function solve (arr) {
  Object.getPrototypeOf(arr).myReduce = function (func, initialVal) {
    let accumolator = initialVal

    for (let i = 0; i < this.length; i++) {
      accumolator = func(accumolator, this[i])
    }

    return accumolator
  }

  let sum = arr.myReduce((a, b) => a + b, 0)       // 6
  let subtract = arr.myReduce((a, b) => a - b, 10) // 4
  let divide = arr.myReduce((a, b) => a / b, 12)   // 2
  let multiply = arr.myReduce((a, b) => a * b, 1)  // 6

  console.log(sum)
  console.log(subtract)
  console.log(divide)
  console.log(multiply)
}

solve([1, 2, 3])