gistfile1.js

r.table('marvel').groupedMapReduce(
    function(hero) { return hero('weightClass'), // group function
    function(hero) { // map function
        return {
            strength: hero('strength'),
            height: hero('height'),
            count: 1
        }
    }, 
    function(left, right) { // reduce function
        return {
            strength: left('strength').add(right('strength')), // sum the strength
            height: left('height').add(right('height'), // sum the height
            count: left('count').add(right('count')) // count
        }
    }
).map( function(weightClass) { // compute the average
    return {
        group: weightClass('group')
        avgStrength: weightClass('reduction')('strength').div(weightClass('reduction')('count')),
        avgHeight: weightClass('reduction')('height').div(weightClass('reduction')('count'))
    }
}).run(conn, callback)