How to use variables in MongoDB Map-reduce map function

You can pass global, read-only data into map-reduce functions using the "scope" parameter on the map-reduce command. It's not very well documented, I'm afraid.

As pointed out by @Dave Griffith, you can use the scope parameter of the mapReduce function.

I struggled a bit to figure out how to properly pass it to the function because, as pointed out by others, the documentation is not very detailed. Finally, I realised that mapReduce is expecting 3 params:

  • map function
  • reduce function
  • object with one or more of the params defined in the doc

Eventually, I arrived at the following code in Javascript:

// I define a variable external to my map and to my reduce functions
var KEYS = {STATS: "stats"};

function m() {
    // I use my global variable inside the map function
    emit(KEYS.STATS, 1);

function r(key, values) {
    // I use a helper function
    return sumValues(values);

// Helper function in the global scope
function sumValues(values) {
    var result = 0;
    values.forEach(function(value) {
        result += value;
    return result;

         out: {inline: 1},
         // I use the scope param to pass in my variables and functions
         scope: {
             KEYS: KEYS,
             sumValues: sumValues // of course, you can pass function objects too