docs/_includes/api/query_database.html - pouchdb - Git at Google

 {% include anchor.html edit="true" title="Map/reduce queries" hash="query_database" %}

 {% highlight js %}
 db.query(fun, [options], [callback])
 {% endhighlight %}

 Invoke a map/reduce function, which allows you to perform more complex queries on PouchDB than what you get with [allDocs()](#batch_fetch), [changes()](#changes), or [find()](#query_index). The [CouchDB documentation for map/reduce](http://docs.couchdb.org/en/latest/couchapp/views/intro.html) applies to PouchDB.

 Since views perform a full scan of all documents, this method may be slow, unless you first save your view in a design document. Read the [query guide](/guides/queries.html) for a good tutorial.

 {% include alert/start.html variant="warning"%}
 {% markdown %}

 **Warning: advanced API.** The map/reduce API is designed for cases that are too complex for Mango queries, which are described in
 [createIndex()](#create_index), [find()](#query_index), [listIndexes()](#list_indexes), and [deleteIndex()](#delete_index).

 Under the hood, Mango indexes are the same as map/reduce indexes. The Mango API is just a simplified user-facing API on top of map/reduce.

 {% endmarkdown %}
 {% include alert/end.html%}

 ### Options

 All options default to `false` unless otherwise specified.

 * `fun`: Map/reduce function, which can be one of the following:
   * A full CouchDB-style map/reduce view: `{map : ..., reduce: ...}`.
   * A map function by itself (no reduce).
   * The name of a view in an existing design document (e.g. `'mydesigndoc/myview'`, or `'myview'` as a shorthand for `'myview/myview'`).
 * `options.reduce`: Reduce function, or the string name of a built-in function: `'_sum'`, `'_count'`, or `'_stats'`.  Defaults to `false` (no reduce).
     * Tip: if you're not using a built-in, [you're probably doing it wrong](http://youtu.be/BKQ9kXKoHS8?t=865s).
     * PouchDB will always call your reduce function with rereduce == false. As for CouchDB, refer to the [CouchDB documentation](http://docs.couchdb.org/en/1.6.1/couchapp/views/intro.html).
 * `options.include_docs`: Include the document in each row in the `doc` field.
     - `options.conflicts`: Include conflicts in the `_conflicts` field of a doc.
   - `options.attachments`: Include attachment data.
     - `options.binary`: Return attachment data as Blobs/Buffers, instead of as base64-encoded strings.
 * `options.startkey` &amp; `options.endkey`: Get rows with keys in a certain range (inclusive/inclusive).
 * `options.inclusive_end`: Include rows having a key equal to the given `options.endkey`. Default: `true`.
 * `options.limit`: Maximum number of rows to return.
 * `options.skip`: Number of rows to skip before returning (warning: poor performance on IndexedDB/LevelDB!).
 * `options.descending`: Reverse the order of the output rows.
 * `options.key`: Only return rows matching this key.
 * `options.keys`: Array of keys to fetch in a single shot.
     - Neither `startkey` nor `endkey` can be specified with this option.
     - The rows are returned in the same order as the supplied `keys` array.
     - The row for a deleted document will have the revision ID of the deletion, and an extra key `"deleted":true` in the `value` property.
     - The row for a nonexistent document will just contain an `"error"` property with the value `"not_found"`.
 * `options.group`: True if you want the reduce function to group results by keys, rather than returning a single result. Defaults to `false`.
 * `options.group_level`: Number of elements in a key to group by, assuming the keys are arrays. Defaults to the full length of the array.
 * `options.stale`: One of `'ok'` or `'update_after'`.  Only applies to saved views. Can be one of:
     * unspecified (default): Returns the latest results, waiting for the view to build if necessary.
     * `'ok'`: Returns results immediately, even if they're out-of-date.
     * `'update_after'`: Returns results immediately, but kicks off a build afterwards.

