The closure (internal function) here allows for the counter to keep track of how many times the function has been called. - In order for it to work, the function first has to be asigned to a variable as a function expression, with that being called instead of the function directly.

Closure example for counter.js

function myFunc() {
  let count = 0;
  return function () {
    if (count < 3) {
      console.log(count);
      count++;
    } else {
      console.error('maximum count has been reached');
      throw 'max count';
    }
  };
}

Example

const instanceOfCount = myFunc();

console.log(instanceOfCount()); // 0
console.log(instanceOfCount()); // 1
console.log(instanceOfCount()); // 2
console.log(instanceOfCount()); // Error: max count // "maximum count has been reached"