please help me, how can I find out the width of an object?

Question

var page_body = document.getElementsByTagName("body")[0];

Hello.
Please tell me. )
I placed a square object on the canvas, and converted it into a symbol.
Gave him a name - chief
Now I'm writing code -

page_body.style.overflow = "hidden";
page_body.style.position = "fixed";
var page_canvas = document.getElementsByTagName("canvas")[0];

var _this = this;

console.log("_this.chief.y - "+_this.chief.y);