JavaScript Functional Programming - Chaining Functions and using an anonymous function

Question

I was doing a question on CodeWars and practicing some functional programming when I encountered a problem while trying to apply a function to a value.

So I made a pass() function that accepts a function as an argument so that I could use an anonymous function to manipulate that value and then return it. So, in this case, it takes the value from reduce and passes it to a function so it can manipulate that value then return it.

It WORKS but I really don't want to add a method to the Object prototype!

How can I do this another way while still maintaining the chaining of functions?

Simple Example

Object.prototype.pass = function(fn) {
  return fn(this);
};

var value = 1;

var new_value = value.pass(function(num){
   return num + 1;
});

console.log(value, new_value); // Outputs: 1 2

CodeWars Problem for context

Object.prototype.pass = function(fn) {
  return fn(this)
};

function digPow(n, p) {
  return n
    .toString()
    .split('')
    .reduce(function(total, num, i) {
      return total + Math.pow(parseInt(num), (p + i))
    }, 0)
    .pass(function(total) {
      return (total % n == 0) ? Math.floor(total / n) : -1;
    });
}

//digPow(89, 1) should return 1 since 8¹ + 9² = 89 = 89 * 1
console.log("Test Case 1 returns (", digPow(89, 1), ") should return 1")

//digPow(92, 1) should return -1 since there is no k such as 9¹ + 2² equals 92 * k
console.log("Test Case 2 returns (", digPow(92, 1), ") should return -1")

//digPow(695, 2) should return 2 since 6² + 9³ + 5⁴= 1390 = 695 * 2
console.log("Test Case 3 returns (", digPow(695, 2), ") should return 2")

//digPow(46288, 3) should return 51 since 4³ + 6⁴+ 2⁵ + 8⁶ + 8⁷ = 2360688 = 46288 * 51
console.log("Test Case 4 returns (", digPow(46288, 3), ") should return 51")

Code Wars Instructions

Some numbers have funny properties. For example:

89 --> 8¹ + 9² = 89 * 1

695 --> 6² + 9³ + 5⁴= 1390 = 695 * 2

46288 --> 4³ + 6⁴+ 2⁵ + 8⁶ + 8⁷ = 2360688 = 46288 * 51 Given a positive integer n written as abcd... (a, b, c, d... being digits) and a positive integer p we want to find a positive integer k, if it exists, such as the sum of the digits of n taken to the successive powers of p is equal to k * n. In other words:

Is there an integer k such as : (a ^ p + b ^ (p+1) + c ^(p+2) + d ^ (p+3) + ...) = n * k If it is the case we will return k, if not return -1.

Note: n, p will always be given as strictly positive integers.

Jauregui

There's no good reason to use method chaining in functional programming. – Lontson

Answer 1

The solution is just to not use method chaining - the functionality you want to apply to the result is not a method of it. There are various ways around:

• A simple variable:

  function digPow(n, p) {
    const total = n
      .toString()
      .split('')
      .reduce(function(total, num, i) {
        return total + Math.pow(parseInt(num), (p + i))
      }, 0);
    return (total % n == 0) ? Math.floor(total / n) : -1;
  }
  

• An IIFE call (this approach works better with static functions):

  function digPow(n, p) {
    return (function(total) {
      return (total % n == 0) ? Math.floor(total / n) : -1;
    }(n
      .toString()
      .split('')
      .reduce(function(total, num, i) {
        return total + Math.pow(parseInt(num), (p + i))
      }, 0));
  }
  

• The experimental bind operator (works better with static "methods" as well):

  function digPow(n, p) {
    return n
      .toString()
      .split('')
      .reduce(function(total, num, i) {
        return total + Math.pow(parseInt(num), (p + i))
      }, 0)
      :: function() {
        return (this % n == 0) ? Math.floor(this / n) : -1;
      }();
  }
  

You also could use any of the approaches with your pass "method".

Lontson

Thanks for your quick Reply :D, The bind operator looks interesting. – Jauregui

Answer 2

You could also use the Identity functor. This gives you a chainable interface but doesn't touch native prototypes

const Identity = x => ({
  runIdentity: x,
  map: f => Identity(f(x))
})

const digPow = (n, p) =>
  Identity(n)
    .map(n => n.toString())
    .map(str => str.split(''))
    .map(nums => nums.reduce((total, num, i) =>
      total + Math.pow(parseInt(num, 10), p + i), 0))
    .map(total => total % n === 0 ? Math.floor(total / n) : -1)
    .runIdentity

console.log(digPow(89, 1))  // 1
console.log(digPow(695, 2)) // 2

---

If you define some reusable functions, it cleans up the code a little bit

const Identity = x => ({
  runIdentity: x,
  map: f => Identity(f(x))
})

const split = x => str => str.split(x)
const reduce = f => y => xs => xs.reduce(f, y)

const digPow = (n, p) =>
  Identity(n)
    .map(String)
    .map(split(''))
    .map(reduce((acc, x, i) => acc + Math.pow(Number(x), p + i)) (0))
    .map(x => x % n === 0 ? Math.floor(x / n) : -1)
    .runIdentity

console.log(digPow(89, 1))  // 1
console.log(digPow(695, 2)) // 2

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