Before You File a Bug Report Please Confirm You Have Done The Following...

• I'm using eslint-plugin-svelte. (*.svelte file linting does not work with the parser alone. You should also use eslint-plugin-svelte with it.)
• I'm sure the problem is a parser problem. (If you are not sure, search for the issue in eslint-plugin-svelte repo and open the issue in eslint-plugin-svelte repo if there is no solution.
• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.

What version of ESLint are you using?

9.3.0

What version of eslint-plugin-svelte and svelte-eslint-parser are you using?

• svelte-eslint-parser@0.36.0
• eslint-plugin-svelte@2.39.0

What did you do?

import tsEslint from 'typescript-eslint'
import epSvelte from 'eslint-plugin-svelte'
import svelteParser from 'svelte-eslint-parser'

export default [
  ...tsEslint.configs.strictTypeChecked,
  ...epSvelte.configs['flat/recommended'],
  {
    languageOptions: {
      parser: tsEslint.parser,
      parserOptions: {
        project: true,
        programs: false,
        extraFileExtensions: ['.svelte']
      },
    },
  },
  {
    files: ['**/*.svelte'],
    languageOptions: {
      parser: svelteParser,
      parserOptions: {
        parser: tsEslint.parser,
      }
    },
  },
]

<script>
  /** @param {number} x  */
  function test(x) {
    return x+1
  }
</script>

While both .svelte and .js files are scanned by ESLint the same function yields different results:

// test.svelte
<script>
  /** @param {number} x */
  function test(x) {
    return x+1
  }
</script>

// test.js
  /** @param {number} x */
  function test(x) {
    return x+1
  }

npx eslint src

test.js
   2:10  error  'test' is defined but never used     @typescript-eslint/no-unused-vars

test.svelte
    3:12  error  'test' is defined but never used  @typescript-eslint/no-unused-vars
    4:5   error  Unsafe return of an `any` typed value  @typescript-eslint/no-unsafe-return
    5:12  error  Invalid operand for a '+' operation. Operands must each be a number or string. Got `any`  @typescript-eslint/restrict-plus-operands

What did you expect to happen?

I expected the same code to produce the same warnings, irrespective of appearing in a .svelte or .js file.

What actually happened?

It seems like the typescript-eslint parser cannot parse type information from the code in the .svelte files. Maybe the svelte-parser strips JSDoc comments from the code before forwarding it to the typescript parser?

Link to GitHub Repo with Minimal Reproducible Example

https://github.com/falco467/svelte-jsdoc-typescript-example

Additional comments

Migrated from eslint-plugin-svelte since it is a parser problem: sveltejs/eslint-plugin-svelte#767