Report useless check when accessing array elements in `no-useless-undefined`

Description

const foo = index >= 0 ? array[index] : undefined;
const foo = index < 0 ? undefined : array[index];

const foo = index <= array.length - 1 ? array[index] : undefined;
const foo = index > array.length - 1 ? undefined : array[index];

const foo = index >= 5 ? array[index - 5] : undefined;

const foo = array[index];

const foo = index >= 0 ? array[index] : bar;

Contributor guide

Tech stack

javascriptnodejs

Domain

developer experiencetooling

Issue type

feature

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-3 hours

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

fresh

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

mostly clear

Prerequisites

basic JavaScriptunderstanding of ESLint rulesfamiliarity with no useless undefined rule

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

60

Research direction

First, examine the existing `no useless undefined` rule implementation in `rules/no useless undefined.js` to understand its logic. Then, decide whether to extend this rule or create a new one based on the maintainer's feedback (the issue author is uncertain). Implement detection of patterns where an array element access (e.g., `array[index]`) is guarded by a ternary with `undefined` on one branch, which could be replaced by just the access. The fix should handle various comparisons like `>=0`, `<0`, `<=array.length 1`, etc. Refer to the linked issue in the comment for discussion.