The reduce function looks like this: The reduce function has two duties: 1. Collect the `prs` and `prevpg` information for each node; 2. Accumulate the total PageRank score sent to each node.

reduce_function.js

var reduce = function(airportId, values) {
    var pg = 0
    , diff = 0
    , prs = {} 
    , prevpg = 0 
    , beta = 0.9
    , totalNodes = 0;

    for (var i in values) {
  // Retrieve the previous pagerank and the probability matrix
        prevPRS = values[i]["prs"]
        for (var key in prevPRS) {
            prs[key] = prevPRS[key];
        }
  prevpg += values[i]["prevpg"];
        // Summation of the pagerank
	pg += values[i]["pg"];
        totalNodes += values[i]["totalNodes"];
    }

    diff = Math.abs(prevpg - pg) / prevpg;
    return {"totalNodes" : totalNodes
        , "pg" : pg 
        , "prs" : prs
        , "diff" : diff
        , "prevpg" : prevpg};
};