Good tooling is essential when learning a new technology. For JavaScript-based MapReduce with MongoDB, comfortable development environments were surprisingly hard to find at the time.

MongoVUE is a decent GUI client that supports writing MapReduce jobs, but the editing experience is not ideal for iterative development.

A better option for rapid prototyping: mongo-mapreduce-webbrowser — a lightweight web tool for testing Map, Reduce, and Finalize functions against small datasets with instant feedback. It ships with examples for common MapReduce patterns and feels similar to jsfiddle.net in its simplicity.