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 ponent of each element:

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

I read the documentation at .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?

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 ponent 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/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?

• javascript
• firefox
• functional-programming
• reduce

3 Answers 3

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.

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.

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