The map function references the variable this to inspect the current object under consideration. A map function calls emit(key,value)any number of times to feed data to the reducer. In most cases you will emit once per input document, but in some cases such as counting tags, a given document may have one, many, or even zero tags. Each emit is limited to 50% of the maximum document size (e.g. 4MB for 1.6.x and 8MB for 1.8.x).