Eloquent Javascript: Higher Order Functions Example

function reduceAncestors(person, f, defaultValue) { function valueFor(person) { if (person == null) return defaultValue; else return f(person, val

Solution 1:

reduceAncestors(ph, sharedDNA, 0) / 4) passes the sharedDNA function as the second argument.

It gets assigned to f here:

functionreduceAncestors(person, f, defaultValue)

f gets called here:

f(person, valueFor(byName[person.mother]), valueFor(byName[person.father]));

… where you can see the second and third arguments which get assigned to fromMother and fromFather:

functionsharedDNA(person, fromMother, fromFather) {

Solution 2:

You could see it as follows:

First, f and sharedDNA are the same during the execution of this particular call.

Secondly, valueOf can return the following values:

• 0, when person == null

• 1, when person == "Pauwels van Haverbeke" (see shareDNA)

• Another value between 0 and 1 by the average of previously calculated values: (fromMother + fromFather) / 2

So if "Pauwels van Haverbeke" is not found in the ancestry of "Philibert Haverbeke" then the final value will be 0.

But if "Pauwels van Haverbeke" is found as an immediate parent of "Philibert Haverbeke" then the final value, before division by 4 in the console.log, is

(0 + 1) / 2 = 1/2

If "Pauwels van Haverbeke" is found as a grand-parent, then the final value is

((0 + 0) / 2 + (0 + 1) / 2) / 2 = 1/4

More generally, if he is found n generations back, the final value is

2 ^ (-n)

Now 2 ^ (-11) happens to be 0.0004882813..., so your (rounded) result 0.00049 means Pauwels van Haverbeke is an ancestor of Philibert Haverbeke, 9 generations earlier, taking into account that there is a division by 4 in console.log.