If you want to add multiple CSS styles to an element in Vanilla JS, you could do something like this:

// Grab a button element.
const button = document.querySelector('button');

button.style.backgroundColor = "red";
button.style.color = "white";
button.style.padding = "20px";

It works but looks a bit tedious and unclean. And it's hard to pass such styles around. Would be nice to put those styles in an object, like this:

const style = {
    backgroundColor: "red",
    color: "white",
    padding: "20px"
};

Now, we need to just mix our style object into button.style, like this:

Object.assign(button.style, style);

And voilà, we're done!

If you find this tutorial helpful, please share it however you can :)

P.S. information for those who while reading this, sputters and tears the hair on the head, shouting: "Use CSS!". This one is not about "Why," it's about "How." Of course, you could and, in most cases, should use CSS for this, but that's another story. Relax.