Map containing array with strict nullable doesn't work right

[delosford]

🔎 Search Terms

map containing array may be null or undefined

🕗 Version & Regression Information

This is the current behavior even in 5.5.1-rc

⏯ Playground Link

No response

💻 Code

function itsaFunction()
{
	let someMap = new Map<(String | null), (Array<String> | null)>();
	const someString = "someString";

	if (someMap.get(someString) !== null && someMap.get(someString) !== undefined)
		console.log(someMap.get(someString)[0]);

}

🙁 Actual behavior

Compiler complains that the result of the Map.get operation could be null or undefined

🙂 Expected behavior

The null and undefined check should be recognized as ensuring that the Map.get operation does not return a null or undefined.

Additional information about the issue

I posted a similar issue recently about multidimensional arrays, but that was solved by 5.5.0 beta/5.5.1-rc. This problem is not solved by them.

[fatcerberus]

.get(someString) from the perspective of the compiler is just another method call. There’s no way for it to know it returns the same result every time. So you can’t narrow it that way, and I probably wouldn’t write the code this way anyway. Call .get() once, assign the result to a variable, and narrow that.

[RyanCavanaugh]

+1 to the above comment. I'm not even sure given the capability, we would do it -Maps are mutable and the key being there on one call isn't a guarantee it's there in another call. FWIW there's another open feature request which I can't find (because the keywords are useless) about using has as a narrowing method for subsequent get calls.

[jakebailey]

That'd be #13086 and kinda #9619.

[typescript-bot]

This issue has been marked as "Design Limitation" and has seen no recent activity. It has been automatically closed for house-keeping purposes.