How to prevent "this" from being overwritten? [duplicate]

So I am working with the ES6 class stuff, which I've not had a chance to mess around with before, and event handlers, and I'm encountering an issue with the value of my this.

Javascript:

class Chat {
    constructor() {
        /*code*/
    }

    handleOnKeyDown() {
        console.log("A key was pressed somewhere on the page!")
        console.log(this)
    }
}

HTML:

let chat = new Chat()

chat.handleOnKeyDown() //this === [Chat Object]

document.onkeydown = chat.handleOnKeyDown //this === #document

How is my this being overwritten, and how can I prevent it from doing so? I don't particularly need the this from the event handler, since it's just literally the entire document, and isn't giving me any extra context.