Javascript - Window resize event in class makes class properties undefined

Question

I'm pretty bad with OOP principles, so this is gonna be hard for me to explain, but here goes.

What I'm trying to do is move a "window.resize" event handler into a class, and have any objects instantiated from said class deal with its own resizing, but the problem is that when I move the handler into the class, and the handler then fires, the object can't seem to access it's own properties - their all undefined.

Let me rather show you what I'm trying to do.

My current (working) code:

// index.js
window.addEventListener('resize', resize);

let thing;
function setup() {
    thing = new CustomThing();
}

function resize() {
    thing.resize();
}

setup();


// CustomThing.js
class CustomThing {
    constructor() {
        this.background = { width:100, height:100 };
    }

    resize() {
        console.log('I\'m successfully resizing. Yay!');
        this.background.width = window.innerWidth;
    }
}

I'm trying to turn the above code into the following:

// index.js
let thing;
function setup() {
    thing = new CustomThing();
}
setup();
// Notice that I don't have the event listener or resize function here anymore


// CustomThing.js
class CustomThing {
    constructor() {
        this.background = { width:100, height:100 };
        window.addEventListener('resize', this.resize);
    }

    resize() {
        console.log('this.background is now undefined :(');
        this.background.width = window.innerWidth;
    }
}

I've also tried changing the above window.addEventListener('resize', this.resize); to window.addEventListener('resize', () => this.resize(this.background));, and resize() into resize(background), and then only using background in the resize function instead of this.background, in which case I can actually access the background's properties. But then it's as if it creates a copy of this.background for me to manipulate, becuase my object doesn't actually resize.

So my question is: How can I refactor the below code, so that I can have the resize event handler in my class, call a function when it triggers, and then successfully access the class' other properties from within the function that got called?

FWIW, this is taken (and simplified) from a PIXI.js application I'm working on. Let me know if I need to include more details or bring in some of the code from my actual PIXI application.