humanSize.js

const humanSize = bytes => {
  if (!bytes || isNaN(bytes) || bytes < 0 ) return null
  
  const metric = {'decimal': 1000,'binary': 1024}  
  const units = ['B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB']
  
  
  /*
    reduce takes a callback and an initial value. 
    callback acts upon every element of the array, and should return at 
    every loop an accumulated value;which is the input for the next loop.
  */ 
  return units.reduce((acc, unit) => {
    if (typeof acc === 'string') {
      // being here means that we have a correct value with its corresponding unit.
      // we have to traverse the whole array so we just return the same result. little caveat 🤷🏻‍♂️
      return acc
    } else if (acc > 1000) {
      // a number bigger than 1000 means that we should use the next bigger unit.       
      return acc = (acc / metric.binary)
    } else {
      // time to attach the corresponding unit, and round decimals.
      // if number is already an Integer just return it, we don't want additional zeroes 😖
      return Number.isInteger(acc) ? acc + unit : acc.toFixed(1) + unit
    }
  } , bytes)
}

humanSize(10000) // 9.8kB
humanSize(999) // 999B
humanSize(1) // 1B
for (let i= 1; i < 10; i++) {
    humanSize(Math.pow(1024, i)) // let's just test each unit
}
humanSize(-2) // null
humanSize("") // null
humanSize(undefined) // null