Styling

Components can be styled using CSS classes or inline styles as you would in when using plain HTML and CSS without React.

Class

className

  • use className not class
  • pass a string to className
  • it is common for CSS classes to be dynamically added or removed based on data in the component props or state
  • vanilla JavaScript can be used to dynamically build the string of CSS classes

  1. Create the file styles.css

  2. Add the following styles:

    .container {
    width: 50rem;
    }
    .alert {
    position: relative;
    padding: 0.75rem 1.25rem;
    margin-bottom: 1rem;
    border: 1px solid transparent;
    border-radius: 0.25rem;
    }
    .alert-danger {
    color: #721c24;
    background-color: #f8d7da;
    border-color: #f5c6cb;
    }
    .alert-success {
    color: #155724;
    background-color: #d4edda;
    border-color: #c3e6cb;
    }
    .alert-warning {
    color: #856404;
    background-color: #fff3cd;
    border-color: #ffeeba;
    }
    .alert-info {
    color: #0c5460;
    background-color: #d1ecf1;
    border-color: #bee5eb;
    }

3) Add a css link tag to the index.html page

<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
+ <link rel="stylesheet" href="styles.css" />
<title></title>
</head>

  1. Create an Alert component in main.js and display the different kinds of alerts

    function Alert() {
    return (
    <div className="container">
    <div className="alert alert-danger">danger</div>
    <div className="alert alert-warning">warning</div>
    <div className="alert alert-success">success</div>
    <div className="alert alert-info">info</div>
    </div>
    );
    }
    ReactDOM.createRoot(document.getElementById("root")).render(<Alert />);

  2. Modify the component to take an Alert type prop to dynamically set the type of alert

    function Alert({ type = "info" }) {
    const alertClasses = `alert alert-${type}`;
    return (
    <div className="container">
    <div className={alertClasses}>This is a message.</div>
    </div>
    );
    }
    ReactDOM.createRoot(document.getElementById("root")).render(
    <Alert type="danger" />
    );

  3. Set the alert type as shown above and confirm it is working

  4. Remove the type prop from <Alert type="danger" />

  5. Verify the alert type correctly defaults to the info type

If you often find yourself writing code to build up a string of CSS classes the classnames package can simplify it. Alternatively, you could use one of the CSS-in-JS libraries below if you prefer.

Style

Using the style attribute as the primary means of styling elements is generally not recommended. In most cases, className should be used to reference classes defined in an external CSS stylesheet. style is most often used in React applications to add dynamically-computed styles at render time.

Are inline styles bad?

  • CSS classes are generally better for performance than inline styles.

style

  • accepts a JavaScript object (not a ; delimited string like HTML)
  • the object properties are camelCased instead of kebab-cased
  • consistent with the DOM property style (not the attribute)
  • more efficient
  • prevents XSS security holes
  • not auto-prefixed
  • to support older browsers you need to supply the corresponding style properties

  1. Override the styles from the CSS Classes in the previous example with inline styles

    function Alert({ type = 'info' }) {
    const alertClasses = `alert alert-${type}`;
    return (
    <div className="container">
    <div
    className={alertClasses}
    + style={{ backgroundColor: 'fuchsia', color: 'white', width: 50 }}
    >
    This is a message.
    </div>
    </div>
    );
    }
    ReactDOM.createRoot(document.getElementById('root')).render(<Alert type="danger" />);

Classnames Library

A simple JavaScript utility for conditionally joining className(s) together.

This library used to be part of React and has been pulled out into a library that can be extremely helpful.

CSS in JS

What is CSS-in-JS?

“CSS-in-JS” refers to a pattern where CSS is composed using JavaScript instead of defined in external files.

Note that this functionality is not a part of React, but provided by third-party libraries. React does not have an opinion about how styles are defined; if in doubt, a good starting point is to define your styles in a separate *.css file as usual and refer to them using className.

CSS-in-JS Libraries

Read a comparison of CSS-in-JS libraries here.

Styled Components

  1. Installation

    npm install styled-components

    index.html

    <body>
    <div id="root"></div>
    <script src="/node_modules/react/umd/react.development.js"></script>
    <script src="/node_modules/react-dom/umd/react-dom.development.js"></script>
    <script src="/node_modules/react-is/umd/react-is.development.js"></script>
    <script src="/node_modules/@babel/standalone/babel.min.js"></script>
    <script src="/node_modules/styled-components/dist/styled-components.min.js"></script>
    <script type="text/babel" src="/main.js"></script>
    </body>

    main.js

    const { styled } = window;

  2. Before styled components. Global styles in styles.css

    main.js

    function Alert() {
    return <div className="alert alert-danger">danger</div>;
    }
    ReactDOM.createRoot(document.getElementById("root")).render(<Alert />);

    styles.css

    .alert {
    position: relative;
    padding: 0.75rem 1.25rem;
    margin-bottom: 1rem;
    border: 1px solid transparent;
    border-radius: 0.25rem;
    }
    .alert-danger {
    color: #721c24;
    background-color: #f8d7da;
    border-color: #f5c6cb;
    }

  3. With styled components

    const { styled } = window;
    const StyledAlert = styled.div`
    position: relative;
    padding: 0.75rem 1.25rem;
    margin-bottom: 1rem;
    border: 1px solid transparent;
    border-radius: 0.25rem;
    `;
    const StyledAlertDanger = styled(StyledAlert)`
    color: #721c24;
    background-color: #f8d7da;
    border-color: #f5c6cb;
    `;
    function Alert() {
    return <StyledAlertDanger>danger</StyledAlertDanger>;
    }
    ReactDOM.createRoot(document.getElementById("root")).render(<Alert />);

  4. With variants:

    const { styled } = window;
    const StyledAlert = styled.div`
    position: relative;
    padding: 0.75rem 1.25rem;
    margin-bottom: 1rem;
    border: 1px solid transparent;
    border-radius: 0.25rem;
    `;
    const StyledAlertDanger = styled(StyledAlert)`
    color: #721c24;
    background-color: #f8d7da;
    border-color: #f5c6cb;
    `;
    const StyledAlertInfo = styled(StyledAlert)`
    color: #0c5460;
    background-color: #d1ecf1;
    border-color: #bee5eb;
    `;
    function Alert() {
    return <StyledAlertInfo>danger</StyledAlertInfo>;
    }
    ReactDOM.createRoot(document.getElementById("root")).render(<Alert />);

  5. With variants driven by props:

    const StyledAlertVariant = styled(StyledAlert)`
    color: ${(props) => props.color};
    background-color: ${(props) => props.backgroundColor};
    border-color: ${(props) => props.borderColor};
    `;
    function Alert() {
    return (
    <StyledAlertVariant
    color="white"
    backgroundColor="lightblue"
    borderColor="black"
    >
    variant
    </StyledAlertVariant>
    );
    }
    ReactDOM.createRoot(document.getElementById("root")).render(<Alert />);

Reference

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