The Problem

Declare a function ‘sums’, which takes no arguments and returns another function. The returned function takes a number as an argument, and when invoked, it will return the total sum of the number passed in and the numbers that have previously been passed to the ‘sums’ function.

This is a simple, straightforward problem that demonstrates the concept of a persistent memory in a closure. count is declared in the outer function counter, and inner has access to count since it’s within counter’s scope.

One solution

// i: number
// o: sum of number already stored and number pased in

// declare function counter
function sums() {
  // declare variable called count set to zero
  let sum = 0;
  // return function with num passed in
  function inner(num) {
    // increment counter by num
    sum += num;
    // return counter
    return sum;
  }
  return inner;
}

const test = sums();
console.log(test(5)); // 5
console.log(test(10)); // 15
console.log(test(100)); // 115