`tsconfig.json` with `allowJs: true` does not downlevel JS

I have the following tsconfig.json in a mixed project (js/ts):

{
  "compilerOptions": {
    "target": "es2019",
    "module": "commonjs",
    "moduleResolution": "node",
    "allowJs": true,
    "esModuleInterop": true,
    "strict": true,
    "outDir": "dist",
    "skipLibCheck": true,
    "sourceMap": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules"]
}

It seems like the ts transpiler is only being applied to the typescript files and the js files are taking a different path.

I only assume this because I have optional chaining in both the js and ts files and only the ts files get the proper conversion from ?. operation to chained null checks.

Start with the tsconfig.json options and trace how allowJs, target, and optional chaining are handled for the mixed JS/TS input described in the issue. Compare the JavaScript and TypeScript transpilation paths; done means JavaScript optional chaining is downleveled consistently with TypeScript when targeting es2019.