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+---
+title: "Getting theme colors in JavaScript using React with DaisyUI and TailwindCSS"
+date: 2023-08-10T00:18:27-05:00
+toc: false
+images:
+tags:
+  - web
+  - dev
+  - react
+---
+
+I've been building a web app using React and TailwindCSS, with DaisyUI. But
+while working on it I hit a minor snag: I'm trying to use Chart.js, and Chart.js
+creates a canvas to render charts. But the canvas can't pick up the CSS
+variables that are defined on the page itself. That means that my charts can't
+use the colors from my theme, unless I manually copy and paste the theme colors!
+This is pretty bad for maintainability because if the theme colors are ever
+changed, or themes are added or removed, you'll have to come back and update the
+colors for the charts as well. Boo!
+
+Luckily, I found out that you can read CSS variables from JavaScript. So I added
+this code:
+
+```ts
+function getPropertyValue(name: string) {
+  return getComputedStyle(document.body).getPropertyValue(name);
+}
+```
+
+This returns the value of any CSS variable you pass it. For example
+`getPropertyValue("--tab-border")` returns `1px` for my theme!
+
+Next, I just looked through the CSS on the page to figure out what CSS variables
+DaisyUI sets for themes. I quickly found the most important ones I needed: the
+primary and secondary colors, and the colors for the text that goes on top of
+them.
+
+```ts
+const primary = getPropertyValue("--p");
+const secondary = getPropertyValue("--s");
+const primaryText = getPropertyValue("--pc");
+const secondaryText = getPropertyValue("--sc");
+```
+
+This is all great! But I had one more concern: I needed a way to change these
+variables and re-render components whenever the user toggles between the light
+and dark themes.
+
+I decided to use SWR for this. SWR is mainly meant to be used to fetch data from
+an API, but there's really nothing stopping you from using it for anything else.
+In this case, SWR will cache all the colors in a primary place, and allow me to
+re-render all the components when the colors change using its `mutate` API.
+Here's how that code looks like:
+
+```ts
+export function useThemeColor() {
+  const themeFetcher = useCallback(() => {
+    const primary = getPropertyValue("--p");
+    const primaryText = getPropertyValue("--pc");
+    const secondary = getPropertyValue("--s");
+    const secondaryText = getPropertyValue("--sc");
+    return { primary, primaryText, secondary, secondaryText };
+  }, []);
+
+  // The key "data:theme" could be anything, as long as it's unique in the app
+  const { data: color, mutate } = useSWR("data:theme", themeFetcher);
+
+  return { ...color, mutate };
+}
+```
+
+It's very easy to use it.
+
+```ts
+export default function Dashboard() {
+  const { primary, primaryContent } = useThemeColor();
+
+  // ... fetch the data and labels ...
+
+  return (
+    <Line
+      data={{
+        labels,
+        datasets: [
+          {
+            data,
+            // borderColor is the color of the line
+            borderColor: `hsl(${primary})`,
+            // backgroundColor is the color of the dots
+            backgroundColor: `hsl(${primaryContent})`,
+          },
+        ],
+      }}
+    />
+  );
+}
+```
+
+Here's what that looks like:
+
+![A screenshot of a web page. At the top there is a dark red colored button labelled "Dashboard". There is a line chart below, which uses the same dark red color as the button.](/img/2023-08-10.chartjs.png)
+
+To keep the colors changing whenever the user toggles the theme, you then just
+have to call the `mutate` function inside your toggle button.
+
+```ts
+export function ThemeToggle() {
+  const { mutate: mutateTheme } = useThemeColor();
+  const [theme, setTheme] = useState("light");
+  const toggleTheme = useCallback(() => {
+    if (theme === "dark") {
+      document.body.dataset.theme = "autumn";
+      setTheme("light");
+    } else {
+      document.body.dataset.theme = "forest";
+      setTheme("dark");
+    }
+    mutateTheme();
+  }, [theme, mutateTheme, setTheme]);
+
+  return (
+    <div className="btn btn-ghost text-xl" onClick={toggleTheme}>
+      <PiLightbulbBold />
+    </div>
+  );
+}
+
+```
+
+Oh and that's a bonus trick for you. You can swap the DaisyUI theme by just
+setting `document.body.dataset.theme`, as long as that theme is enabled in the
+settings.