Looking for a pure Javascript Functional programming solution

Question

I've been challenging myself with solving this in pure functional programming style, in Javascript. The problem is this:

Input:

var src = [{n:3, x:2}, {n:6, x:1}, {n:2, x:0}, {n:10, x:5}, {n:5, x:2}, {n:1, x:44}];

Problem definition:

Group the items in the src array using a either a predicate or a hash function and sum the item.x values for each group. For example, in the output I pasted the grouping is the remainder division of item.n with 2

Output:

var trgt = [
  {sum:48, items: [{n:3, x:2}, {n:5, x:2}, {n:1, x:44}]},
  {sum:6, items: [{n:6, x:1}, {n:2, x:0}, {n:10, x:5}]}
]

The goal here is to find as pure a functional programming solution as possible.

Here is my solution

function moduloTwo(val) {
  return (val.n % 2);
}
function makeObjectAndAdd(hashFn) {
  return function (result, curr) {
    if (result[hashFn(curr)] === undefined) {
      result[hashFn(curr)] = {sum: 0, items: []};
    }
    var newObj = result[hashFn(curr)];
    newObj.items.push(curr);
    newObj.sum += curr.x;

    return result;
  };
}

var result = src.reduce(makeObjectAndAdd(moduloTwo), {});

I feel that it could be made to be more "functional".

Thanks!

the link points to the output, not your solution. output is not "functional". Code is. — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Dec 3, 2014 at 7:30
You should paste the relevant code into the question directly, not only a link to it. Maybe the linked site is not reachable (anymore) and then the question couldn't be answered or makes no sense at all :) — Andreas
– Andreas, Commented Dec 3, 2014 at 7:33
I don't see the point of isDividedByTwo as a separate function. It should be called isEven and could be simply return !(val%2) since the name infers that it will return a boolean. — RobG
– RobG, Commented Dec 3, 2014 at 7:34
@RobG Like I said, you can either return a predicate or a hash function. For example, what if you would want to group by the result of %3? It wouldn't work with true/false. At-least not for this solution. I agree that the function name is misleading though. — Ben
– Ben, Commented Dec 3, 2014 at 7:37

Aadit M Shah · Accepted Answer · 2015-05-04 14:34:43Z

2

You can write your code more functionally using Ramda.js as follows:

Given an input:

var input = [
    { n:  3, x:  2 },
    { n:  6, x:  1 },
    { n:  2, x:  0 },
    { n: 10, x:  5 },
    { n:  5, x:  2 },
    { n:  1, x: 44 }
];

We first separate them into odd and even groups based on the property n:

var oddN = R.compose(odd, R.prop("n"));

var groups = R.groupBy(oddN, input);

function odd(n) {
    return n % 2 === 1;
}

Then we get the values of all the groups, pluck the x property of each item and summate:

var output = R.map(count, R.values(groups));

function count(items) {
    return {
        sum:   R.sum(R.pluck("x", items)),
        items: items
    };
}

See the online demo for yourself: http://bit.ly/1IawqzD

answered May 4, 2015 at 14:34

Aadit M Shah

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3 Comments

Ben Over a year ago

I like Ramda a lot. I talk about it when explaining Functional Programming to Javascript devs

Aadit M Shah Over a year ago

I don't like Ramda, but it's the least bad functional library we've got.

Ben Over a year ago

Within the confides of JavaScript I think it does a good job.

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