What are MapReduce views in CouchDB and how do you define a map function?

MapReduce views are CouchDB's primary indexing mechanism. A view has a map phase (mandatory) and an optional reduce phase. The map function is a JavaScript function that CouchDB runs against every document in the database. For each document it emits zero or more key-value pairs. CouchDB stores these emissions in a B-tree index, kept sorted by key. The reduce function (when present) aggregates values within a key range.

Views are defined inside design documents under the views key:

{
  "_id": "_design/products",
  "views": {
    "by_category": {
      "map": "function(doc) { if (doc.type === 'product' && doc.category) { emit(doc.category, { name: doc.name, price: doc.price }); } }"
    },
    "price_by_category": {
      "map": "function(doc) { if (doc.type === 'product') { emit(doc.category, doc.price); } }",
      "reduce": "_sum"
    },
    "by_compound_key": {
      "map": "function(doc) { if (doc.type === 'order') { emit([doc.year, doc.month, doc.day], 1); } }"
    }
  }
}

Key points about map functions:

• The emit(key, value) call adds an entry to the index. A single document can emit multiple times, creating multiple index entries.
• Keys can be strings, numbers, arrays, or null. Arrays support compound-key queries — range queries on [year, month] work naturally.
• The value can be any JSON. Emitting null as the value and using include_docs=true in the query avoids duplicating the full document in the index.
• Map functions must be pure (no side effects, no external HTTP calls) and deterministic.

Views are built lazily on first query and updated incrementally on subsequent queries — only documents changed since the last index update are re-processed.