Why does reduceRight return NaN in Javascript?

Question

I'm using Firefox 3.5.7 and within Firebug I'm trying to test the array.reduceRight function, it works for simple arrays but when I try something like that I get a NaN. Why?

>>> var details = [{score : 1}, {score: 2}, {score: 3}];
>>> details
[Object score=1, Object score=2, Object score=3]
>>> details.reduceRight(function(x, y) {return x.score + y.score;}, 0)
NaN

I also tried map and at least I can see the .score component of each element:

>>> details.map(function(x) {console.log (x.score);})
1
2
3
[undefined, undefined, undefined]

I read the documentation at https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Objects/Array/reduceRight but apparently I can't get it work to sum up all the score values in my details array. Why?

Answer 1

The first argument given to the function is the accumulated value. So the first call to the function will look like f(0, {score: 1}). So when doing x.score, you're actually doing 0.score which doesn't work of course. In other words you want x + y.score.

Answer 2

try this (will convert to numbers as side effect)

details.reduceRight(function(previousValue, currentValue, index, array) {
  return previousValue + currentValue.score;
}, 0)

or this

details.reduceRight(function(previousValue, currentValue, index, array) {
  var ret = { 'score' : previousValue.score + currentValue.score} ;
  return ret;
}, { 'score' : 0 })

Thanks to @sepp2k for pointing out how { 'score' : 0 } was needed as a parameter.

Answer 3

The reduce function should combine two objects with a property "score" into a new object with a property "score." You're combining them into a number.