React Express

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React Components
React Components
Components are the building blocks of any React UI. React manages the mapping from components created with JSX (e.g. <div />) to the native browser DOM which is actually rendered.
The entire UI of the application is specified by declaring which components to render, and in what order. Components are nested inside other components, forming a tree data structure. The top level component, or the root of the tree, is known as the root component. Nested components are called children components.
DOM Components
ReactDOM provides us built-in DOM-node components that look just like their corresponding HTML tags, e.g. <div />, <input />. These generally accept the same attributes as their HTML counterparts, like title and style.
A few key differences to keep in mind:
  • Since class is a reserved word in JavaScript, we use className instead.
  • Styling elements with style is quite common in idiomatic React code, unlike in plain HTML. The style attribute takes an object which uses the JavaScript names of CSS rules. For example, we would write style={{paddingTop: 20}} in React, to achieve the same thing as style="padding-top: 20px;" in plain HTML. We'll cover styling in greater depth later.
Here are a few examples of DOM components rendered using React.
Custom Components
Often we want to render more complex components which encapsulate logic, styles, and multiple DOM components.
To declare a custom component, we extend React.Component and write a render method which returns other components. We can then instantiate our component just as we would a DOM component. By convention, custom component names are capitalized (to distinguish them from DOM components, which are lowercase).
Notice how the Card component encapsulates multiple DOM components and styles, while also supporting custom parameters - we can pass a custom color and children components (these are accessed within the component from this.props). Custom components let us reuse code extremely easily.
Ultimately, our Card component doesn't actually insert an HTML element called <Card /> in the DOM. Custom components are just an abstract representation that React keeps track of, making it easier for us to write complex apps. Only built-in DOM components are actually inserted as elements in the DOM.

In the next sections, we'll cover two important aspects of using components, the Component API and the Component Lifecycle API.