• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

Playground

{
  "rules": {
      "@typescript-eslint/no-unnecessary-condition": [
      "error"
    ],
  }
}

let foo: string;

const setFoo = (): void => {
  if (foo?.length < 1) {
    foo = 'foo';
  }
};

tsconfig

{
  "compilerOptions": {
    "outDir": "./lib",
    "module": "commonjs",
    "target": "es2015",
    "lib": ["es6", "es7"],
    "sourceMap": true,
    "allowJs": false,
    "moduleResolution": "node",
    "forceConsistentCasingInFileNames": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noImplicitAny": true,
    "strict": true,
    "strictNullChecks": true,
    "suppressImplicitAnyIndexErrors": true,
    "noUnusedLocals": true,
    "noErrorTruncation": true,
    "pretty": true,
    "declaration": true
  },
  "include": [
    "src/**/*"
  ]
}

Expected Result

Existence check on a possibly uninitialized variable should not trigger no-unnecessary-condition

For sure, the type might be extended to let foo = string | undefined; as workaround, but that actually looks a bit uncommon for a let definition, where not initializing might be intended.

Actual Result
Unnecessary optional chain on a non-nullish value @typescript-eslint/no-unnecessary-condition

Versions