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]?

• Kinglish: const superIds = supervisors.map(s => s.userId);

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.

• Jetro Olowole: Thanks @Barmar I used it as described and I got the error UnhandledPromiseRejectionWarning: TypeError: q.supervisors.map is not a function
• Barmar: I assumed q was the object you showed in the question. If not, replace it with the correct variable.
• Jetro Olowole: in my case I assume q will be req.body so I replace it and still got the error req.body.supervisors.map is not a function
• Jetro Olowole: please take a look again at the question I have updated it with more details
• Barmar: The client didn't send the parameters correctly. They concatenated the supervisor objects, which converted them to strings [object Object].

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);
}

• Jetro Olowole: Thanks @Rounin when I do I got the error UnhandledPromiseRejectionWarning: ReferenceError: myObject is not defined
• Rounin: myObject is an example name (like foo and bar) - in the example above, it's a stand-in for whatever variable name you're actually using.
• Jetro Olowole: When I do, I got a long undefined value. Meanwhile I updated the question with more details supervisors => [ undefined ] supervisors => [ undefined, undefined ] supervisors => [ undefined, undefined, undefined ]
• Jetro Olowole: please take a look again at the question I have updated it with more details
• Rounin: @Wacademy - you've been given two clear answers on this page showing you how to build a simple array from an object: 1) using the map() function; 2) using a for loop . You surely have enough to write your own working code at this point. Are there any parts of either of those two answers which are not making sense to you?