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CSS Variables React hook


css-vars-hook contains React hooks to set and manipulate CSS custom properties (variables).

Demo

dev.to article

Highlights

- CSS Variables in React: manage your component design in a fast and convenient way.
- Dynamic Theming: create and manage themes for your application. Apply multiple CSS variables to any HTML element.
- TypeScript Support: The library is written in TypeScript, offering type safety and enhancing developer experience.
- Zero Dependencies: It operates independently without the need for additional libraries, ensuring a lightweight integration.
- Performance: The hook is optimized for performance, with a small footprint that does not impact application speed.

Install

``shell script
npm install css-vars-hook
`

Usage

css-vars-hook exposes two hooks: useRootTheme, useLocalTheme. Both of them provide developer a bridge between React Component state and CSS Custom Properties.

useRootTheme

useRootTheme applies application level themes. API consists of two elements: the hook itself and RootThemeProvider component which acts as :root selector. Directly applying theme to the :root is not compatible with Server side rendering (SSR).

Manipulate theme

$3

In order to set global theming you need to wrap your application with RootThemeProvider on highest possible level.

`jsx
// App.js
import React from 'react';
import {RootThemeProvider} from 'css-vars-hook';

// Theme object contains dictionary of CSS variables you will use later in your application
const theme = {
boxColor: 'purple',
borderColor: 'violet',
}

export const App = () => (
theme={theme}>
{/.../}

);
`

$3

To avoid unnecessary reconciliations and re-renders theme object has to preserve referential equality during component lifecycle.

#### Wrong examples

Arbitrary objects are recreated every time React component reconciles. Avoid this when defining theme object.

`tsx
// Don't do this!!!
const Component: FC = () => {
//...
const theme = {
foo: 'bar'
}

return {/.../}
}
`

`tsx
// Don't do this!!!
const Component: FC = () => {
//...
return {/.../}
}
`

#### Correct examples

Set theme object externally to Component or wrap with useMemo.

`tsx
// Correct!
const theme = {
foo: 'bar'
}

const Component: FC = () => {
return {/.../}
}
`

`tsx
// Correct! Theme will preserve until foo property change
const Component: FC<{foo: string}> = ({foo}) => {

const theme = useMemo(() => ({foo}), [foo])

return {/.../}
}
`

$3

Theme changing methods (setTheme, setVariable, removeVariable) are implemented as effects. They will apply after component re-render. You'll have to wrap the side effect with useEffect or put in inside callback to move it out of the rendering calculation.
`jsx
// Component.jsx
import React, { useEffect, useCallback } from "react";
import { useRootTheme } from 'css-vars-hook';

const theme = {
boxColor: 'red',
borderColor: 'green',
}

const Component = () => {
const { setTheme, setVariable, removeVariable } = useRootTheme();

// Set theme value inside useEffect hook
useEffect(() => {
// Theme changing effects can be applied like this. The change will happen after render.
setTheme(theme);
}, [theme, setTheme])

// Set theme value inside callback
const handleVariable = useCallback(() => {
setVariable('boxColor', 'pink');
}, [])

return Change variable;
}
`

$3

`jsx
//...
const Component = () => {
const { setTheme } = useRootTheme();

// This will not work!
setTheme(theme)

//...
}
`

The reason this code isn’t correct is that it tries to do something with the DOM node during rendering. In React, rendering should be a pure calculation of JSX and should not contain side effects like modifying the DOM. Moreover, when Component is called for the first time, its DOM does not exist yet, so there is no theme container to operate with.

$3

Developers can provide theme type to useRootTheme hook as a TypeScript Generic.

`tsx
import { FC } from "react";

type Theme = {
boxColor: 'yellow' | 'blue';
borderColor: string;
};

const themeYellow: Theme = {
boxColor: 'yellow',
borderColor: 'blue',
};

const Component: FC = () => {
const {setTheme, getTheme, setVariable} = useRootTheme();
const doSomething = () => {
// theme value will be properly typed this way
console.log('root theme', getTheme().boxColor);
};
//...
}
`

$3

CSS variables set by RootThemeProvider are available globally across all application.

#### In CSS

`postcss
// Component.css

.box {
background: var(--boxColor);
border: 1px solid var(--borderColor)
}
`

#### In JS

`js
import {useRootTheme} from 'css-vars-hook';

const {
/* Get current theme /
getTheme,
/* Get variable value within active theme /
getVariable,
} = useRootTheme();

console.log(getVariable('boxColor')) // => 'purple'
console.log(getTheme()) // => theme object
`

useLocalTheme

useLocalTheme applies theme locally to the wrapped React components.

$3

In order to set local theme you need to wrap your component with LocalRoot component which is returned by useLocalTheme hook.

`jsx
import { useLocalTheme } from 'css-vars-hook';
import { useCallback } from "react";

const theme = { boxColor: 'yellow' };
const darkTheme = {boxColor: 'darkYellow'};

const Component = () => {
const { LocalRoot, setTheme } = useLocalTheme();
const setDarkMode = useCallback(() => {
setTheme(darkTheme)
}, []);
return {/.../}
}
`

Outside different wrapping strategies this hook is similar to useRootTheme.

$3

By default LocalRoot is rendered as a div HTMLElement. You can provide custom element type (button, span, e. t. c.) by changing as prop of LocalRoot.

`jsx
import {useLocalTheme} from 'css-vars-hook';

const theme = {boxColor: 'yellow'};
const darkTheme = {boxColor: 'darkYellow'};

const Component = () => {
const {LocalRoot: Button, setTheme} = useLocalTheme();
const setDarkMode = useCallback(() => {
setTheme(darkTheme)
}, [])
return (
theme={theme}
as="button"
onClick={setDarkMode}>
Set dark mode

)
}
`

$3

Local theme type is inferred from corresponding LocalRoot` prop.