How to use javascript reduce function to calculate average of items meeting a specific condition?

Question

So assume I have the following array of objects:

var arr = [
  {"name": "John", "score": "8.8"},
  {"name": "John", "score": "8.6"},
  {"name": "John", "score": "9.0"},
  {"name": "John", "score": "8.3"},
  {"name": "Tom",  "score": "7.9"}
];

var count = 0;
var avgScore = arr.reduce(function (sum,person) {
  if (person.name == "John") {
    count+=1;
    return sum + parseFloat(person.score);
  }
  return sum;
},0)/count);

Question: Is there a way way to calculate the average score for "John" without creating a global count variable. Ideally, the count would be internal to the anonymous function in the arr.reduce.

Answer 1

You could return an object with the average in it, calculated on every loop with an update.

var arr = [{ name: "John", score: "8.8" }, { name: "John", score: "8.6" }, { name: "John", score: "9.0" }, { name: "John", score: "8.3" }, { name: "Tom", score: "7.9" }],
    avgScore = arr.reduce(function (r, person) {
        if (person.name === "John") {
            r.sum += +person.score;
            r.avg = r.sum / ++r.count;
        }
        return r;
    }, { sum: 0, count: 0, avg: 0 }).avg;

console.log(avgScore);

A version with a closure and a direct return of the average.

var arr = [{ name: "John", score: "8.8" }, { name: "John", score: "8.6" }, { name: "John", score: "9.0" }, { name: "John", score: "8.3" }, { name: "Tom", score: "7.9" }],
    avgScore = arr.reduce(function (sum, count) {
        return function (avg, person) {
            if (person.name === "John") {
                sum += +person.score;
                return sum / ++count;
            }
            return avg;
        };
    }(0, 0), 0);

console.log(avgScore);

Above as ES6

var arr = [{ name: "John", score: "8.8" }, { name: "John", score: "8.6" }, { name: "John", score: "9.0" }, { name: "John", score: "8.3" }, { name: "Tom", score: "7.9" }],
    avgScore = arr.reduce(((sum, count) => (avg, person) => person.name === "John" ? (sum += +person.score) / ++count : avg)(0, 0), 0);

console.log(avgScore);

Answer 2

Here is yet another ES6 variant, which (ab)uses the third argument of reduce as temporary storage, and calls reduce again for a chained calculation of the average from the sum and count:

const arr = [
  {"name": "John", "score": "8.8"},
  {"name": "John", "score": "8.6"},
  {"name": "John", "score": "9.0"},
  {"name": "John", "score": "8.3"},
  {"name": "Tom",  "score": "7.9"}
];

const avg = arr.reduce( ([sum, count], {name, score}, i) =>
                        (i = name == 'John', [sum + i * score, count + i]), [0, 0] )
               .reduce( (sum, count) => sum/count );

console.log(avg);