Unexpected narrowing of optional fields via Object.entries

[defunctzombie]

🔎 Search Terms

object.entires incorrect type narrowed

🕗 Version & Regression Information

I can experience this on v5.8.3. I also checked back to 5.0.4 on the playground and see the same behavior

⏯ Playground Link

https://www.typescriptlang.org/play/?target=10&ts=5.8.3#code/GYVwdgxgLglg9mABMMAKYc4C5EG9EBGAhgE4D8OYIAtgQKYmIA+i4AJncDGHW4gL4BKPAChEiAPQTEUAJ4AHOgGcIJGPKgyAFtwDWS7TANHEAAwDaSqGrABzADSIqtBgF1zr04gDuWukiJoECIAGxDZRBhNCCIkejNLa24HJxp6RhZ2Tm5ed08xRAgEK0RSRgBeRAB5AgArOmgAOn8k5XRMQQBuAqKwJTgQukaQuFtUMq6RfhERFFRcAuISHCyuHjYprqA

💻 Code

function fn(foo: { bar?: number | undefined }) {
  // typescript thinks _arr_ is `[string, number][]` when actually it is `[string, number | undefined][]`
  const arr = Object.entries(foo);
  console.log(arr);
}

fn({
  bar: undefined
});

🙁 Actual behavior

The type from Object.entries is narrowed

🙂 Expected behavior

Object.entries(foo) produces a type output that is [string, number | undefined][] when foo has type { bar?: number | undefined }

Additional information about the issue

No response

[MartinJohns]

You need to enable exactOptionalPropertyTypes.

[defunctzombie]

You need to enable exactOptionalPropertyTypes.

@MartinJohns could you elaborate on why that option would change the behavior here? From my perspective Object.entries has dropped the notion that one of the fields could be set to undefined and that would be a valid value for it. I understand the idea that it could be omitted entirely - but not being present is different than being set to undefined.

[snarbles2]

Without exactOptionalPropertyTypes there is no distinction between {prop?: someType} and {prop?: someType | undefined}; the latter behaves as if the former.

not being present is different than being set to undefined

That is the distinction that exactOptionalPropertyTypes allows for.