If you’re looking for a way to write easily maintainable and portable CSS in React, you may be interested in using the styled-components library. It’s one of a number of CSS in JS libraries that let you skip some of the pains that you may otherwise experience dealing with large and unwieldy .css files.

What is CSS in JS, anyway?

When using plain CSS, you’d define your CSS in one file:

.saveButton {
  color: green;
}

And refer to it via the className prop in your React code:

export const App = () => <button className='saveButton'>Save</button>

As your codebase grows in size, managing these CSS files can become unwieldy, and the naming of your CSS classes will become super important (and sometimes complicated) - you have to remember that your defined CSS is global to your entire app, and to make sure you don't end up with naming conflicts!

On the other hand, CSS in JS libraries let you define your CSS inside of your JavaScript files. For example using styled-components it will look like this:

import styled from 'styled-components';

// Creates a <button> with the style "color: green"
const SaveButton = styled.button`
  color: green;
`;

export const App = () => <SaveButton>Save</SaveButton>

The benefit here is that when using SaveButton, this CSS will only apply to what it wraps around, and nothing else. Of course there are other cool benefits too like being able to use props, which we'll explore later in this guide.

Where did the class names go?

You may have noticed that we're no longer defining CSS class names - styled-components does it for us behind the scenes! If you took a look at your React app's source code, you would see something like this:

<style>
  .cPQVKZ { color: green; }
</style>

<!-- Other stuff goes here... -->

<button class="sc-AxjAm cPQVKZ">Save</button>

There will be two randomly generated class names for every React component:

• The first one (starting with sc-) is a reference to the SaveButton styled component itself and won't ever have any styles attached to it
• The second one (in this example cPQVKZ) will contain the styles defined inside of SaveButton

Using the styled API

So far we’ve seen styled.button, but we can also use any HTML tag in its place, like h1 or div:

const Header = styled.h1`
  font-size: 16px;
`;

const CoolDiv = styled.div`
  padding: 8px;
`;

We can use other styled components too, and inherit all of their styles (and override any that are the same):

const BoringButton = styled.button`
  color: blue;
`;

const CoolButton = styled(BoringButton)`
  color: pink;
`;

Passing in props

Another part of the magic of styled components is that you can pass in props!

const CoolButton = styled.button`
  color: ${props => props.color};
`;

export const App = () => (
  <>
    <CoolButton color="yellow"/>
    <CoolButton color="green"/>
  </>
);

The source code will end up looking like this:

<style>
  .jUNwe { color: yellow; }
  .krQJtN { color: green; }
</style>

<!-- Other stuff goes here... -->

<button color="yellow" class="sc-AxjAm jUNwe">Save</button>
<button color="green" class="sc-AxjAm krQJtN">Save</button>

You can see that for each value we use for the color prop, a new CSS class will be created. We can also see that the sc-AxjAm class in each button is the same - since this class name refers to the CoolButton itself, and doesn’t change regardless of props.

Using the css helper function

We can define CSS styles using the css helper function:

import { css } from 'styled-components';

const whiteColor = css`
  color: white;
`;

This can come in handy if you have a common set of styles that you want to use in multiple styled components:

const Button = styled.button`
  ${whiteColor};
  background-color: red;
`;

const AnotherButton = styled.button`
  ${whiteColor};
  background-color: green;
`;

The styles defined in whiteColor will not be its own separate class, but will be added to both of the CSS classes generated for Button and AnotherButton.

Modifying the styles of a styled component’s children

If you’ve created a styled component, and you need to change the styles of one of its children, you can add selectors:

const Button = styled.button`
  .h1 {
    color: pink;
  }
`;

Referring to .h1 will apply to children at any depth - if you only want to refer to any direct children, you’ll need to use > .h1 instead.

Referring to other styled components

Instead of just referring to tag names, you can also refer to other styled components!

const ChildButton = styled.button`
    color: pink;
`;

const CoolDiv = styled.div`
  ${ChildButton} {
    color: green;
  }
`;

export const App = () => (
  <>
    <CoolDiv>
      <ChildButton>Click me</ChildButton>
    </CoolDiv>
  </>
);

This will override the original styles of ChildButton.

The styled-components library is the biggest and most popular CSS in JS library at the moment, so it’s very easy to get started with all the documentation and help available for it on the internet.

However if you’re still shopping around for a CSS in JS library, I’d also encourage you to check out some alternatives such as emotion and linaria to see if they’re right for you.

Thanks for reading!