Get the Sum of the result of map() ES6

Question

Here's the updated code that reads all the contents in subfiles, returns all integers in a single array and returns the sum of all the items in the array:

const fs = require('fs')

let output = (file) => {
  let data = fs.readFileSync(file, 'utf8')
    .split('\n')
    .reduce((array, i) => {
      if (i.match(/.txt$/)) {
        let intArr = array.concat(output(i))
        return intArr
      } else if (i.match(/^\d+$/)) {
        array.push(parseInt(i, 10));
      }
      return array;
    }, [])

    return data
}

console.log(output('a.txt')) // single array

const sum = output('a.txt')

console.log(sum.reduce((a, b) => a + b, 0)) // sum of array contents

---

I'm trying to get the result of the following code into a single array so I can get the sum of all the numbers. Consider that the contents of the file a.txt are

1
b.txt

the contents of b.txt are

2
c.txt

and the contents of c.txt are

3

The code is as follows:

const fs = require('fs')
let output = (file) => {
  let data = fs.readFileSync(file, 'utf8')
    .split('\n')
    .map((i) => {
      return i
    })

  let subData = data
    .filter(value => /.txt$/.test(value))
    .map((i) => {
      output(i)
      return i
    })

  let intData = data
    .filter(value => /^\d+$/.test(value))
    .map((i) => {
      return parseInt(i, 10)
    })
  console.log(intData)
}

output('a.txt')

This outputs:

[3]
[2]
[1]

Which would be the best way of getting the sum of these numbers?

Answer 1

You can modify your code by using the .reduce method instead of .map :

The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

const fs = require('fs')
let output = (file) => {
  let data = fs.readFileSync(file, 'utf8')
    .split('\n')
    .reduce((sum, i) => {
      if (i.match(/.txt$/)) {
        sum += output(i);
      } else if (i.match(/^\d+$/)) {
        sum += parseInt(i, 10);
      }
      return sum;
    }, 0)
}

---

If you want to save the integers in an array you could reduce it to an array instead:

const fs = require('fs')
let output = (file) => {
  let data = fs.readFileSync(file, 'utf8')
    .split('\n')
    .reduce((array, i) => {
      if (i.match(/.txt$/)) {
        array.concat(output(i));
      } else if (i.match(/^\d+$/)) {
        array.push(parseInt(i, 10));
      }
      return array;
    }, [])
}

Then if you want to get the sum of the file a.txt do:

const sum = output('a.txt').map((sum,i) => sum + i);

Answer 2

Below uses the promisified version of readFile, I've left it intentionally a bit verbose to demonstrate what's going on, you could obviously simplify/shorten that a bit.

However, it takes an array of files, filters out every number and sums all of them

const fs = require('fs');
const {promisify} = require('util');
const readFileAsync = promisify(fs.readFile); 

function readAndSum(file) {
  return readFileAsync(file, 'utf8')
    .then(f => f.split('\n'))
    .then(lines => {
      return lines.map(line => {
        let newFile = line.match(/\w+\.txt/);
        if (newFile) {
            return readAndSum(newFile[0]);
        }
        return Number(line);
      })
    })
    .then(t => Promise.all(t.reduce((a, b) => a.concat(b), [])))
    .then(numbers => numbers.reduce((a, b) => a + b, 0))
}

readAndSum('foo.txt').then(sum => console.log(sum));