How to decompose a return?

What you're returning is an Object , so you can access the object's properties xand yalter them however you want.

function someFunction(){
  return {
          x: 199,
          y: 200
         };
}

var obj = someFunction();
obj.x += 20;
obj.y += 20;
console.log(obj.x +","+obj.y);

In your question you refer to

How to decompose a return?

Even if you ask something slightly different later in the text, for completeness it is appropriate to talk about destructuring . An option introduced in ES6 to easily "unpack" objects. In the example that you put, if what we were interested in was really "decomposing" the object returned by the return, you could do something like this:

function foo() {
    return {
        x: 199,
        y: 200,
    }
}

let {x, y} = foo();
x += 20; 
y += 20;
console.log(x); // 219
console.log(y); // 220

It's best to create a function that adds functionality.

Imagine a function as follows

function Original(x,y)
{
  return {x,y}
}

You want to add a quick mechanism to add a certain number to both values, so you create another function that abstracts the process, first copies the output of the Original function and defines a more comfortable prototype

   function EnvolturaSumar(i)
    {
      this.x += i
      this.y += i
    }

Take a look at the full example:

function Original(x,y)
{
  return {x,y}
}

function Envoltura(x,y)
{
  if (this == window)
    throw new Error('debe utilizarse el operador new')
  Object.assign(this, Original(x,y)) 
}

function EnvolturaSumar(i)
{
  this.x += i
  this.y += i
}

Envoltura.prototype = Object.create(Object.prototype, {
   sumar: {value: EnvolturaSumar}
})

var x = new Envoltura(2,3)
console.log(x)
x.sumar(20)
console.log(x)
x.sumar(-10)
console.log(x)

function changeValorJson(){
  var a={};
  
          a.x= 1;
          a.y= 2;
         
         return a;
         
}
var valorJson = changeValorJson();
console.log(valorJson.x );
valorJson.x =valorJson.x + 100;

console.log(valorJson.x );

to access an attribute of a json object you must use the ., example

valorJson.x