Each Call Gets Its Own Variables

In this video, you saw why a counter declared outside a function gives a wrong count on the second call, and how declaring it inside the function fixes that. Here’s what to keep from it.

Key Points

• When the counter is declared outside the function, every call shares that one count. The delivery call leaves it at 2, so the billing call starts from 2 and reports 3 instead of 1. Nothing crashes, so the wrong number can slip into a report.
• The fix is to move one line: declare let count = 0; inside the function, at the top of its body.
• Each call gets its own variables, like a fresh room: the parameters messages and topic hold that call’s arguments, and count starts at 0. When the call returns, the room closes and its count is gone.
• count is declared inside countTopic(), so it can only be used inside that function’s braces. The code that calls the function can’t read it, so a line after the calls that logs count stops with ReferenceError: count is not defined. To use a count outside, return it and keep it in deliveryCount or billingCount.
• The rule: declare a variable inside the function that needs it, and every call gets a fresh one. When you read a reporting function, whoever wrote it, check where each counter is declared.

Final Code

const messages = [
  { id: "M-101", topic: "delivery", text: "Box came late" },
  { id: "M-102", topic: "billing", text: "Charged twice" },
  { id: "M-103", topic: "delivery", text: "Wrong address" },
];

function countTopic(messages, topic) {
  let count = 0;
  for (const message of messages) {
    if (message.topic === topic) {
      count = count + 1;
    }
  }
  return count;
}

const deliveryCount = countTopic(messages, "delivery");
const billingCount = countTopic(messages, "billing");
console.log("delivery", deliveryCount);
console.log("billing", billingCount);