Suppose the following class:

class MyClass {
    private keysPressed: string[] = [];

    configure() {
        window.addEventListener('keyup', this.handleKeyUp);
    }

    handleKeyUp(e: KeyboardEvent) {
        this.keysPressed.push(e.key);
    }
}

The following throws an error:

const instance = new MyClass();
instance.configure(); // TypeError: Cannot read properties of undefined (reading 'push')

If I add console.log(this.keysPressed) in configure, it sees keysPressed as expected, however handleKeyUp seems to behave differently.

Why is this happening?