Reduce returning undefined?

Question

I have an object called student, with two properties, name and score. I am trying to perform calculations using the score property but am having trouble accessing the property from the array of students. Currently, I'm trying to get the sum of the scores with the following code:

var sum = students.reduce(function(a, b) { 
                    return {sum: a.score + b.score}
                    })

This returns an undefined value and causes display weirdness in firefox. I can't seem to find the error.

Is there no way to just access parameters simply, (i.e. var myVar = myArray. myObject.myProperty;)?

Answer 1

I think you have a misunderstanding about how reduce works. For each element in the array, it executes the provided function. The first argument is an "accumulator"; something that's passed along as each element is visited (actually, it's the return value of the function from the last element, but it's usually used as an accumulator). The second argument is the element being visited. So what I think you want is this:

var sum = students.reduce(function(a, s) {
    a.sum += s.score;
    return a;
}, { sum: 0 });

When you invoke reduce, you can provide the initial value of the accumulator (otherwise it takes on the value of the first element, and the visitation starts with the second element). Here, we provide an object with a sum property set to zero.

If all you want is the sum (not an object with a sum property), it can be even simpler:

var sum = students.reduce(function(a, s) {
    return a += s.score;
}, 0);