JavaScript Scoping Can't Access setInterval

You have to store the timer handle somewhere. :-)

You have lots of options:

Modules

You could use modules. Then a top-level declaration of interval wouldn't be a global, it would only be accessible to the module:

let interval = 0;
export function beginSlideshow() {
    interval = setInterval(function () {
        //Slideshow content here
    }, someValue);
}

export function revertSlideshow() {
    clearInterval(interval);
    interval = 0;
}

In a closure's scope

Similar concept to the module above, but without using modules:

const { beginSlideshow, revertSlideshow } = (() => {
    let interval = 0;
    function beginSlideshow() {
        interval = setInterval(function () {
            //Slideshow content here
        }, someValue);
    }

    function revertSlideshow() {
        clearInterval(interval);
        interval = 0;
    }

    return { beginSlideshow, revertSlideshow };
})());

In the caller's scope

You could make this the problem of the person calling beginSlideshow by returning the function to stop it:

function beginSlideshow() {
    const interval = setInterval(function () {
        //Slideshow content here
    }, someValue);
    return () => {
        clearInterval(interval);
    };
}

The caller would use that like this:

const revertSlideshow = beginSlideShow();
// ...
revertSlideshow();

Another way to store it in the caller's scope is to wrap this up in a class and have the handle be a data property:

class Slideshow {
    interval = 0;

    begin() {
        this.interval = setInterval(/*...*/);
    }

    revert() { // I'd call it "end"
        clearInterval(this.interval);
        this.interval = 0;
    }
}