React中尖括号和星号的含义是什么？表达式中该类符号解析

Hey there! Let's break down these two questions with clear, practical examples—since syntax nuances can be tricky when you're working with React.

Angle Brackets (<>) #

These are foundational to React's JSX syntax, with three key use cases:

• Rendering Components/HTML Elements
  This is the most common scenario. <Button /> renders a custom React component named Button, while <p>Hello World</p> renders a native HTML paragraph element. Use self-closing tags <Component /> for components that don't need child content, and open/closed tags <Component>Child content</Component> when you need to wrap elements or text.
• TypeScript Generic Parameters
  If you're using TypeScript with React, angle brackets define generic types for type safety. For example, const [username, setUsername] = useState<string>('') uses <string> to tell TypeScript that the username state should always be a string—catching type errors early.
• React.Fragment Shorthand
  When you need to return multiple elements without adding an extra wrapper div, use empty angle brackets <>...</> as a shorthand for React.Fragment. Like this:

  return (
    <>
      <h1>My Page</h1>
      <p>Welcome to my site!</p>
    </>
  )
  

Asterisks (*) #

Asterisks aren't native React syntax, but they show up frequently in React codebases via JavaScript or companion libraries:

• JavaScript Multiplication Operator
  Inside JSX expression braces ({}), the asterisk acts like regular JavaScript multiplication. For example, <div>5 * 4 = {5 * 4}</div> renders as 5 * 4 = 20.
• React Router Wildcard Matching
  In React Router (both v5 and v6), asterisks are path wildcards:
  • <Route path="/products/*" element={<ProductDetails />} /> (v6) matches all subpaths under /products, like /products/123 or /products/category/shoes.
  • <Route path="*" element={<NotFoundPage />} /> catches any path that isn't matched by other routes—perfect for 404 pages.
• Generator Functions (Rare)
  If you're using generator functions with React Suspense for async workflows, you'll see asterisks in function declarations (e.g., function* fetchUserData() {}), but this is pure JavaScript, not React-specific.

Since you didn't share a specific expression, here are examples of high-frequency use cases:

Example 1: useState<number>(0) #

• Angle Brackets <number>: TypeScript generic that enforces the state value is a number. This ensures setState only accepts numeric values.
• No asterisk here, but if you wrote useState<number>(2 * 3), the * would be JavaScript multiplication, setting the initial state to 6.

Example 2: <Route path="/dashboard/*" element={<Dashboard />} /> #

• Angle Brackets <Route> and <Dashboard />: JSX syntax for rendering React Router's Route component and your custom Dashboard component.
• Asterisk *: React Router wildcard that matches all subpaths under /dashboard.

Example 3: {[...Array(3)].map((_, i) => <ListItem key={i} />)} #

• Angle Brackets <ListItem />: JSX to render a custom list item component, repeated 3 times via the array map.
• If you changed this to {[...Array(2 * 4)].map(...)}, the * would multiply 2 and 4 to create an array of length 8, rendering 8 list items.

Example 4: const LazyComponent = React.lazy<typeof import('./LazyComponent').default>(() => import('./LazyComponent')) #

• Angle Brackets <typeof import(...)>: TypeScript generic that defines the type of the lazily loaded component, maintaining type safety.
• No asterisk here, but this shows how angle brackets extend React functionality with TypeScript.

内容的提问来源于stack exchange，提问作者arnoldbird