Bug: [no-unnecessary-condition] Unnecessary conditional when trying to use coalesce operator on null | undefined

[darkbasic]

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=5.5.2&fileType=.tsx&code=GYVwdgxgLglg9mABAWwJ4DFwQBQEMBOA5gFyJggA2FiAPouACYCmwMYTDAlKYy2x4gDeAKESJ8TKCHxIChRAH4F9MM1bsGwgL5A&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tiacTJTIAhtEK0yHJgBNK%2BSpPRgA2uGyRE0aB2iQANKuyYDaofHgcA7gGFJyfEJYAZDh2I3ps%2BU1QZ8cRMYQAL7GALqqIUFAA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Repro Code

function myFunc(arg: null | undefined): undefined {
  return arg ?? undefined
}

ESLint Config

module.exports = {
  parser: "@typescript-eslint/parser",
  rules: {
    "@typescript-eslint/no-unnecessary-condition": [
    "error",
      {
      "allowConstantLoopConditions": true
      }
    ]
  },
};

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

Expected Result

arg ?? undefined limits the union type null | undefined to just undefined so it's not unnecessary.

Actual Result

arg ?? undefined errors out like if it didn't restrict the union type a subset of its values.

Additional Info

No response

[kirkwaiblinger]

The rule is trying to point out that you could write this like

function myFunc(arg: null | undefined): undefined {
  return undefined
}

it reports both always-true and always-false conditions as unnecessary, too, like

declare const alwaysTrue: true;
declare const alwaysFalse: false;
// reported by the rule
if (alwaysTrue) {
}
// reported by the rule
if (alwaysFalse) {
}

[darkbasic]

Makes sense. My bad but I'm in the process of refactoring a big codebase with eslint-config-love and I'm a bit overflowed by over-reaching rules right now :)

[darkbasic]

@kirkwaiblinger sorry for reusing the same issue but I've also noticed the following:

let api: Axios

export function useApi(options: AxiosRequestConfig = {}): Axios {
  // Unnecessary conditional, the types have no overlap.eslint[@typescript-eslint/no-unnecessary-condition]
  if (api == null) {
    createApi(options)
  }
  return api
}

Typescript should be able to notice that no value has been assigned for certain to api and thus it can be undefined, right?

[kirkwaiblinger]

Ah - this is a pet peeve of mine - see #9565.

In short - no. let x: Axios means "x will be defined by the time I use it in a closure" and let x: Axios | undefined means "x may or may not be defined by the time it's accessed". This is a textbook case of where you must use let x: Axios | undefined in order to get correct types.

[kirkwaiblinger]

For future reference, these sort of general usage/support questions are best asked in our discord server 🙂

[kirkwaiblinger]

In short - no.

Or, rather - it should complain, but it doesn't! 😆

(note that I have yet to file the TS issue that I intended to file)