Guarding against object value not being undefined fails when object value is typed with `Partial<T> | undefined`

[bencooper222]

Bug Report

🔎 Search Terms

• full partial type guard union
• enum as keys to interface with values possibly undefined or
• undefined guard not compatible with Partial<T> | undefined

🕗 Version & Regression Information

I reproduced this on TS4.5, 4.8, 4.9, and 5.1.0-dev.20230306

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Partials and undefined type guards

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.1.0-dev.20230306#code/JYOwLgpgTgZghgYwgAgCoQM5mQbwFDLIBGANnABYBcyWUoA5gNwHIwD2b1tDzAvnnlCRYiFAjbg4oaLhYBtANYQAnlzB0Q9ALrUACnChhgcEgB50WAHzIAPsgCuIACYQY0p3wHiQWZGyIAVtTeYFIgMgC8soRw1PiEhKQU1ABEcCkANCyE7JzIKUSZLLxZvMx43r7qyqjkmChRcmmZ+YVa5exQyAAUldhgbKhQyn4wyNW19QCU0cjAY93+AXIDQ8payACEUY4ubuFOM-EJlWwkEAB0JGz0i4Erg8NaF0nkU8yE-PxAA

💻 Code

interface Test {
  blah: string;
  foo: string;
}

interface container {
  [key: string]: Partial<Test> | undefined;
}

const obj: container = {
  a: {
    blah: "a",
    foo: "b",
  },
};

const tryThese = ["a", "b"];

for (const toTry of tryThese) {
  if (obj[toTry] != undefined) {
    // next line has error "Object is possibly 'undefined'.(2532)"
    console.log(obj[toTry].blah);
  }
}

🙁 Actual behavior

I specifically check if obj[toTry] is not undefined but TS still throws an error that obj[toTry] is potentially undefined. I suspect this is because Partial<T> is a superset of undefined.

🙂 Expected behavior

I want TS to not throw an error that obj[toTry] is undefined.

[fatcerberus]

You have to assign obj[toTry] to a variable and test that. It's a known limitation that TS can't narrow expressions of the form obj[key] unless key is a hard-coded literal.

for (const toTry of tryThese) {
    const value = obj[toTry];
    if (value != undefined) {
        console.log(value.blah);  // now it works
    }
}

I suspect this is because Partial<T> is a superset of undefined.

No, Partial<T> only makes properties optional; you can't assign undefined to it.

[RyanCavanaugh]

See #10530

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.