Calculated/transient fields in JavaScript

Question

I am trying to achieve something like the following using OO JavaScript:

class Sample
{
  public int x {get; set;}
  public int y {get; set;}

  public int z
  {
    get {return x+y;}
  }
}

I could not understand on how to implement property 'z' in above class.

That looks like Java, not JavaScript. Are you saying you want to implement something like that in JavaScript? (Also the syntax for the values for "x" and "y" is unfamiliar.) — Pointy, Jul 07 '12 at 15:45
Looks more like C# to me. But however, how did you implement the rest of the class? — Niko, Jul 07 '12 at 15:46
The code is in C# just to explain what I am looking for in JavaScript (not Java). — user203687, Jul 07 '12 at 16:05

score 5 · Accepted Answer · edited May 23 '17 at 12:04

You have to use a function. As of ECMAScript 5th edition (ES5), that function can be a "getter" for a property that you access in the normal non-function way; prior to that you have to use an explicit function call.

Here's the ES5 way, using defineProperty: Live copy | source

function Sample()
{
    // Optionally set this.x and this.y here

    // Define the z property
    Object.defineProperty(this, "z", {
        get: function() {
            return this.x + this.y;
        }
    });
}

Usage:

var s = new Sample();
s.x = 3;
s.y = 4;
console.log(s.z); // "7"

With ES3 (e.g., earlier versions):

function Sample()
{
    // Optionally set this.x and this.y here
}
Sample.prototype.getZ = function() {
    return this.x + this.y;
};

Usage:

var s = new Sample();
s.x = 3;
s.y = 4;
console.log(s.getZ()); // "7"

Note that you have to actually make the function call getZ(), whereas ES5 makes it possible to make it a property access (just z).

Note that JavaScript doesn't (yet) have a class feature (although it's a reserved word and one is coming). You can do classes of objects via constructor functions and prototypes, as JavaScript is a prototypical language. (Well, it's a bit of a hybrid.) If you start getting into hierarchies, there starts to be some important, repetitive plumbing. See this other answer here on Stack Overflow for more about that.

Calculated/transient fields in JavaScript

1 Answers1