

EDIT: This error doesn't happen if files are .js. Luca suggests that it is TSC that treats dynamically imported file as a script not a module.

[nayeemrmn]

This is an example of where simply ignoring TS2306:

deno/cli/tsc/99_main_compiler.js

Lines 145 to 147 in ae8874b

	// TS2306: File 'file:///Users/rld/src/deno/cli/tests/subdir/amd_like.js' is
	// not a module.
	2306,

is not enough, and we need a real way of telling the TypeScript compiler out-of-band that all source files should be treated as ES modules. Unfortunately no such compiler option is provided by TypeScript, so we tell users to work around this by adding export {}; in the imported module.

[nayeemrmn]

Ref microsoft/TypeScript#18232.

[kitsonk]

That old problem. There is no easy way around it except export {}, but is dynamically importing a module that doesn't export anything actually a common realistic problem?

I suggest we simply add a note to the docs FAQs about this know TypeScript issue and be done with it.

[nayeemrmn]

Ref microsoft/TypeScript#18232 (comment).

[kitsonk]

As stated in the response, the option doesn't fix the issue and can't fix the issue as tsc is currently artchitected.