Why the answer is NaN?

Question

class enemy{
constructor(height,strength){
this.height=height;
this.strength=strength;}
}
let chris=new enemy(100,50);
let{prop1,prop2} = chris;
console.log(prop1 + prop2)

Answer 1

Use the actual names of the properties when object deconstructing

let{height, strength} = chris;

if you want/need to use an alias then you need both the original and the substitute names.

let{height: prop1,strength: prop2} = chris;

Answer 2

prop1 and prop2 are undefined, hence you get NaN when trying to add them.

Have you considered using
let [prop1, prop2] = [chris.height, chris.strength];
?

Answer 3

We'd probably need to see more than just that.
Do you have all your code wrapped in a window.onload?