"delete" does not change the type but only reports error

[kkmuffme]

🔎 Search Terms

object delete TS2790 optional property

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries

⏯ Playground Link

https://www.typescriptlang.org/play?#code/FDBmFcDsGMBcEsD2kAEpGIBQoIYCcBzALhQCMMAbAUx1QEoUBvYFVlaZAZ1jXiooAmnFAF4mwAJASOUWHgCeJAOQ4lAGkkTuOWFWWqNU6PFiKUK9ZoAOibhwF7zBzTgEC8VTpwD6ARn2WEgC+ANwgbCjwoCiY+ASiImJy4FQMzBERDtS6vPxCAHSu7p4+vmERQeFsHJDcKAAeormCnGFBQA

💻 Code

function foo( arg: boolean ) {
    const fields = {
		country: 'a',
		state: 'a',
		city: 'a',
		postcode: 'a',
		address_1: 'a',
	};

    if (arg === true) {
        delete fields.address_1;
    }

    const x = fields;
}

🙁 Actual behavior

address_1 in x is non-optional

🙂 Expected behavior

address_1 in x should be optional with ? since in some cases it is deleted.

Typescript reports an error for this, however that doesn't mean that delete doesn't change the type.
e.g. if errors are ignored for this, the type will be wrong and lead to bugs downstream.

Additional information about the issue

Followup to #13783

[fatcerberus]

if errors are ignored for this, the type will be wrong and lead to bugs downstream.

I mean, this is true of pretty much all TS errors, since that’s the entire point of the type checker. “If you ts-ignore this error you might shoot yourself in the foot” isn’t really a good argument for changing the behavior.

[MartinJohns]

This sounds like a duplicate of #53697. Or more accurately #51340.

if errors are ignored for this, the type will be wrong and lead to bugs downstream.

That basically applies to all errors.