How long should a function be — the right answer

There's no single correct line count. But there are strong signals that a function has grown past its natural size.

The scroll test: if you have to scroll to read the entire function in your editor, it's too long. Your working memory degrades as you scroll. The entire function should fit on one screen — roughly 20-30 lines maximum.

The indentation test: count the levels of indentation. More than two levels (a loop inside a condition, for example) is a sign the function is doing too much.

// Three levels deep — extract something
function processOrders(orders) {
  for (const order of orders) {           // level 1
    if (order.status === 'pending') {     // level 2
      if (order.items.length > 0) {      // level 3
        // now doing work
      }
    }
  }
}

Extract the innermost logic:

function hasPendingItems(order) {
  return order.status === 'pending' && order.items.length > 0;
}

function processOrders(orders) {
  for (const order of orders) {
    if (hasPendingItems(order)) {
      processOrder(order);
    }
  }
}

Aim for 5-15 lines per function as a healthy default. Functions shorter than five lines are often fine — don't add padding. Functions longer than 20 lines need scrutiny but aren't automatically wrong.

The real metric is cognitive complexity: how much do you have to hold in your head while reading this function? If the answer is "a lot," the function is too long regardless of its line count.