I solved an exercise from the book “Eloquent Javascript”. I want to share with you guys the solution. The exercise is from chapter 5. Summing up, I have to find the average age from the ancestors array per century. Here is my solution:

var ancestry = JSON.parse(require('../resources/ancestry'))  //Top-down approach, functions are defined after their use //Necessary side-effect in function to print the output function printAverageAgePerCentury(ancestry){     var averageAgePerCentury = getAverageAgePerCentury(ancestry);     Object.keys(averageAgePerCentury).forEach(function(century){         console.log(century.concat(": ").concat(averageAgePerCentury[century]))     }) }  function getAverageAgePerCentury(ancestry){     var agesPerCentury = getAgesPerCentury(ancestry);      return agesPerCentury.reduce(function(averageAgePerCentury, agePerCentury){         if(averageAgePerCentury[agePerCentury.century]){             averageAgePerCentury[agePerCentury.century] = [average(averageAgePerCentury[agePerCentury.century].concat(agePerCentury.age))];             return averageAgePerCentury;         } else {             averageAgePerCentury[agePerCentury.century] = [agePerCentury.age];             return averageAgePerCentury;         }     }, {}) }  function getAgesPerCentury(ancestry){     return ancestry.map(function(person){         return {             century : whichCentury(person),             age : person.died - person.born         }     }) }  function average(array){     function plus(a,b){ return a + b}     return array.reduce(plus) / array.length }  function whichCentury(person){     return Math.ceil(person.died / 100) }  //--------------------------------//  printAverageAgePerCentury(ancestry); 

1. What do you think about the top-down approach? Do you prefer the opposite? Where functions are defined before their use.
2. Is possible to make this solution more readable, more elegant?
3. What could you do to make it better?