 For details, see the [CouchDB query options documentation](http://wiki.apache.org/couchdb/HTTP_view_API#Querying_Options).

 #### Example Usage:

 {% include code/start.html id="query1" type="callback" %}
 {% highlight js %}
 // create a design doc
 var ddoc = {
   _id: '_design/index',
   views: {
     index: {
       map: function mapFun(doc) {
         if (doc.title) {
           emit(doc.title);
         }
       }.toString()
     }
   }
 }

 // save the design doc
 db.put(ddoc, function (err) {
   if (err && err.name !== 'conflict') {
     return console.log(err);
   }
   // ignore if doc already exists
   // find docs where title === 'Lisa Says'
   db.query('index', {
     key: 'Lisa Says',
     include_docs: true
   }, function (err, result) {
     if (err) { return console.log(err); }
     // handle result
   });
 });
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query1" type="async" %}
 {% highlight js %}
 // create a design doc
 var ddoc = {
   _id: '_design/index',
   views: {
     index: {
       map: function mapFun(doc) {
         if (doc.title) {
           emit(doc.title);
         }
       }.toString()
     }
   }
 }

 // save the design doc
 try {
   try {
     await db.put(ddoc);
   } catch (err) {
     if (err.name !== 'conflict') {
       throw err;
     }
     // ignore if doc already exists
   }
   // find docs where title === 'Lisa Says'
   var result = await db.query('index', {
     key: 'Lisa Says',
     include_docs: true
   });
 } catch (err) {
   console.log(err);
 }
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query1" type="promise" %}
 {% highlight js %}
 // create a design doc
 var ddoc = {
   _id: '_design/index',
   views: {
     index: {
       map: function mapFun(doc) {
         if (doc.title) {
           emit(doc.title);
         }
       }.toString()
     }
   }
 }

 // save the design doc
 db.put(ddoc).catch(function (err) {
   if (err.name !== 'conflict') {
     throw err;
   }
   // ignore if doc already exists
 }).then(function () {
   // find docs where title === 'Lisa Says'
   return db.query('index', {
     key: 'Lisa Says',
     include_docs: true
   });
 }).then(function (result) {
   // handle result
 }).catch(function (err) {
   console.log(err);
 });
 {% endhighlight %}
 {% include code/end.html %}

 #### Example Response:
 {% highlight js %}
 {
   "offset" : 0,
   "rows": [{
     "id": "doc3",
     "key": "Lisa Says",
     "value": null,
     "doc": {
       "_id": "doc3",
       "_rev": "1-z",
       "title": "Lisa Says"
     }
   }],
   "total_rows" : 4
 }
 {% endhighlight %}

 In the result,`total_rows` is the total number of possible results in the view. The response is very similar to that of `allDocs()`.

 **Note:** you can also pass in the map function instead of saving a design doc first, but this is slow because it has to do a full database scan. The following examples will use this pattern for simplicity's sake, but you should normally avoid it.

 #### Complex keys

 You can also use [complex keys](https://wiki.apache.org/couchdb/Introduction_to_CouchDB_views#Complex_Keys) for fancy ordering:

 {% include code/start.html id="query2" type="callback" %}
 {% highlight js %}
 function map(doc) {
   // sort by last name, first name, and age
   emit([doc.lastName, doc.firstName, doc.age]);
 }
 db.query(map, function (err, response) {
   if (err) { return console.log(err); }
   // handle result
 });
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query2" type="async" %}
 {% highlight js %}
 function map(doc) {
   // sort by last name, first name, and age
   emit([doc.lastName, doc.firstName, doc.age]);
 }
 try {
   var result = await db.query(map);
 } catch (err) {
   console.log(err);
 }
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query2" type="promise" %}
 {% highlight js %}
 function map(doc) {
   // sort by last name, first name, and age
   emit([doc.lastName, doc.firstName, doc.age]);
 }
 db.query(map).then(function (result) {
   // handle result
 }).catch(function (err) {
   console.log(err);
 });
 {% endhighlight %}
 {% include code/end.html %}

