db.command.aggregate.reduce

1. Operator Description

Function: Similar to JavaScript's reduce method, applies an expression to each element of an array and reduces them into a single element.

Declaration: db.command.aggregate.reduce({ input: array, initialValue: expression, in: expression })

2. Operator Parameters

Field	Type	Required	Description
input	Expression	Required	The input array, which can be any expression that resolves to an array.
initialValue	Expression	Required	Initial value
in	Expression	Required	An expression applied to each element. Two variables are available in in: value represents the accumulated value, and this represents the current array element.

3. Sample Code

Simple String Concatenation

Suppose the collection player contains the following records:

{ "_id": 1, "fullname": [ "Stephen", "Curry" ] }
{ "_id": 2, "fullname": [ "Klay", "Thompsom" ] }

Get the full names of all players and prefix them with Player:.

// Sample code in the Cloud Function environment
const tcb = require('@cloudbase/node-sdk')
const app = tcb.init({
  env: 'xxx'
})

const db = app.database()
const $ = db.command.aggregate
const _ = db.command

exports.main = async (event, context) => {
  const res = await db
    .collection('player')
    .aggregate()
    .project({
      info: $.reduce({
        input: '$fullname',
        initialValue: 'Player:',
        in: $.concat(['$$value', ' ', '$$this'])
      })
    })
    .end()
  console.log(res.data)
}

The returned result is as follows:

{ "_id": 1, "info": "Player: Stephen Curry" }
{ "_id": 2, "info": "Player: Klay Thompson" }

Get the full names of all players without any prefix.

const $ = db.command.aggregate
db.collection('player')
  .aggregate()
  .project({
    name: $.reduce({
      input: '$fullname',
      initialValue: '',
      in: $.concat([
        '$$value',
        $.cond({
          if: $.eq(['$$value', '']),
          then: '',
          else: ' '
        }),
        '$$this'
      ])
    })
  })
  .end()

The returned result is as follows:

{ "_id": 1, "name": "Stephen Curry" }
{ "_id": 2, "name": "Klay Thompson" }