db.command.aggregate.map

1. Operator Description

Function: Similar to the map method on JavaScript Array, it transforms each element of a given array according to a specified transformation method to produce a new array.

Declaration: db.command.aggregate.map({ input: expression, as: string, in: expression })

2. Operator Parameters

Field	Type	Required	Description
input	Expression	Required	An expression that resolves to an array.
as	string	No	Optional. The variable representing each element in the array, defaults to this.
in	Expression	Required	An expression that can be applied to each element of a given array. The element name is determined by the as parameter (parameter names must be prefixed with $$, e.g., $$this).

3. Sample Code

Suppose the collection stats contains the following records:

{
  "_id": 1,
  "sales": [ 1.32, 6.93, 2.48, 2.82, 5.74 ]
}
{
  "_id": 2,
  "sales": [ 2.97, 7.13, 1.58, 6.37, 3.69 ]
}

Truncate each number to an integer and then sum them up.

// 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

exports.main = async (event, context) => {
  const res = await db
    .collection('stats')
    .aggregate()
    .project({
      truncated: $.map({
        input: '$sales',
        as: 'num',
        in: $.trunc('$$num')
      })
    })
    .project({
      total: $.sum('$truncated')
    })
    .end()
  console.log(res.data)
}

The returned result is as follows:

{ "_id": 1, "index": 16 }
{ "_id": 2, "index": 19 }