 #### Example Response:
 {% highlight js %}
 {
   "offset": 0,
   "rows": [{
       "id"  : "bowie",
       "key" : ["Bowie", "David", 67]
     }, {
       "id"  : "dylan",
       "key" : ["Dylan", "Bob", 72]
     }, {
       "id"  : "younger_dylan",
       "key" : ["Dylan", "Jakob", 44]
     }, {
       "id"  : "hank_the_third",
       "key" : ["Williams", "Hank", 41]
     }, {
       "id"  : "hank",
       "key" : ["Williams", "Hank", 91]
     }],
   "total_rows": 5
 }
 {% endhighlight %}

 **Tips:**

 * The sort order is `[nulls, booleans, numbers, strings, arrays, objects]`, so `{startkey: ['Williams'], endkey: ['Williams', {}]}` would return all people with the last name `'Williams'` because objects are higher than strings. Something like `'zzzzz'` or `'\uffff'` would also work.
 * `group_level` can be very helpful when working with complex keys.  In the example above, you can use `{group_level: 1}` to group by last name, or `{group_level: 2}` to group by last and first name. (Be sure to set `{reduce: true, group: true}` as well.)

 #### Linked documents

 PouchDB fully supports [linked documents](https://wiki.apache.org/couchdb/Introduction_to_CouchDB_views#Linked_documents). Use them to join two types of documents together, by simply adding an `_id` to the emitted value:

 {% include code/start.html id="query3" type="callback" %}
 {% highlight js %}
 function map(doc) {
   // join artist data to albums
   if (doc.type === 'album') {
     emit(doc.name, {_id : doc.artistId, albumYear : doc.year});
   }
 }
 db.query(map, {include_docs : true}, function (err, response) {
   if (err) { return console.log(err); }
   // handle result
 });
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query3" type="async" %}
 {% highlight js %}
 function map(doc) {
   // join artist data to albums
   if (doc.type === 'album') {
     emit(doc.name, {_id : doc.artistId, albumYear : doc.year});
   }
 }
 try {
   var result = await db.query(map, {include_docs : true});
 } catch (err) {
   console.log(err);
 }
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query3" type="promise" %}
 {% highlight js %}
 function map(doc) {
   // join artist data to albums
   if (doc.type === 'album') {
     emit(doc.name, {_id : doc.artistId, albumYear : doc.year});
   }
 }
 db.query(map, {include_docs : true}).then(function (result) {
   // handle result
 }).catch(function (err) {
   console.log(err);
 });
 {% endhighlight %}
 {% include code/end.html %}

 #### Example response:

 {% highlight js %}
 {
     "offset": 0,
     "rows": [
         {
             "doc": {
                 "_id": "bowie",
                 "_rev": "1-fdb234b78904a5c8293f2acf4be70d44",
                 "age": 67,
                 "firstName": "David",
                 "lastName": "Bowie"
             },
             "id": "album_hunkeydory",
             "key": "Hunky Dory",
             "value": {
                 "_id": "bowie",
                 "albumYear": 1971
             }
         },
         {
             "doc": {
                 "_id": "bowie",
                 "_rev": "1-fdb234b78904a5c8293f2acf4be70d44",
                 "age": 67,
                 "firstName": "David",
                 "lastName": "Bowie"
             },
             "id": "album_low",
             "key": "Low",
             "value": {
                 "_id": "bowie",
                 "albumYear": 1977
             }
         },
         {
             "doc": {
                 "_id": "bowie",
                 "_rev": "1-fdb234b78904a5c8293f2acf4be70d44",
                 "age": 67,
                 "firstName": "David",
                 "lastName": "Bowie"
             },
             "id": "album_spaceoddity",
             "key": "Space Oddity",
             "value": {
                 "_id": "bowie",
                 "albumYear": 1969
             }
         }
     ],
     "total_rows": 3
 }
 {% endhighlight %}

 #### Closures

 If you pass a function to `db.query` and give it the `emit` function as the second argument, then you can use a closure. (Since PouchDB has to use `eval()` to bind `emit`.)

