prefer-at

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Keywords: Array.at(), negative index, last element, ESLint rule, ES2022, auto-fix, LLM-optimized

Prefer using Array.at() for accessing elements, especially with negative indices

Prefer using Array.at() for accessing elements, especially with negative indices. This rule is part of eslint-plugin-modernization.

Quick Summary

Aspect	Details
Severity	Warning (modern JavaScript)
Auto-Fix	✅ Yes (converts to .at())
Category	Modernization
ESLint MCP	✅ Optimized for ESLint MCP integration
Best For	ES2022+ codebases, cleaner array access

Rule Details

Array.at() provides a cleaner way to access array elements, especially the last element or elements from the end.

Why This Matters

Issue	Impact	Solution
📖 Readability	`arr[arr.length - 1]` is verbose	Use `arr.at(-1)`
🐛 Off-by-one errors	Easy to make mistakes	Negative indices
🔄 Consistency	Multiple patterns in codebase	Standardize on .at()

Examples

❌ Incorrect

// Accessing last element
const last = array[array.length - 1];

// Second to last
const secondLast = array[array.length - 2];

// Dynamic negative access
const item = array[array.length - offset];

✅ Correct

// Clean last element access
const last = array.at(-1);

// Second to last
const secondLast = array.at(-2);

// Dynamic negative access
const item = array.at(-offset);

// Also works with strings
const lastChar = string.at(-1);

Configuration Examples

Basic Usage

{
  rules: {
    'architecture/prefer-at': 'warn'
  }
}

prefer-node-protocol - Modern Node.js imports

Known False Negatives

The following patterns are not detected due to static analysis limitations:

Dynamic Variable References

Why: Static analysis cannot trace values stored in variables or passed through function parameters.

// ❌ NOT DETECTED - Value from variable
const value = externalSource();
processValue(value); // Variable origin not tracked

Mitigation: Implement runtime validation and review code manually. Consider using TypeScript branded types for validated inputs.

Imported Values

Why: When values come from imports, the rule cannot analyze their origin or construction.

// ❌ NOT DETECTED - Value from import
import { getValue } from './helpers';
processValue(getValue()); // Cross-file not tracked

Mitigation: Ensure imported values follow the same constraints. Use TypeScript for type safety.