I assign a property of an object to be the result of the sum of two other properties. The result is not what I expect. Why is this?

Question

Here's my code:

var obj = {
  x: 25,
  y: 24,
  z: this.x + this.y,

  a: function() {
    return this.x + this.y;
  }

}
console.log(obj.z) //-> NaN
console.log(obj.a()) //-> 49

I expect console.log(obj.z) to output what console.log(obja()) outputs. But this is not the case. I suspect that there is a good reason not to assign a value of a property to depend on other properties. If so, why?