Docs: [prefer-nullish-coalescing] doesn't work with strictNullChecks: false.

[jsalvata]

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

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=4.8.4&sourceType=module&code=DYUwLgBAHgXBDOYBOBLAdgcwgHwgVzQBMQAzdEQgbgCgodcByBmoA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Y6RAM0WlqYSNkAC1pkA9gEMkyMswDm6KL2jjokcGAC+ILUA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylAGYBDVCXTgwAXxCygA

Repro Code

let x: string | undefined;
x || '';

ESLint Config

module.exports = {
  parser: "@typescript-eslint/parser",
  "rules": {
    "@typescript-eslint/prefer-nullish-coalescing": "error"
  }
};

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": false
  }
}

Expected Result

An error should be reporting for the ||, which should be replaced with ??.

Actual Result

No error is reported.

Additional Info

The same rule works if strictNullChecks is changed to true.

This makes sense because strictNullChecks: false essentially erases | null and | undefined from all types.

Ideally the rule should work anyway, assuming all types to be nullable. If that's not possible, a note should be added to the documentation so people do not spend a whole afternoon trying to understand why the rule doesn't work, as I did.

Versions

package	version
@typescript-eslint/eslint-plugin	5.39.0
@typescript-eslint/parser	5.39.0
TypeScript	4.4.4
ESLint	7.32.0
node	12.22.6

[bradzacher]

This is expected.
With the compiler option turned off TS essentially erases the nullish types from everything. So when we inspect the types - they don't include the nullish types.

Use the flag - it's a VERY bad practice to not use it.

[jsalvata]

Even if working as intended, which I agrree with, a note should be added to the documentation for the sake of the rule user's.

Bad practice indeed, but the flag is there for a reason: sometimes it is unavoidable, at least temporarily.

[JoshuaKGoldberg]

True! Marking as accepting PRs for a docs update to mention this on https://typescript-eslint.io/rules/prefer-nullish-coalescing.

At least one other rule has a similar limitation. You can probably copy the phrasing from it/them.

[bradzacher]

We should give it the same treatment as rules like strict-boolean-expressions where the rule will explicitly error if you don't have the option turned on.

https://typescript-eslint.io/rules/strict-boolean-expressions/#allowruletorunwithoutstrictnullchecksiknowwhatiamdoing

I would say we can do that change without a breaking change because the rule is LITERALLY useless without the option.

[JoshuaKGoldberg]

For anybody interested in this as a good first issue: feel free to do either or both of:

• Adding an explicit notice to the docs page (prefer-nullish-coalescing.md)
• Adding a similar option to the rule (prefer-nullish-coalescing.ts)

We can file a followup issue if you only tackle one and not the other. 🙂

[JoshuaKGoldberg]

Reopening so @cparros can tackle the second part 😄