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ESM Loader / TSX SourceMap Issue Demo

Steps to reproduce

Node.js

Use node v18 or v20. Node v16 is not affected by the issue.

Setup

cd tsx-sm-package
npm install
npm run tsc:build

cd ..

cd tsx-sm-app
npm install

Issue

The npm run tsx:withDynImport and the npm run esmLoader:withDynImport will show the notok file as a .js file with the line numbers corresponding to the state in build/notok.js:

TSX

> [email protected] start
> tsx ts-w-dyn-imp.ts

/.../tsx-sm-issue/tsx-sm-package/build/notok.js:2
    throw new Error('x');
          ^

Error: x
    at foo (/.../tsx-sm-issue/tsx-sm-package/build/notok.js:2:11)
    at <anonymous> (/.../tsx-sm-issue/tsx-sm-app/ts-w-dyn-imp.ts:4:1)
    at ModuleJob.run (node:internal/modules/esm/module_job:192:25)
    at async CustomizedModuleLoader.import (node:internal/modules/esm/loader:228:24)
    at async loadESM (node:internal/process/esm_loader:40:7)
    at async handleMainPromise (node:internal/modules/run_main:66:12)

ESM Loader

(node:85624) ExperimentalWarning: Custom ESM Loaders is an experimental feature and might change at any time
/.../tsx-sm-issue/tsx-sm-package/build/notok.js:2
    throw new Error('x');
          ^

Error: x
    at foo (/.../tsx-sm-issue/tsx-sm-package/build/notok.js:2:11)
    at file:///.../tsx-sm-issue/tsx-sm-app/js-w-dyn-imp.js:4:1
    at ModuleJob.run (node:internal/modules/esm/module_job:192:25)
    at async CustomizedModuleLoader.import (node:internal/modules/esm/loader:228:24)
    at async loadESM (node:internal/process/esm_loader:40:7)
    at async handleMainPromise (node:internal/modules/run_main:66:12)

Comparing to importing file without dynamic import

However, running npm run tsx:withoutDynImport or npm run esmLoader:withoutDynImport will show the ok file as a .ts file with the line numbers corresponding to the state in src/ok.tsx:

TSX

> [email protected] start2
> tsx ts-wo-dyn-imp.ts

/.../tsx-sm-issue/tsx-sm-package/src/ok.ts:4
  throw new Error('x')
        ^

Error: x
    at foo (/.../tsx-sm-issue/tsx-sm-package/src/ok.ts:4:9)
    at <anonymous> (/.../tsx-sm-issue/tsx-sm-app/ts-wo-dyn-imp.ts:4:1)
    at ModuleJob.run (node:internal/modules/esm/module_job:192:25)
    at async CustomizedModuleLoader.import (node:internal/modules/esm/loader:228:24)
    at async loadESM (node:internal/process/esm_loader:40:7)
    at async handleMainPromise (node:internal/modules/run_main:66:12)

ESM Loader

(node:85460) ExperimentalWarning: Custom ESM Loaders is an experimental feature and might change at any time

/.../tsx-sm-issue/tsx-sm-package/src/ok.ts:4
  throw new Error('x')
        ^

Error: x
    at foo (/.../tsx-sm-issue/tsx-sm-package/src/ok.ts:4:9)
    at file:///.../tsx-sm-issue/tsx-sm-app/js-wo-dyn-imp.js:4:1
    at ModuleJob.run (node:internal/modules/esm/module_job:192:25)
    at async CustomizedModuleLoader.import (node:internal/modules/esm/loader:228:24)
    at async loadESM (node:internal/process/esm_loader:40:7)
    at async handleMainPromise (node:internal/modules/run_main:66:12)

The only difference between the two cases is that notok.ts has a dynamic import (await import('some-package')), while ok.tsx has none.

Comparing to native Node.js behaviour

Indeed, we may assume the issue is with Node.js instead of TSX / ESM Loader, however if the below two other scripts are executed, we can see that sourcemaps are working fine with pure Node.js even with dynamic imports in the dependency tree:

npm run node:withDynImport

> [email protected] node:withDynImport
> node --enable-source-maps js-w-dyn-imp.js

/.../tsx-sm-issue/tsx-sm-package/src/notok.ts:4
  throw new Error('x')
        ^

Error: x
    at foo (/.../tsx-sm-issue/tsx-sm-package/src/notok.ts:4:9)
    at file:///.../tsx-sm-issue/tsx-sm-app/js-w-dyn-imp.js:4:1
    at ModuleJob.run (node:internal/modules/esm/module_job:192:25)
    at async DefaultModuleLoader.import (node:internal/modules/esm/loader:228:24)
    at async loadESM (node:internal/process/esm_loader:40:7)
    at async handleMainPromise (node:internal/modules/run_main:66:12)

The output shows the notok file as a .ts file with the line numbers corresponding to the state in src/notok.ts

npm run node:withoutDynImport Same as above, but this works even with TSX so only included for completeness: The ok file as a .ts file with the line numbers corresponding to the state in src/ok.ts

Note

The target files of the two Node.js and the two TSX scripts are identical, except for the file extension.