Typescript: `| undefined` should be part of type when getting items from an array

[lonewarrior556]

Search Terms: Array type

Code

Simplest code:

var a: string[] = ['a'];

var shouldBeString = a[0] // string 
var couldBeString = a[10000]; // string 
var notAString = a[Infinity]; // string 
var alsoNotAString = a[NaN]; // string 

I get the difficulty in discerning 0, 10, NaN and Infinity since the type of all those is number.. but wouldn't it make sense then to make the return type from these queries string | undefined?
For those who don't want that they can turn off strictNullChecks

Tuples are only slightly better

// tuple 
var str = '';
var aa = [str, str] as const

var _shouldBeString = aa[0] //string
var _notString1 = aa[10000]; // type error
var _notString2 = aa[Infinity]; // no error
var _notString3 = aa[NaN]; // no error

Expected behavior:

var arr = [''];
var item = arr[10];
// typeof item = string | undefined;

Actual behavior:

var arr = [''];
var item = arr[10];
// typeof item = string;

Playground Link:
TS-Playground

Related Issues:

[MartinJohns]

Duplicate of #13778.

[RyanCavanaugh]

Duplicate #13778

[RyanCavanaugh]

@MartinJohns easy dupe day eh

[MartinJohns]

@RyanCavanaugh I spend way too much time reading these issues in this repository, for quite some time already (using TypeScript since preview version), so I often recognize repeated issues. Just sometimes I fail to find the original issue.

[lonewarrior556]

@RyanCavanaugh @MartinJohns
Fine, yes part one complete overlap..

However I did not see this tuple behavior being address in the other thread.. Can't see how those arguments would apply here:

// tuple 
var str = '';
var aa = [str, str] as const

var _shouldBeString = aa[0] //string
var _notString1 = aa[10000]; // type error
var _notString2 = aa[Infinity]; // no error, type string
var _notString3 = aa[NaN]; // no error, type string

[RyanCavanaugh]

Infinity and NaN aren't specially typed; they're just number. See #32277 and #28682

[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.