ESLint 200-Line Max File Size Rule for Better AI Coding

custom-rules.js

/**
 * 🧠 My game-changing ESLint rule that makes AI coding 10x better:
 * - Enforces 200-line max file size
 * - Counts only actual code (ignores comments)
 * - Gives helpful refactoring suggestions
 * - Works perfectly with Cursor AI's "Fix in Chat"
 * 
 * Custom ESLint rule to limit file size to 200 lines
 * @type {import("eslint").Rule.RuleModule}
 */
export const maxFileLines = {
  meta: {
    type: 'suggestion',
    docs: {
      description: 'Enforce maximum file line count of 200',
      category: 'Best Practices',
      recommended: true,
    },
    schema: [], // no options
    messages: {
      tooManyLines:
        'File has too many lines ({{count}}). Maximum allowed is 200 lines. Please split this file into smaller, more focused modules with single responsibilities. Consider extracting utility functions, separating components, or moving complex logic into dedicated files.',
    },
  },
  create(context) {
    return {
      Program(node) {
        const sourceCode = context.sourceCode()
        const lines = sourceCode.lines || sourceCode.text.split(/\r?\n/)

        // Count non-empty, non-comment lines
        let codeLineCount = 0
        const comments = sourceCode.getAllComments()
        const commentLines = new Set()

        // Mark all comment lines
        comments.forEach(comment => {
          const startLine = comment.loc.start.line
          const endLine = comment.loc.end.line

          for (let i = startLine; i <= endLine; i++) {
            commentLines.add(i)
          }
        })

        // Count actual code lines
        for (let i = 0; i < lines.length; i++) {
          const lineNumber = i + 1
          const line = lines[i].trim()

          if (line.length > 0 && !commentLines.has(lineNumber)) {
            codeLineCount++
          }
        }

        if (codeLineCount > 200) {
          context.report({
            node,
            messageId: 'tooManyLines',
            data: {
              count: codeLineCount,
            },
          })
        }
      },
    }
  },
}

/**
 * Plugin object with our custom rules
 */
export const customRules = {
  rules: {
    'max-file-lines': maxFileLines,
  },
}

eslint.config.js

import js from '@eslint/js'
import { customRules } from './custom-rules.js'

/**
 * A minimal shared ESLint configuration with custom rules.
 *
 * @type {import("eslint").Linter.Config[]}
 */
export const config = [
  js.configs.recommended,
  {
    plugins: {
      'custom-rules': customRules,
    },
    rules: {
      'custom-rules/max-file-lines': 'warn',
    },
  },
]