JSX In Depth

JSX is a syntactic sugar for React.createElement(component, props, ...children)

<MyButton color="blue" shadowSize={2}>
  Click Me
</MyButton>

React.createElement(
  MyButton,
  {color: 'blue', shadowSize: 2},
  'Click Me'
);

Specifying The React Element Type

The first part of a JSX tag determines the type of the React element.

User-Defined Components Must Be Capitalized

The first part of a JSX tag determines the type of the React element.

• Capitalized: React component

• Uncapitalized: HTML tag (e.g. <div>, <span>)

React Must Be in Scope

Since JSX compiles into calls to React.createElement, the React library must also always be in scope from your JSX code.

Choosing the Component at Runtime

To render a different component based on a prop, assign the component type to a capitalized variable first.

const components = {
  photo: PhotoStory,
  video: VideoStory
};

const Story = ({ storyType }) => {
  const SpecificStory = components[storyType];
  return <SpecificStory story={props.story} />;
};

Props in JSX

JavaScript Expressions as Props

You can pass any JavaScript expression as a prop, by surrounding it with {}: <MyComponent foo={1 + 2 + 3 + 4} />. if statements and for loops are not expressions in JavaScript.

Props Default to "True"

If you pass no value for a prop, it defaults to true. Therefore, <MyTextBox autocomplete /> is equivalent to <MyTextBox autocomplete={true} />.

Spread Attributes

If you already have props as an object, and you want to pass it in JSX, you can use ... to pass the whole props object.

const App1 = () => <Greeting firstName="Ben" lastName="Hector" />;

const App2 = () => {
  const props = {
    firstName: 'Ben',
    lastName: 'Hector'
  };
  return <Greeting {...props} />;
};

Children in JSX

In JSX expressions that contain both an opening tag and a closing tag, the content between those tags is passed as a special prop: props.children.

Booleans, Null, and Undefined Are Ignored

false, null, undefined, and true are valid children. They simply don’t render. To make them appear in the output, convert them to string first.