Array.at() shows weird results when used with tuple

[ronaldruzicka]

Bug Report

I found multiple issues with .at() when used with tuples. Since tuple is strictly typed array by indexes, I think that the type inference with .at() should be better.

🔎 Search Terms

• array
• array.at()
• at()
• tuple
• array.at() with tuple
• tuple at()

🕗 Version & Regression Information

Version: 4.8.2 (also 4.9.0-dev.20220921)

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about arrays and tuples with .at()

⏯ Playground Link

Playground link with relevant code

💻 Code

const tuple: [string, number] = ['1', 2]

// 1. Shows that "a" can be undefined, but should be a string
// 2. Shows that "a" can be number, but should be string
const a = tuple.at(0)
const b = tuple[0] // this is correct

// 3. Shows type error with bracket notation but no type error with .at()
const d = tuple.at(3)
const c = tuple[3] // shows type error which is correct

🙁 Actual behavior

When I use .at() with tuple, it wrongly infers type.

• It shows string | number | undefined when used with an existing index.
• It doesn't show a type error when used with an index that doesn't exist

🙂 Expected behavior

I would expect, that it works the same as with bracket notation [] when used with a tuple. Because it knows what type is at what index. Also if that index exists in that tuple.

[MartinJohns]

Duplicate of #47660. Used search terms: at tuple in:title

[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.