Conceptually, components are like JavaScript functions. They accept arbitrary inputs (called “props”) and return React elements describing what should appear on the screen. Function and Class Components . The simplest way to define a component is to write a JavaScript function:

How do I bind a function to a component instance? There are several ways to make sure functions have access to component attributes like this.props and this.state , depending on which syntax and build steps you are using.

A Hook is a special function that lets you “hook into” React features. For example, useState is a Hook that lets you add React state to function components. We’ll learn other Hooks later. When would I use a Hook? If you write a function component and realize you need to add some state to it, previously you had to convert it to a class.

Function Components . If you are using function components in your regular development, you may want to make some small changes to allow PropTypes to be properly applied. Let’s say you have a component like this:

React lets you define components as classes or functions. Components defined as classes currently provide more features which are described in detail on this page. To define a React component class, you need to extend React.Component:

Hooks are functions that let you “hook into” React state and lifecycle features from function components. Hooks don’t work inside classes — they let you use React without classes. (We don’t recommend rewriting your existing components overnight but you can start using Hooks in the new ones if you’d like.)

概念上來說，component 就像是 JavaScript 的 function，它接收任意的參數（稱之為「props」）並且回傳描述畫面的 React element。 Function Component 與 Class Component . 定義 component 最簡單的方法即是撰寫一個 Javascript function：

React has a powerful composition model, and we recommend using composition instead of inheritance to reuse code between components. In this section, we will consider a few problems where developers new to React often reach for inheritance, and show how we can solve them with composition.

You can only call Hooks while React is rendering a function component: Call them at the top level in the body of a function component. Call them at the top level in the body of a custom Hook. Learn more about this in the Rules of Hooks.

A React component that subscribes to context changes. Using this component lets you subscribe to a context within a function component. Requires a function as a child. The function receives the current context value and returns a React node.