feat: adjust TS2691 message for .ts import sources

[ctjlewis]

The advice given by error TS2691 is confusing, and following its instructions produces output which throws ERR_MODULE_NOT_FOUND at runtime when outputting an ES module, i.e. when moduleKind >= ModuleKind.ES2015.

An import for a module located at ./test.ts must be specified as import ... from './test.js' in order to get a valid ES module out, but users are directed to rename it to ./test. This error message should be updated so users are given the correct advice based on their use case, and are not confused why their output throws despite the source being perfectly valid TS. (The ./test format can be used to generate valid CommonJS modules, just not ES modules.)

Closes #42151.

Current behavior

import { test } from './test.ts'

Currently throws TS2691 which says:

An import path cannot end with a '.ts' extension. Consider importing './test' instead.ts(2691)

This adjustment does silence the compiler, but the compiled JS looks like:

import { test } from './test';
console.log(test);

Which is not a valid ES module, as the import source is located at ./test.js. Executing the output throws ERR_MODULE_NOT_FOUND.

Suggested changes

This PR adjusts checker.ts to detect if the output is an ES module, and if so advises the user to replace ./import.ts with ./import.js instead of ./import.

After these changes, trying to import ./test.js while outputting an ES module throws:

An import path cannot end with a '.ts' extension. Consider importing './test.js' instead.ts(2691)

Which will compile to a valid ES module that behaves as expected.

[andrewbranch]

Can you add a test that shows this error? Thanks!

[ctjlewis]

Sure @andrewbranch - if you see this in time, is there a way to run just a single test in this workspace, rather than running the whole suite with gulp test?

[andrewbranch]

npx gulp runtests --tests substringOfTestFileName

[andrewbranch]

super sorry for pinging you

Don’t be! Contributions are always appreciated and we expect first-time contributors to have some questions 👍

[ghost]

All CLA requirements met.

[ctjlewis]

Suggested change for CJS module target.

[ctjlewis]

Suggested change for ES module target.

[andrewbranch]

Thanks @ctjlewis!