Narrowing does not apply to array member / object element but should

[wbt]

Bug Report

🔎 Search Terms

narrowing array member object element

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about "Common "Bugs" That Aren't Bugs" and "Common Feature Requests"

⏯ Playground Link

Playground link with relevant code

💻 Code

const typeSafeJSFailsInTS = function(inputArray: ('' | Date)[], correspondingNames: string[]) {
	let result : {[index: string]: Date} = {};
	for(let index in inputArray) {
		if(inputArray[index] !== '') {
			result[correspondingNames[index]] = inputArray[index];
		}
	}
	return result;
}
const workaround = function(inputArray: ('' | Date)[], correspondingNames: string[]) {
	let result : {[index: string]: Date} = {};
	for(let index in inputArray) {
		const temporaryValue = inputArray[index];
		if(temporaryValue !== '') {
			result[correspondingNames[index]] = temporaryValue;
		}
	}
	return result;
}

🙁 Actual behavior

Error on line 5 (the most indented line):

Type '"" | Date' is not assignable to type 'Date'. Type 'string' is not assignable to type 'Date'. ts(2322)

This is wrong because it's inside a conditional block that narrows out the empty-string type from inputArray[index].

🙂 Expected behavior

The first example works just fine, with narrowing of the type working just as it does in the second.

Related issues

I don’t think this is a duplicate of #9998, despite how common duplicates are. This isn’t about other functions modifying the state or possibly expanding the type of a variable beyond a narrowing; there aren’t even multiple function calls involved. It’s not asynchronous, which was recently held to be a distinguishing feature of #9998.

I don’t think this is a duplicate of #18758 which is about narrowing the type of the parent based on the type/presence of deeper members, which is more complicated than what’s sought here, which is narrowing a specific member of an array rather than its parent.

[MartinJohns]

Duplicate of #10530. Type narrowing does not occur for indexed access forms e[k] where k is not a literal.