[swc] Allow keepClassNames configuration, or default to true

[ldiqual]

Desired Behavior

In https://github.com/TypeStrong/ts-node/blob/main/src/transpilers/swc.ts#L57-L88 there is no reference to keepClassNames which is required for popular libraries like TypeORM to work.

Proposal is to either:

1. Make jsc.keepClassNames = true by default
2. Allow setting keepClassNames somehow (maybe in ts-node.compilerOptions.keepClassNames)

Is this request related to a problem?

With ts-node v10 and swc 1.2.58, running typeorm migrations result in this kind of error:

Error during migration run:
Error: Entity metadata for Entity#field was not found. Check if you specified a correct entity object and if it's connected in the connection options.
    at /Users/ldiqual/Siteline/siteline/src/metadata-builder/EntityMetadataBuilder.ts:671:23
    at Array.forEach (<anonymous>)
    at EntityMetadataBuilder.computeInverseProperties (/Users/ldiqual/Siteline/siteline/src/metadata-builder/EntityMetadataBuilder.ts:666:34)
    at /Users/ldiqual/Siteline/siteline/src/metadata-builder/EntityMetadataBuilder.ts:118:56
    at Array.forEach (<anonymous>)
    at EntityMetadataBuilder.build (/Users/ldiqual/Siteline/siteline/src/metadata-builder/EntityMetadataBuilder.ts:118:25)
    at ConnectionMetadataBuilder.buildEntityMetadatas (/Users/ldiqual/Siteline/siteline/src/connection/ConnectionMetadataBuilder.ts:66:111)
    at Connection.buildMetadatas (/Users/ldiqual/Siteline/siteline/src/connection/Connection.ts:517:59)
    at Connection.<anonymous> (/Users/ldiqual/Siteline/siteline/src/connection/Connection.ts:193:18)
    at step (/Users/ldiqual/Siteline/siteline/node_modules/typeorm/node_modules/tslib/tslib.js:143:27)

This is what keepClassNames is for: swc-project/swc#1279

[cspotcode]

Thank you for the detailed report.

Your proposed option 1. seems the easiest for everyone. Are there any situations where a user would want to disable keepClassNames?

If we need to implement option 2, I think we can support it like this. The example below does not work today, but the necessary code change is small.

// tsconfig.json
{
  "ts-node": {
    // This is undocumented, but transpilers can accept an options object
    "transpiler": ["ts-node/transpilers/swc-experimental", {
      "jsc": {
        "keepClassNames": true
      }
    }]
  }
}

[cspotcode]

In the swc docs I see this:

    // Requires v1.2.50 or upper and requires target to be es2016 or upper.
    "keepClassNames": false

But that is no problem; we can automatically set it to false if target is too low.

[ldiqual]

I can't think of a case where one would not want to keep class names – other than maybe performance impact at the swc-transpilation level if they're doing extra processing. Solution 1. feels like a good path forward and I like the idea of setting the flag automatically depending on target.

[cspotcode]

other than maybe performance impact at the swc-transpilation level if they're doing extra processing.

Ok, I don't think anyone will notice a couple extra processor cycles in rust.

Thank you for the quick reply. I created a fix in #1344. I need to add tests before I can merge and publish to npm, but in the meantime, I included installation instructions on the pull request so you can test it today.