 {% include code/start.html id="query4" type="callback" %}
 {% highlight js %}
 // BAD! will throw error
 var myId = 'foo';
 db.query(function(doc) {
   if (doc._id === myId) {
     emit(doc);
   }
 }, function(err, results) {
   // you'll get an error here
 });

 // will be fine
 var myId = 'foo';
 db.query(function(doc, emit) {
   if (doc._id === myId) {
     emit(doc);
   }
 }, function(err, results) {
   if (err) { return console.log(err); }
   // handle result
 });
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query4" type="async" %}
 {% highlight js %}
 // BAD! will throw error
 var myId = 'foo';
 try {
   var result = await db.query(function(doc) {
     if (doc._id === myId) {
       emit(doc);
     }
   });
 } catch (err) {
   // you'll get an error here
 }

 // will be fine
 var myId = 'foo';
 try {
   var result = await db.query(function(doc, emit) {
     if (doc._id === myId) {
       emit(doc);
     }
   });
 } catch (err) {
   console.log(err);
 }
 {% endhighlight %}
 {% include code/end.html %}

 {% include code/start.html id="query4" type="promise" %}
 {% highlight js %}
 // BAD! will throw error
 var myId = 'foo';
 db.query(function(doc) {
   if (doc._id === myId) {
     emit(doc);
   }
 }).catch(function (err) {
   // you'll get an error here
 }

 // will be fine
 var myId = 'foo';
 db.query(function(doc, emit) {
   if (doc._id === myId) {
     emit(doc);
   }
 }).then(function (result) {
   // handle result
 }).catch(function (err) {
   console.log(err);
 });
 {% endhighlight %}
 {% include code/end.html %}

 Note that closures are only supported by local databases with temporary views. So if you are using closures, then you must use the slower method that requires a full database scan.
	{% include anchor.html edit="true" title="Map/reduce queries" hash="query_database" %}

	{% highlight js %}
	db.query(fun, [options], [callback])
	{% endhighlight %}

	Invoke a map/reduce function, which allows you to perform more complex queries on PouchDB than what you get with [allDocs()](#batch_fetch), [changes()](#changes), or [find()](#query_index). The [CouchDB documentation for map/reduce](http://docs.couchdb.org/en/latest/couchapp/views/intro.html) applies to PouchDB.

	Since views perform a full scan of all documents, this method may be slow, unless you first save your view in a design document. Read the [query guide](/guides/queries.html) for a good tutorial.

	{% include alert/start.html variant="warning"%}
	{% markdown %}

	Warning: advanced API. The map/reduce API is designed for cases that are too complex for Mango queries, which are described in
	[createIndex()](#create_index), [find()](#query_index), [listIndexes()](#list_indexes), and [deleteIndex()](#delete_index).

	Under the hood, Mango indexes are the same as map/reduce indexes. The Mango API is just a simplified user-facing API on top of map/reduce.

	{% endmarkdown %}
	{% include alert/end.html%}

	### Options

	All options default to `false` unless otherwise specified.

