concatinate nested array of object ids

Question

There are two chat rooms and each one is having two owners which are user object IDs. i would like to concatinate both the owners into one array so that i can find details of owners other than current user

  [ { owners: [ 5d6caefdbb6f2921f45caf1d, 5d6caee9bb6f2921f45caf1b ],
   _id: 5d6ccf5d55b38522a042dbb2,
    createdAt: 2019-09-02T08:14:21.734Z,
    updatedAt: 2019-09-02T08:14:21.734Z,
     __v: 0,
    id: '5d6ccf5d55b38522a042dbb2' },

  { owners: [ 5d6dfcd6e3b11807944348b8, 5d6caefdbb6f2921f45caf1d ],
    _id: 5d6dfd48e3b11807944348ba,
     createdAt: 2019-09-03T05:42:32.572Z,
     updatedAt: 2019-09-03T05:42:32.572Z,
      __v: 0,
   id: '5d6dfd48e3b11807944348ba' } ]


    const chatrooms = await ChatRoom.find({owners:{$all:[user._id]}})
    //the above code was run to get result in problem statement

    const owners = []
    chatrooms.forEach((chatroom) => {
        owners.push(chatroom.owners)
    })

i am expecting something like this

owners= [ 5d6caefdbb6f2921f45caf1d, 5d6caee9bb6f2921f45caf1b, 
5d6dfcd6e3b11807944348b8, 5d6caefdbb6f2921f45caf1d]

Manish · Accepted Answer · 2019-09-06 19:19:13Z

2

On newer browsers and node 11, you could use the flatMap method.

Something like this would work:

chatrooms.flatMap(room => room.owners)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatMap

And for older version, you'd do something like this:

const owners = []
chatrooms.forEach((chatroom) => {
  chatroom.owners.forEach(owner => owners.push(owner))
})

answered Sep 6, 2019 at 19:19

Manish

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3 Comments

vithu shaji Over a year ago

can you please explain the flat map, how it works here

Manish Over a year ago

@vithushaji From the link: The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. So it takes all room owners and into an array and then flattens the array, resulting in single array with all owner ids.

vithu shaji Over a year ago

if i want to remove user id from the newly created array. how shall we do it?

Vitor Ribeiro · Accepted Answer · 2019-09-06 19:42:19Z

0

If you want to go all es6 like with javascript (making sure you are using recent browsers) you can use the array function reduce with the array concat method. Like so:

const owners = chatrooms.reduce((acc, chatroom) => acc.concat(chatroom.owners));

answered Sep 6, 2019 at 19:42

Vitor Ribeiro

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