Styling

There are two primary ways to style React components

Inline styles with the style prop
Regular CSS with the className prop

Before we get into those, you need to know something about element properties and HTML attributes.

Element properties and HTML attributes

In HTML, you have elements and attributes. For example, the div element has a class attribute:

<div class="my-class"></div>

In the DOM, the div element has a className property:

<div id="my-div" class="my-class"></div>
<script>
	const myDiv = document.getElementById('my-div')
	console.log(myDiv.class) // undefined
	console.log(myDiv.className) // "my-class"
</script>

In JSX, you use the property name rather than the attribute name:

<div className="my-class" />

This applies to a number of other attributes as well. For example, for in HTML is htmlFor in DOM (and JSX). Others include tabindex and readonly (which are, respectively, tabIndex and readOnly in JSX).

Most differences between HTML attributes and JSX props are due to the fact that props are a reference to "properties" not "attributes", so understanding the difference is handy!

Inline styles

Inline styles are set differently in JSX from HTML as a result of how they are applied to the DOM. In HTML, you'd pass a string of CSS, but you access it as an object in the DOM:

<div id="my-div" style="margin-top: 20px; background-color: blue;"></div>
<script>
	const myDiv = document.getElementById('my-div')
	console.log(myDiv.style.marginTop) // "20px"
	console.log(myDiv.style.backgroundColor) // "blue"
</script>

As a result, in JSX, you pass an object of CSS rather than a string:

<div style={{ marginTop: 20, backgroundColor: 'blue' }} />

This actually makes them much easier to work with. Though the style prop has some significant limitations (like lack of media queries and pseudo-selectors), so it's not always the best choice.

Note that in React the {{ and }} is actually a combination of a JSX expression and an object expression. The same example above could be written like so:

const myStyles = {marginTop: 20, backgroundColor: 'blue'}
<div style={myStyles} />

Note also that the property names are camelCased rather than kebab-cased. This matches the style property of DOM nodes (which is a CSSStyleDeclaration object).

Class names

The most common mechanism for styling web applications is through the use of CSS and class names.

As mentioned earlier, in HTML, you apply a class name to an element with the class attribute. In JSX, you use the className prop:

<div className="my-class" />

Then you'll load a CSS file onto the page:

<link rel="stylesheet" href="styles.css" />

And that can contain the styles for .my-class:

.my-class {
	margin-top: 20px;
	background-color: blue;
}