max-function-length.md

llm-core/max-function-length

📝 Enforce a maximum number of lines per function to encourage decomposition.

💼 This rule is enabled in the following configs: 🌐 all, 🧮 complexity, ✅ recommended.

Enforce a maximum number of lines per function to encourage decomposition.

Rule Details

LLMs frequently generate monolithic functions with 100+ lines. This rule limits function length, forcing the LLM to extract helpers and decompose logic into focused, testable units.

Examples

Incorrect

// 200-line function doing everything
function processOrder(order: Order) {
  // validate input (20 lines)
  // check inventory (30 lines)
  // calculate pricing (40 lines)
  // apply discounts (25 lines)
  // process payment (35 lines)
  // send confirmation (20 lines)
  // update analytics (30 lines)
}

Correct

function processOrder(order: Order) {
  validateOrder(order);
  const inventory = checkInventory(order.items);
  const pricing = calculatePricing(order, inventory);
  const total = applyDiscounts(pricing, order.coupon);
  await processPayment(order.payment, total);
  await sendConfirmation(order.email, total);
  trackOrderAnalytics(order);
}

Options

max

Maximum allowed lines per function. Default: 50.

skipBlankLines

Whether to skip blank lines when counting. Default: true.

skipTestFiles

Whether to skip test files (.test.ts, .spec.ts, etc.). Default: true.

Test functions are intentionally verbose — setup, mocking, assertions — and benefit from being self-contained rather than decomposed.

{
  "llm-core/max-function-length": [
    "error",
    {
      "max": 40,
      "skipBlankLines": true,
      "skipTestFiles": true
    }
  ]
}

Error Messages

Error messages follow a structured teaching format designed for LLM self-correction:

1. What's wrong — identifies the function and its line count
2. Why — explains that long functions are hard to understand and test
3. How to fix — three decomposition strategies: extract helpers, single responsibility, separate setup/logic/cleanup