When we have a loop running inside another loop, we call that a nested loop.

One use of a nested for loop is to compare the elements in two arrays. For each round of the outer for loop, the inner for loop will run completely.

Below, we have an example of a nested for loop.

const listOne = ["1", "2", "3", "4"]

const listTwo = ["2", "3", "6", "8", "12"]

const mutualNums = []

for (let i = 0; i < listOne.length; i++) {
  for (let j = 0; j < listTwo.length; j++) {
    if (listOne[i] === listTwo[j]) {
      mutualNums.push(listOne[i])
    }
  }
}

console.log(mutualNums)
// ['2', '3']

In the example above, for each element in the outer loop array, listOne, the inner loop will run in its entirety comparing the current element from the outer array, listOne[i], to each element in the inner array, listTwo[j]. When it finds a match, it pushes the string to the mutualNums array.