mongodb-word-count.js

var map = function() {  
    var summary = this.message_soup_text;
    if (summary) { 
        // quick lowercase to normalize per your requirements
        summary = summary.toLowerCase().split(" "); 
        for (var i = summary.length - 1; i >= 0; i--) {
            // might want to remove punctuation, etc. here
            if (summary[i])  {      // make sure there's something
               emit(summary[i], 1); // store a 1 for each word
            }
        }
    }
};

var reduce = function( key, values ) {    
    var count = 0;    
    values.forEach(function(v) {            
        count +=v;    
    });
    return count;
}

// full thing
// db.support_emails.mapReduce(map, reduce, { out: "word_count" })

// subset
db.support_emails.mapReduce(map, reduce, {limit: 1000, out: "word_count" })
db.word_count.find().sort({ value: -1 }).limit(10)

http://stackoverflow.com/questions/16174591/mongo-count-the-number-of-word-occurences-in-a-set-of-documents