Node.js, MongoDB and Angular Web Development: The definitive guide to using the MEAN stack to build web applications (Developer's Library) by Brad Dayley & Brendan Dayley & Caleb Dayley

Author:Brad Dayley & Brendan Dayley & Caleb Dayley [Dayley, Brad]
Language: eng
Format: azw3
Publisher: Pearson Education
Published: 2017-10-11T04:00:00+00:00

query: Query object that defines which documents to be included in the initial set. See Table 15.1 for a list of query options.

initial: Specifies an initial group object to use when aggregating data while grouping. An initial object is created for each distinct set of keys. The most common use is a counter that tracks a count of items that match the keys. For example:

{"count":0}

reduce: This is a function(obj, prev) with two parameters, obj and prev. This function is executed on each document that matches the query. The obj parameter is the current document, and the prev parameter is the object created by the initial parameter. You can then use the obj object to update the prev object with new values such as counts or sums. For example, to increment the count value you would use:

function(obj, prev) { prev.count++; }

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