removeDuplicateInstances.js

const removeDuplicateInstances = (arrayList, val) => {
  return Array.isArray(arrayList) === false
    ? "Your first argument must be an array"
    : arrayList.length <= 0
    ? "Your array must contain at least one or more values"
    : !val
    ? "You must pass in the second argument"
    : arrayList.filter((index) => index != val).length;
};

console.log(removeDuplicateInstances([5, 2, 2, 5, 3], 5));