nodejs: calculate mean with highland streams

Question

I am just getting started with highland.js and streams in node and I am stuck trying to calculate the min/max/mean of some numbers. This is what I have so far:

const _ = require('highland');

const input = [
 { val: 1 },
 { val: 2 },
 { val: 3 },
];

_(input)
  .reduce((acc, { val }) => {
    if (typeof acc.min === 'undefined' || val < acc.min) {
      acc.min = val;
    }
    if (typeof acc.max === 'undefined' || val > acc.max) {
      acc.max = val;
    }
    acc.count = (acc.count || 0) + 1;
    acc.sum = (acc.sum || 0) + val;
    return acc;
  }, {});

If I then do, say, toCallback and console.log the result I get {min: 1, max: 3, count: 3, sum: 6} but I am not interested in the count and sum fields, I want the object like {min: 1, max: 3, mean: 2}.

However, since the return of the reduce is an object, there's nothing highland can do with it - I can only consume it but I would like to do the average in highland land.

How can I continue from here or how should I refactor the code to get that average?

Answer 1

You could try using highland .map method like this:

_(input)
 .reduce({}, (acc, {val}) => {
  if (typeof acc.min === 'undefined' || val < acc.min) {
   acc.min = val;
  }
  if (typeof acc.max === 'undefined' || val > acc.max) {
   acc.max = val;
  }
  acc.count = (acc.count || 0) + 1;
  acc.sum = (acc.sum || 0) + val;
  return acc;
 })
 .map(stats => {
  return {
   min: stats.min,
   max: stats.max,
   mean: stats.sum / stats.count
  };
 })
 .toCallback(function (err, data) {
  //Contains the data structure you need
  console.log(data);
 });