	* `fun`: Map/reduce function, which can be one of the following:
	* A full CouchDB-style map/reduce view: `{map : ..., reduce: ...}`.
	* A map function by itself (no reduce).
	* The name of a view in an existing design document (e.g. `'mydesigndoc/myview'`, or `'myview'` as a shorthand for `'myview/myview'`).
	* `options.reduce`: Reduce function, or the string name of a built-in function: `'_sum'`, `'_count'`, or `'_stats'`. Defaults to `false` (no reduce).
	* Tip: if you're not using a built-in, [you're probably doing it wrong](http://youtu.be/BKQ9kXKoHS8?t=865s).
	* PouchDB will always call your reduce function with rereduce == false. As for CouchDB, refer to the [CouchDB documentation](http://docs.couchdb.org/en/1.6.1/couchapp/views/intro.html).
	* `options.include_docs`: Include the document in each row in the `doc` field.
	- `options.conflicts`: Include conflicts in the `_conflicts` field of a doc.
	- `options.attachments`: Include attachment data.
	- `options.binary`: Return attachment data as Blobs/Buffers, instead of as base64-encoded strings.
	* `options.startkey` & `options.endkey`: Get rows with keys in a certain range (inclusive/inclusive).
	* `options.inclusive_end`: Include rows having a key equal to the given `options.endkey`. Default: `true`.
	* `options.limit`: Maximum number of rows to return.
	* `options.skip`: Number of rows to skip before returning (warning: poor performance on IndexedDB/LevelDB!).
	* `options.descending`: Reverse the order of the output rows.
	* `options.key`: Only return rows matching this key.
	* `options.keys`: Array of keys to fetch in a single shot.
	- Neither `startkey` nor `endkey` can be specified with this option.
	- The rows are returned in the same order as the supplied `keys` array.
	- The row for a deleted document will have the revision ID of the deletion, and an extra key `"deleted":true` in the `value` property.
	- The row for a nonexistent document will just contain an `"error"` property with the value `"not_found"`.
	* `options.group`: True if you want the reduce function to group results by keys, rather than returning a single result. Defaults to `false`.
	* `options.group_level`: Number of elements in a key to group by, assuming the keys are arrays. Defaults to the full length of the array.
	* `options.stale`: One of `'ok'` or `'update_after'`. Only applies to saved views. Can be one of:
	* unspecified (default): Returns the latest results, waiting for the view to build if necessary.
	* `'ok'`: Returns results immediately, even if they're out-of-date.
	* `'update_after'`: Returns results immediately, but kicks off a build afterwards.

	For details, see the [CouchDB query options documentation](http://wiki.apache.org/couchdb/HTTP_view_API#Querying_Options).

	#### Example Usage:

	{% include code/start.html id="query1" type="callback" %}
	{% highlight js %}
	// create a design doc
	var ddoc = {
	_id: '_design/index',
	views: {
	index: {
	map: function mapFun(doc) {
	if (doc.title) {
	emit(doc.title);
	}
	}.toString()
	}
	}
	}

	// save the design doc
	db.put(ddoc, function (err) {
	if (err && err.name !== 'conflict') {
	return console.log(err);
	}
	// ignore if doc already exists
	// find docs where title === 'Lisa Says'
	db.query('index', {
	key: 'Lisa Says',
	include_docs: true
	}, function (err, result) {
	if (err) { return console.log(err); }
	// handle result
	});
	});
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query1" type="async" %}
	{% highlight js %}
	// create a design doc
	var ddoc = {
	_id: '_design/index',
	views: {
	index: {
	map: function mapFun(doc) {
	if (doc.title) {
	emit(doc.title);
	}
	}.toString()
	}
	}
	}

	// save the design doc
	try {
	try {
	await db.put(ddoc);
	} catch (err) {
	if (err.name !== 'conflict') {
	throw err;
	}
	// ignore if doc already exists
	}
	// find docs where title === 'Lisa Says'
	var result = await db.query('index', {
	key: 'Lisa Says',
	include_docs: true
	});
	} catch (err) {
	console.log(err);
	}
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query1" type="promise" %}
	{% highlight js %}
	// create a design doc
	var ddoc = {
	_id: '_design/index',
	views: {
	index: {
	map: function mapFun(doc) {
	if (doc.title) {
	emit(doc.title);
	}
	}.toString()
	}
	}
	}

