importsNotUsedAsValues in JS files elides imports incorrectly

[alan-agius4]

TypeScript Version: 4.0.0-dev.20200508

Search Terms:
importsNotUsedAsValues
import eliding

Code

In TypeScript 3.8, a new importsNotUsedAsValues feature/compilerOption was added. This, however is producing different output based on if the file is .ts or .js.

tsconfig.json

{
  "compilerOptions": {
    "allowJs": true,
    "noResolve": true,
    "importsNotUsedAsValues": "preserve",
    "target": "ES2015",
    "outDir": "./out",
    "types": []
  }
}

test.js

// @ts-nocheck
import * as moment from 'moment';
import rollupMoment__default from 'moment';
export const moment = rollupMoment__default || moment;

Output

// @ts-nocheck
import * as moment from 'moment';
import 'moment';
export const moment = rollupMoment__default || moment;

Removing the importsNotUsedAsValues option, which is equivalent of having the value of remove will produce:

// @ts-nocheck
import * as moment from 'moment';
export const moment = rollupMoment__default || moment;

Changing the file extension from .js to .ts will produce the correct output for both cases

// @ts-nocheck
import * as moment from 'moment';
import rollupMoment__default from 'moment';
export const moment = rollupMoment__default || moment;

Expected behavior:

• Used imports are not elided.
• Given the same input it should produce the same output, irrespective of the file extension.

Actual behavior:

• Given a JS input the import is elided incorrectly.

Playground Link: Playground doesn't have the importsNotUsedAsValues options.
Related Issues: None that I can find.

[alan-agius4]

Note: this bug is also present in 3.8 & 3.9.