Array Type Inference

[cinderblock]

TypeScript Version: 3.3.0-dev.20181222

Search Terms:

• array
• type
• inference
• index

Code
Playground Link

const typed = ['a', 1] as [string, number];
typed[0].length; // Works
typed[1].toPrecision; // Works
typed[2]; // Errors as expected

const array = ['a', 1];
array[0].length; // Error
array[1].toPrecision; // Error
array[2]; // Should error

Expected behavior:

TypeScript infers type of array as [string, number].

Actual behavior:

TypeScript infers type of array as (string | number)[].

Related Issues:
#25516

[fatcerberus]

Known issue - array literals are inferred as a union of their member types. I don't think it's considered a bug.

[cinderblock]

I see. I looked through the FAQ pages of other bugs and did not find exactly equivalent.

I'll let them close this if this is desired behavior.

I'm curious what reasons there are for this not being the desired behavior!

[fatcerberus]

It's been discussed on the TypeScript gitter several times in the past, and I was pretty sure there was a GitHub issue that described the design challenges at length, but I can't find it now...

See also #27179, #16656.

[weswigham]

Aye, the two links @fatcerberus has - #27179 and #16656 are where this problem is being discussed.

[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.