size-sum.js

const fs = require('fs')

fs.writeFile('a.txt', 'some text', err => {
  if (err) throw err

  fs.writeFile('b.txt', 'some more text', err => {
    if (err) throw err

    fs.writeFile('c.txt', 'even more text', err => {
      if (err) throw err

      fs.stat('a.txt', (err, statsA) => {
        if (err) throw err

        fs.stat('b.txt', (err, statsB) => {
          if (err) throw err

          fs.stat('c.txt', (err, statsC) => {
            if (err) throw err

            const totalSize = statsA.size + statsB.size + statsC.size
            console.log(`Total size: ${totalSize} bytes`)
          })
        })
      })
    })
  })
})