Hello. I was making a code and I just found this error which really bugs me.

Question

I am trying to make a code, and I made an array of objects with different properties. It is something like:
[{x:W, y: 200, h: this.y-100}] and etc. The issue I have is that h is evaluated to NaN because this.y is undefined. Why is that? How could I define the property h being property y - 100? I always avoided this, but I do not know what is the problem. I would appreciate some help.

Answer 1

You cannot do that with an object litteral assignement: the object properties are only accessible once the object is created (ie: after you close the curly braket)... The solution is to assign your object values in two times, or create a JS class-like function (an object constructor):

var my_litteral_obj = {x:W, y:200};
my_litteral_obj.h = my_litteral_obj.y-100;

... or:

function MyClassObj(_x,_y) {
this.x = _x;
this.y = _y;
this.h = this.y-100;
}
var my_class_obj = new MyClassObj(W,200);

In the first case, there's no related 'this' object, while in second case, the constructor is called with the 'new' keyword, so 'this' is initialized to a new empty object reference... and is already accessible in the constructor scope ;)
In a certain point of view, it's near to do:

var my_obj = {}; // create an empty object
my_obj.prop = 42;
my_obj.value = my_obj.prop;

... but from inside the object itself, with help of the special object 'this' :P

Answer 2

Thank you guys. I will use the constructor function then :)

Answer 3

Try to use constructor function

var obj = [new function() {
this.W=1;
this.x = this.W;
this.y = 200;
this.h = this.y-100;
}];

console.log(obj[0].x);
console.log(obj[0].y);
console.log(obj[0].h);

https://code.sololearn.com/W7MXx7FeT2t7/?ref=app

Answer 4

I am not a profisional but try to creat h in new array after craeting y