Rule proposal: Prefer array.at(...) over array[...] as T | undefined

[JoshuaKGoldberg]

Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

Array index lookups aren't type-safe without the often-overly-strict noUncheckedIndexedAccess: they give back T, not T | undefined. A lot of TS code that predates Array.prototype.at() had to look like [index] as T | undefined instead of .at(index). Example in our repo:

typescript-eslint/packages/eslint-plugin/src/rules/prefer-optional-chain-utils/analyzeChain.ts

Line 97 in eef257b

const nextOperand = chain[index + 1] as ValidOperand | undefined;

Now that we have .at(), I think it makes sense to enforce that: if an array index lookup uses a type assertion to add | undefined, it should use .at instead.

Fail Cases

declare const values: string[];
declare const index: number;

values[index] as string | undefined;

Pass Cases

declare const values: string[];
declare const index: number;

values.at(index);

Additional Info

This is vaguely familiar to the idea of @typescript-eslint/no-unnecessary-type-assertion. But it includes a runtime change.

[kirkwaiblinger]

big +1 ❤️

• Should this also have have any options around prohibiting .at(i)!, or preferring [i] or .at(i) in general? I'm thinking not since .at(i)! is much more useful (e.g. with negative indices), and changing to [i] is a bigger, less safe logic change than [i] as T | undefined to .at(i). And as you noted, noUncheckedIndexAccess, already exists.

• Due to the runtime changes, let's be sure to provide a suggested fix, and not an autofix?

[bradzacher]

We didn't used to use this cos we supported old nodejs versions!
Up until our v7 release our min was just v16.0.0 - but .at was only added in v16.6.0
But now that our min is v18 we can safely use this everywhere!