MongoDB MapReduce: Global variables within map function instance?

Question

I've written a MapReduce in MongoDB and would like to use a global variable as a cache to write to/read from. I know it is not possible to have global variables across map function instances - I just want a global variable within each function instance. This type of functionality exists in Hadoop's MapReduce so I was expecting it to be there in MongoDB. But the following does not seem to work:

var cache = {}; // Does not seem to work!
function () {
  var hashValue = this.varValue1 + this.varValue2;
  if(typeof(cache[hashValue])!= 'undefined') {
    // Do nothing, we've processed at least one input record with this hash
  } else {
    // Process the input record
    // Cache the record
    cache[hashValue] = '1';
  }
}

Is this not allowed in MongoDB's MapReduce implementation, or am I doing something wrong in JavaScript (not experienced in JS)?

Answer 1

Looking at the docs, I'm finding the following:

db.runCommand(
 { mapreduce : <collection>,
   map : <mapfunction>,
   reduce : <reducefunction>
   [, scope : <object where fields go into javascript global scope >]
 }
);

I think that "scope" variable is what you need.

There's a test / example on Github that uses the "scope" variable.

I'm still new to this stuff, but hopefully that's enough to get you started.