	// save the design doc
	db.put(ddoc).catch(function (err) {
	if (err.name !== 'conflict') {
	throw err;
	}
	// ignore if doc already exists
	}).then(function () {
	// find docs where title === 'Lisa Says'
	return db.query('index', {
	key: 'Lisa Says',
	include_docs: true
	});
	}).then(function (result) {
	// handle result
	}).catch(function (err) {
	console.log(err);
	});
	{% endhighlight %}
	{% include code/end.html %}

	#### Example Response:
	{% highlight js %}
	{
	"offset" : 0,
	"rows": [{
	"id": "doc3",
	"key": "Lisa Says",
	"value": null,
	"doc": {
	"_id": "doc3",
	"_rev": "1-z",
	"title": "Lisa Says"
	}
	}],
	"total_rows" : 4
	}
	{% endhighlight %}

	In the result,`total_rows` is the total number of possible results in the view. The response is very similar to that of `allDocs()`.

	Note: you can also pass in the map function instead of saving a design doc first, but this is slow because it has to do a full database scan. The following examples will use this pattern for simplicity's sake, but you should normally avoid it.

	#### Complex keys

	You can also use [complex keys](https://wiki.apache.org/couchdb/Introduction_to_CouchDB_views#Complex_Keys) for fancy ordering:

	{% include code/start.html id="query2" type="callback" %}
	{% highlight js %}
	function map(doc) {
	// sort by last name, first name, and age
	emit([doc.lastName, doc.firstName, doc.age]);
	}
	db.query(map, function (err, response) {
	if (err) { return console.log(err); }
	// handle result
	});
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query2" type="async" %}
	{% highlight js %}
	function map(doc) {
	// sort by last name, first name, and age
	emit([doc.lastName, doc.firstName, doc.age]);
	}
	try {
	var result = await db.query(map);
	} catch (err) {
	console.log(err);
	}
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query2" type="promise" %}
	{% highlight js %}
	function map(doc) {
	// sort by last name, first name, and age
	emit([doc.lastName, doc.firstName, doc.age]);
	}
	db.query(map).then(function (result) {
	// handle result
	}).catch(function (err) {
	console.log(err);
	});
	{% endhighlight %}
	{% include code/end.html %}

	#### Example Response:
	{% highlight js %}
	{
	"offset": 0,
	"rows": [{
	"id" : "bowie",
	"key" : ["Bowie", "David", 67]
	}, {
	"id" : "dylan",
	"key" : ["Dylan", "Bob", 72]
	}, {
	"id" : "younger_dylan",
	"key" : ["Dylan", "Jakob", 44]
	}, {
	"id" : "hank_the_third",
	"key" : ["Williams", "Hank", 41]
	}, {
	"id" : "hank",
	"key" : ["Williams", "Hank", 91]
	}],
	"total_rows": 5
	}
	{% endhighlight %}

	Tips:

	* The sort order is `[nulls, booleans, numbers, strings, arrays, objects]`, so `{startkey: ['Williams'], endkey: ['Williams', {}]}` would return all people with the last name `'Williams'` because objects are higher than strings. Something like `'zzzzz'` or `'\uffff'` would also work.
	* `group_level` can be very helpful when working with complex keys. In the example above, you can use `{group_level: 1}` to group by last name, or `{group_level: 2}` to group by last and first name. (Be sure to set `{reduce: true, group: true}` as well.)

	#### Linked documents

	PouchDB fully supports [linked documents](https://wiki.apache.org/couchdb/Introduction_to_CouchDB_views#Linked_documents). Use them to join two types of documents together, by simply adding an `_id` to the emitted value:

