How do I get array of values from a nested object in Javascript

Question

I am trying to create record from a form data. When I console.log(req.body) I get the following record.

req.body => [Object: null prototype] {
  id: '1',
  key1: 'Value1',
  key2: 'Value2',
  supervisors: '[object Object],[object Object],[object Object]'
}

So I checked the database where the record is stored and see that supervisors is stored as:

supervisors: Array(3)
  0:
    name: "Reporter"
    userId: 4
  1:
    name: "Officer 1"
    userId: 5
  2:
    name: "Coordinator"
    userId: 2

I will like to get the values of userId as an array in supervisors field in the req.body so that my req.body will look like:

req.body => [Object: null prototype] {
  id: '1',
  key1: 'Value1',
  key2: 'Value2',
  supervisors: '[4, 5, 2]'
}

1. I did const supervisors = JSON.stringify(req.body.supervisors)) and I got [object Object],[object Object],[object Object] when console logged

2. I did const supervisors = q.supervisors ? JSON.parse(q.supervisors) : []; and I got SyntaxError: Unexpected token o in JSON at position 1 at JSON.parse when console logged.

3. I did const supervisors = req.body.supervisors.map(sup => sup.userId); and I got req.body.supervisors.map is not a function when console logged.

How can I get the supervisors value as [2, 4, 5]?

Answer 1

Use map()

const supervisors = q.supervisors.map(sup => sup.userId);

You don't need to use any JSON functions, as the data you show has already been parsed into an array of objects.

Answer 2

As mentioned above, map() is the correct one-line approach.

map() using an arrow function (which has an implicit return) is also the ideal approach if you need to chain further transformations.

The alternative (verbose but also lightning-fast) approach is (that old work-horse) the for loop.

---

Working Example:

// THE OBJECT
const myObject = {

  supervisors: [

    {
      name: 'Reporter',
      userId: 4
    },

    {
      name: 'Officer 1',
      userId: 5
    },

    {
      name: 'Coordinator',
      userId: 2
    }
  ]
};

// THE SUPERVISORS ARRAY (NOT YET POPULATED)
const supervisors = [];

// LOOP THROUGH THE ARRAY IN THE OBJECT TO POPULATE THE SUPERVISORS ARRAY
for (let i = 0; i < myObject.supervisors.length; i++) {

  supervisors.push(myObject.supervisors[i].userId);
  
  console.log(supervisors);
}