How to assign a property in object to the value of some property in same object in javascript?

Question

In code below I'm creating a circle objects and giving it's keys some values. I'm setting the radius property of the circle object to it's diameter divided by 2. When we console.log it's value, it returns NAN. How to fix this problem?

let circle = { posX: 40, posY: 70, diameter: 30, radius: this.diameter/2 } console.log(circle.radius)

this does not refer to circle. You could call circle.radius = circle.diameter/2 after the initial assignment or add it as a function to the object. — Lain
– Lain, Commented Jul 26, 2021 at 13:57
May I ask why you are setting both the radius and the diameter attributes, when you can easily calculate one from the other if/when you need it? — secan
– secan, Commented Jul 26, 2021 at 14:01

AlexSp3 · Accepted Answer · 2021-07-26 14:42:52Z

You need a method inside the object in order to do it, because you are using the this keyword, and it needs a function to work:

let circle = { posX: 40, posY: 70, diameter: 30, radius: function () { return this.diameter/2; } } console.log(circle.radius())

Sleepwalker · Accepted Answer · 2021-07-26 14:01:04Z

You can use a class:

class Circle { posX; posY; diameter; radius; constructor(posX, posY, diameter){ this.posX = posX; this.posY = posY; this.diameter = diameter; this.radius = diameter / 2; } }

Then when you instanciate it like the following, the radius is automatically set to diameter/2

let circle = new Circle(40, 70, 30); // circle.radius is 15

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