	{% include code/start.html id="query3" type="callback" %}
	{% highlight js %}
	function map(doc) {
	// join artist data to albums
	if (doc.type === 'album') {
	emit(doc.name, {_id : doc.artistId, albumYear : doc.year});
	}
	}
	db.query(map, {include_docs : true}, function (err, response) {
	if (err) { return console.log(err); }
	// handle result
	});
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query3" type="async" %}
	{% highlight js %}
	function map(doc) {
	// join artist data to albums
	if (doc.type === 'album') {
	emit(doc.name, {_id : doc.artistId, albumYear : doc.year});
	}
	}
	try {
	var result = await db.query(map, {include_docs : true});
	} catch (err) {
	console.log(err);
	}
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query3" type="promise" %}
	{% highlight js %}
	function map(doc) {
	// join artist data to albums
	if (doc.type === 'album') {
	emit(doc.name, {_id : doc.artistId, albumYear : doc.year});
	}
	}
	db.query(map, {include_docs : true}).then(function (result) {
	// handle result
	}).catch(function (err) {
	console.log(err);
	});
	{% endhighlight %}
	{% include code/end.html %}

	#### Example response:

	{% highlight js %}
	{
	"offset": 0,
	"rows": [
	{
	"doc": {
	"_id": "bowie",
	"_rev": "1-fdb234b78904a5c8293f2acf4be70d44",
	"age": 67,
	"firstName": "David",
	"lastName": "Bowie"
	},
	"id": "album_hunkeydory",
	"key": "Hunky Dory",
	"value": {
	"_id": "bowie",
	"albumYear": 1971
	}
	},
	{
	"doc": {
	"_id": "bowie",
	"_rev": "1-fdb234b78904a5c8293f2acf4be70d44",
	"age": 67,
	"firstName": "David",
	"lastName": "Bowie"
	},
	"id": "album_low",
	"key": "Low",
	"value": {
	"_id": "bowie",
	"albumYear": 1977
	}
	},
	{
	"doc": {
	"_id": "bowie",
	"_rev": "1-fdb234b78904a5c8293f2acf4be70d44",
	"age": 67,
	"firstName": "David",
	"lastName": "Bowie"
	},
	"id": "album_spaceoddity",
	"key": "Space Oddity",
	"value": {
	"_id": "bowie",
	"albumYear": 1969
	}
	}
	],
	"total_rows": 3
	}
	{% endhighlight %}

	#### Closures

	If you pass a function to `db.query` and give it the `emit` function as the second argument, then you can use a closure. (Since PouchDB has to use `eval()` to bind `emit`.)

	{% include code/start.html id="query4" type="callback" %}
	{% highlight js %}
	// BAD! will throw error
	var myId = 'foo';
	db.query(function(doc) {
	if (doc._id === myId) {
	emit(doc);
	}
	}, function(err, results) {
	// you'll get an error here
	});

	// will be fine
	var myId = 'foo';
	db.query(function(doc, emit) {
	if (doc._id === myId) {
	emit(doc);
	}
	}, function(err, results) {
	if (err) { return console.log(err); }
	// handle result
	});
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query4" type="async" %}
	{% highlight js %}
	// BAD! will throw error
	var myId = 'foo';
	try {
	var result = await db.query(function(doc) {
	if (doc._id === myId) {
	emit(doc);
	}
	});
	} catch (err) {
	// you'll get an error here
	}

	// will be fine
	var myId = 'foo';
	try {
	var result = await db.query(function(doc, emit) {
	if (doc._id === myId) {
	emit(doc);
	}
	});
	} catch (err) {
	console.log(err);
	}
	{% endhighlight %}
	{% include code/end.html %}

	{% include code/start.html id="query4" type="promise" %}
	{% highlight js %}
	// BAD! will throw error
	var myId = 'foo';
	db.query(function(doc) {
	if (doc._id === myId) {
	emit(doc);
	}
	}).catch(function (err) {
	// you'll get an error here
	}

	// will be fine
	var myId = 'foo';
	db.query(function(doc, emit) {
	if (doc._id === myId) {
	emit(doc);
	}
	}).then(function (result) {
	// handle result
	}).catch(function (err) {
	console.log(err);
	});
	{% endhighlight %}
	{% include code/end.html %}

	Note that closures are only supported by local databases with temporary views. So if you are using closures, then you must use the slower method that requires a full database scan.