CSS Variables - Swapping values? - css

Question

I have a very simple problem with CSS variables. I would like to swap two CSS variables, basically the CSS equivalent of [a, b] = [b, a] in ES6. Here's a simple example:
<p>White background</p>
<button>Black background</button>
<div>
<p>Black background</p>
<button>White background</button>
</div>
:root {
--primary-color: #fff;
--secondary-color: #000;
}
body {
background-color: var(--primary-color);
}
button {
background-color: var(--secondary-color);
}
div {
/* i'd like to do the following: */
--primary-color: var(--secondary-color);
--secondary-color: var(--primary-color);
/* so here, `--primary-color` would be `--secondary-color` from `:root`
* and any children have these colors swapped as well
*/
background-color: var(--primary-color);
}
However, this fails because CSS var()s are live bindings. Am I missing something here? Or is this the way the spec currently works?

Answer 1

You are creating a cyclic dependence because you are defining each property using the other one and this won't work. Instead you may try something like this by introducing more variables:
:root {
--p:#fff;
--s:#000;
--primary-color: var(--p);
--secondary-color: var(--s);
}
body {
background-color: var(--primary-color);
}
button {
background-color: var(--secondary-color);
}
div {
/* i'd like to do the following: */
--primary-color: var(--s);
--secondary-color: var(--p);
background-color: var(--primary-color);
}
<p>White background</p>
<button>Black background</button>
<div>
<p>Black background</p>
<button>White background</button>
</div>