Unexpected type error for accessing known existent members in a ReadonlyArray with noUncheckedIndexedAccess

[chrisirhc]

Bug Report

🔎 Search Terms

non-existent members, ReadonlyArray
Referenced #13778 (comment) .

🕗 Version & Regression Information

noUncheckedIndexedAccess was introduced in 4.1.

⏯ Playground Link

Playground link with relevant code

💻 Code

const x: ReadonlyArray<number> = [0, 1, 2];
const c: number = x[0]; /* Unexpected `Type 'number | undefined' is not assignable to type 'number'.
  Type 'undefined' is not assignable to type 'number'.`
*/

🙁 Actual behavior

It returned number | undefined type for all members of the ReadonlyArray.

🙂 Expected behavior

Expected known members to be typed number.

[tadhgmister]

I think this is expected, x[10] is also number | undefined since your type doesn't retain length information. [0,1,2] as const retains the length information and therefore x[0] is typed as number.

[fatcerberus]

That's right, ReadonlyArray is just a generic array type and doesn't include the length of the array as part of the type, nor does the control flow analyzer track that information (hence why noUncheckedIndexedAccess is optional). If you had gotten this array as an argument to a function call, for example, nothing in its typing says those elements have to be present, so the error is correct.

[RyanCavanaugh]

ReadOnly also doesn't mean immutable. You could get back an array from a function that still is able to mutate it later.

The behavior you expect can be achieved from

const x = [0, 1, 2] as const;
const c: number = x[0]; 

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.