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=4.9.5&sourceType=module&code=MYewdgzgLgBCBGArAXDA3jADgJxJg-KgNpgCuAtvAKbYC6MAvjALzoMDcAUJwGaljAoAS3AwIVKAHkBVABQA3AIYAbUlVRlKNAJTFN1Ouk4wY2CaWxg4SAHQ48MfPlZElqqrXYwA9N5gBCGAABKABPTCoIYGwhTCgAWkjlITAobzAQeP4wKmBIiEVsUPjQMAATIWFwVAAiAFUwHLyIAqKYUoqqsBUAGhgqAA8IwSoymGUqHgSAC0VysSEyqjgeGAADJzW4COxFKBBsGH2YaiwQFqF4ZVCYMmVlOEP+JZ4U0ZsazgZOIA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tiacTJTIAhtEK0yHJgBNK+SpPRRE0aB2iRwYAL4gtQA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoAkNZOgB4CG0euBxTqACprIljwBPACpCBZSnkwBXdABpwEKCWnJWAORmpUAYQAW6aAGtJYaXKUBfEFaA

Repro Code

const obj: { prop?: [number] } = {};

function setOnce(value: number): [number] {
    return obj.prop ??= [value]; // ! @typescript-eslint/no-unnecessary-condition: "Unnecessary conditional, expected left-hand side of `??` operator to be possibly null or undefined."
}

ESLint Config

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

tsconfig

{
  "compilerOptions": {
    "exactOptionalPropertyTypes": true,
    "strictNullChecks": true
  }
}

Expected Result

I expected the expression obj.prop ??= [value] to not report an error.

Since the property hasn’t been set yet, it is actually undefined at runtime.

Actual Result

The expression obj.prop ??= [value] is reported as an error.

typescript-eslint thinks obj.prop is non-nullish since exactOptionalPropertyTypes is on. We don’t want to turn it off because we still don’t want to allow statements like obj.prop = undefined;.

Additional Info

Related: #3553, #5344, #6632