How to add new key/value to a JavaScript object?

How to add new element to Javascript object? In this article we will show you simple way to add key/value pair to Javascript object by using spread operator where you can add new object element in just one line. Here is how:

So, let's say we have a object which contains online users:

let onlineUsers = {
  1421:{ // user id
    name: "foo"  // user name
  }
}

And now we want to add new user to this object,
first let's create new object:

let newUser = {
  1439:{ // user id
    name: "bar"  // user name
  }
}

and now we can simply add new element to object by using spread operator:

onlineUsers = {...onlineUsers, ...newUser};

and now your object should look like this:

{
1421:{ 
    name: "foo" 
  },
1439:{
    name: "bar"
  }
}

Here is another example, how to add just one new key/value to existing object element. Here is example of the object:

let user = {
 name:'tester',
 role:'user'
}

and now we want to add property lastName to existing user, we will take existing object and just add new key with value to it:

user = {...user,'lastName':'testoso'};

and now our object looks like this:

{
  name: "tester", 
  role: "user", 
  lastName: "testoso"
}

and that is it.
happy coding