motherChildAgeDifference.js

/* Mother-child age difference */

/*
Using the example data set from this chapter, compute the average age difference between mothers and children (the age 
of the mother when the child is born). You can use the average function defined earlier in this chapter.

Note that not all the mothers mentioned in the data are themselves present in the array. The byName object, which makes 
it easy to find a person’s object from their name, might be useful here.
*/

function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}

var byName = {};
ancestry.forEach(function(person) {
  byName[person.name] = person;
});

// Your code here.
function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}

var byName = {};
ancestry.forEach(function(person) {
  byName[person.name] = person;
});

// Your code here.
var hasKnownMother = function(person) {
    return person.mother in byName;
};
 
function getAgeDiff(person) {
    return person.born - byName[person.mother].born;
}
 
console.log(average(ancestry.filter(hasKnownMother).map(getAgeDiff)));
// → 31.2

// → 31.2