In JavaScript, you can't do some kind of query selector like:

document.querySelector("div::after");

But, with the power of CSS variables, you can still change the styles of those selectors with JavaScript!

In your CSS, pick a variable name and assign it to something:

div::after {
    /* 50px is the default value if --somewidth doesn't exist */
    width: var(--somewidth, 50px);
}

And in your JavaScript, you use the setProperty function to assign a value to that variable!

// this is just grabbing a div, you can change it to select any element
const element = document.getElementsByTagName("div")[0];

element.style.setProperty("--somewidth", "50%");

So there ya go! You can obviously make this more complex as needed. Here's an example of all of this in action! It's a template I made for tracking fundraising efforts (feel free to use the template on CodePen).
Specifically note line 38 in the CSS, and line 25 in the JavaScript!