src/plugins/aggregation.js
function (property, callback) {
        var sum = 0
        this.each(function(e){
            if (e[property]) sum += e[property]
        })
        this.fn('sum', callback).call(this, sum)
    }
function (property, callback) {
        var sum = 0
        this.each(function(e){
            if (e[property]) sum += e[property]
        })
        this.fn('sum', callback).call(this, sum)
    }
function ( property , callback ) { var sum = 0 this . each ( function ( e ) { if ( e [ property ] ) sum += e [ property ] } ) this . fn ( 'sum' , callback ) . call ( this , sum ) }
adds up property and returns sum
adds up property and returns sum