Yesterday, I was migrating type-fest’s XO configuration from package.json to a dedicated xo.config.js file. During this process, I noticed something odd: the XO VSCode extension stopped respecting @typescript-eslint rules when the configuration was moved to xo.config.js, even though it worked perfectly when the config was inside package.json.

And, after spending an absurd amount of time debugging, I realised that xo was mutating the config (as the PR already mentions) in setXoConfig (L213 and L216) while segregating @typescript-eslint rules from other rules.

Here's a minimal reproducible example that highlights the issue well:

// xo.config.js

const xoConfig = [
  {
    rules: {
      '@typescript-eslint/no-empty-object-type': 'off',
      '@stylistic/function-paren-newline': 'off',
    },
  },
];

export default xoConfig;

async function main() {
  const XO = (await import('xo')).default;

  const xo = new XO({ cwd: 'some/path' });
  await xo.lintText('', { filePath: 'some/path' });

  console.log(xo.xoConfig);
}

main().then(() => main()); // Call `main` twice 

The output for the first console.log has the rules correctly split:

[
  {},
  {
    rules: { '@typescript-eslint/no-empty-object-type': 'off' },
    files: ['**/*.{ts,tsx,cts,mts}'],
  },
  {
    rules: { '@stylistic/function-paren-newline': 'off' },
    files: ['**/*.{js,jsx,mjs,cjs,ts,tsx,cts,mts}'],
  },
];

However, during the split, the original config gets mutated:

• L213 removes the @typescript-eslint rules from the original rules object.
• And similarly, L216 adds the files property to the original object.

So, when the second instance of Xo is created, and setXoConfig is invoked, the flatOptions that it receives is the mutated one.

Hence, the second console.log prints the following:

[
  {},
  {
    rules: { '@stylistic/function-paren-newline': 'off' },
    files: ['**/*.{js,jsx,mjs,cjs,ts,tsx,cts,mts}'],
  },
];

So, here's a fix I came up with:

- // Set the other rules to the original config
- config.rules = Object.fromEntries(otherRules);

- // These rules should still apply to all files
- config.files = [allFilesGlob];

- return [tsConfig, config];

+ const otherConfig: XoConfigItem = {
+     ...config,
+     // Set the other rules to the original config
+     rules: Object.fromEntries(otherRules),
+ };

+ // These rules should still apply to all files
+ otherConfig.files = [allFilesGlob];

+ return [tsConfig, otherConfig];

It simply avoids mutating the original config, and this also seems to pass all the newly added tests.

Wish I read seen this PR before delving into this 😅, but anyways, thought I'd share my debugging here and it might add some value.

@som-sm Thanks much for digging in!

Funny enough this wasn't happening in the pre-release, something changed when the deps were updated for this v1 release but its not clear to me what would have caused it.