Find the amount of numbers between two positive integers whose digits sum equals another given positive integer

digit-sums.js

/**
 * Find the amount of numbers between 10 and 1000 whose digits sum equals 9
 */

// lodash gives us lots of useful magic functions for arrays, objects, etc
const _ = require('lodash');

// set up our variables
let start = 10, end = 1000;
let digitTotal = 9;
let counter = 0;

// loop through intergers between start and finish
for(let i = start + 1; i < end; ++i) {
  // convert current number to string to make getting digits much easier
  let currentNumber = i.toString();
  // turn into array of digits and convert them to integers
  let digits = currentNumber
                     .split('')
                     .map((x) => { return parseInt(x); });
  // print the current number
  // console.log(currentNumber);
  // magic summing of numbers
  let sum = _.reduce(digits,
                     (memo, num) => {
                        return memo + num;
                      },
                     0);
  // if the digits are equal to our target
  // increase counter by one, display counter,
  // and display the current number
  if( sum === digitTotal ) {
    ++counter;
    console.log(counter + ": " + currentNumber);
  }
}

console.log("A total of " + counter + " integers between " + start + " and " + end + ' whose digits add up to ' + digitTotal);