Описание UI

Ваш первый компонент

Приложения, написанные при помощи React, состоят из изолированных блоков UI, называемых компонентами. React-компонент представляет из себя JavaScript функцию, в которую вы можете добавлять разметку. Компоненты могут быть маленькими, как кнопка, или большими, как целая страница. Ниже показан компонент Gallery, который отображает три компонента Profile:

function Profile() {
  return (
    <img
      src="https://i.imgur.com/MK3eW3As.jpg"
      alt="Кэтрин Джонсон"
    />
  );
}

export default function Gallery() {
  return (
    <section>
      <h1>Удивительные ученые</h1>
      <Profile />
      <Profile />
      <Profile />
    </section>
  );
}

Ready to learn this topic?

Прочитайте статью Ваш первый компонент, чтобы узнать, как объявлять и использовать React-компоненты.

Read More

---

Импорт и экспорт компонентов

You can declare many components in one file, but large files can get difficult to navigate. To solve this, you can export a component into its own file, and then import that component from another file:

import Profile from './Profile.js';

export default function Gallery() {
  return (
    <section>
      <h1>Amazing scientists</h1>
      <Profile />
      <Profile />
      <Profile />
    </section>
  );
}

Ready to learn this topic?

Read Importing and Exporting Components to learn how to split components into their own files.

Read More

---

Пишем разметку при помощи JSX

Each React component is a JavaScript function that may contain some markup that React renders into the browser. React components use a syntax extension called JSX to represent that markup. JSX looks a lot like HTML, but it is a bit stricter and can display dynamic information.

If we paste existing HTML markup into a React component, it won’t always work:

export default function TodoList() {
  return (
    // This doesn't quite work!
    <h1>Hedy Lamarr's Todos</h1>
    <img
      src="https://i.imgur.com/yXOvdOSs.jpg"
      alt="Hedy Lamarr"
      class="photo"
    >
    <ul>
      <li>Invent new traffic lights
      <li>Rehearse a movie scene
      <li>Improve spectrum technology
    </ul>

If you have existing HTML like this, you can fix it using a converter:

export default function TodoList() {
  return (
    <>
      <h1>Hedy Lamarr's Todos</h1>
      <img
        src="https://i.imgur.com/yXOvdOSs.jpg"
        alt="Hedy Lamarr"
        className="photo"
      />
      <ul>
        <li>Invent new traffic lights</li>
        <li>Rehearse a movie scene</li>
        <li>Improve spectrum technology</li>
      </ul>
    </>
  );
}

Ready to learn this topic?

Read Writing Markup with JSX to learn how to write valid JSX.

Read More

---

Использование JavaScript внутри JSX при помощи фигурных скобок

JSX lets you write HTML-like markup inside a JavaScript file, keeping rendering logic and content in the same place. Sometimes you will want to add a little JavaScript logic or reference a dynamic property inside that markup. In this situation, you can use curly braces in your JSX to “open a window” to JavaScript:

const person = {
  name: 'Gregorio Y. Zara',
  theme: {
    backgroundColor: 'black',
    color: 'pink'
  }
};

export default function TodoList() {
  return (
    <div style={person.theme}>
      <h1>{person.name}'s Todos</h1>
      <img
        className="avatar"
        src="https://i.imgur.com/7vQD0fPs.jpg"
        alt="Gregorio Y. Zara"
      />
      <ul>
        <li>Improve the videophone</li>
        <li>Prepare aeronautics lectures</li>
        <li>Work on the alcohol-fuelled engine</li>
      </ul>
    </div>
  );
}

Ready to learn this topic?

Read JavaScript in JSX with Curly Braces to learn how to access JavaScript data from JSX.

Read More

---

Передача пропсов в компонент

React components use props to communicate with each other. Every parent component can pass some information to its child components by giving them props. Props might remind you of HTML attributes, but you can pass any JavaScript value through them, including objects, arrays, functions, and even JSX!

import { getImageUrl } from './utils.js'

export default function Profile() {
  return (
    <Card>
      <Avatar
        size={100}
        person={{
          name: 'Katsuko Saruhashi',
          imageId: 'YfeOqp2'
        }}
      />
    </Card>
  );
}

function Avatar({ person, size }) {
  return (
    <img
      className="avatar"
      src={getImageUrl(person)}
      alt={person.name}
      width={size}
      height={size}
    />
  );
}

function Card({ children }) {
  return (
    <div className="card">
      {children}
    </div>
  );
}

Ready to learn this topic?

Read Passing Props to a Component to learn how to pass and read props.

Read More

---

Условный рендеринг

Your components will often need to display different things depending on different conditions. In React, you can conditionally render JSX using JavaScript syntax like if statements, &&, and ? : operators.

In this example, the JavaScript && operator is used to conditionally render a checkmark:

function Item({ name, isPacked }) {
  return (
    <li className="item">
      {name} {isPacked && '✅'}
    </li>
  );
}

export default function PackingList() {
  return (
    <section>
      <h1>Sally Ride's Packing List</h1>
      <ul>
        <Item
          isPacked={true}
          name="Space suit"
        />
        <Item
          isPacked={true}
          name="Helmet with a golden leaf"
        />
        <Item
          isPacked={false}
          name="Photo of Tam"
        />
      </ul>
    </section>
  );
}

Ready to learn this topic?

Read Conditional Rendering to learn the different ways to render content conditionally.

Read More

---

Рендерим списки

You will often want to display multiple similar components from a collection of data. You can use JavaScript’s filter() and map() with React to filter and transform your array of data into an array of components.

For each array item, you will need to specify a key. Usually, you will want to use an ID from the database as a key. Keys let React keep track of each item’s place in the list even if the list changes.

import { people } from './data.js';
import { getImageUrl } from './utils.js';

export default function List() {
  const listItems = people.map(person =>
    <li key={person.id}>
      <img
        src={getImageUrl(person)}
        alt={person.name}
      />
      <p>
        <b>{person.name}:</b>
        {' ' + person.profession + ' '}
        known for {person.accomplishment}
      </p>
    </li>
  );
  return (
    <article>
      <h1>Scientists</h1>
      <ul>{listItems}</ul>
    </article>
  );
}

Ready to learn this topic?

Read Rendering Lists to learn how to render a list of components, and how to choose a key.

Read More

---

Поддерживаем компоненты чистыми

Some JavaScript functions are pure. A pure function:

• Minds its own business. It does not change any objects or variables that existed before it was called.
• Same inputs, same output. Given the same inputs, a pure function should always return the same result.

By strictly only writing your components as pure functions, you can avoid an entire class of baffling bugs and unpredictable behavior as your codebase grows. Here is an example of an impure component:

let guest = 0;

function Cup() {
  // Bad: changing a preexisting variable!
  guest = guest + 1;
  return <h2>Tea cup for guest #{guest}</h2>;
}

export default function TeaSet() {
  return (
    <>
      <Cup />
      <Cup />
      <Cup />
    </>
  );
}

You can make this component pure by passing a prop instead of modifying a preexisting variable:

function Cup({ guest }) {
  return <h2>Tea cup for guest #{guest}</h2>;
}

export default function TeaSet() {
  return (
    <>
      <Cup guest={1} />
      <Cup guest={2} />
      <Cup guest={3} />
    </>
  );
}

Ready to learn this topic?

Read Keeping Components Pure to learn how to write components as pure, predictable functions.

Read More

---

Ваш UI как дерево

React uses trees to model the relationships between components and modules.

A React render tree is a representation of the parent and child relationship between components.

Components near the top of the tree, near the root component, are considered top-level components. Components with no child components are leaf components. This categorization of components is useful for understanding data flow and rendering performance.

Modelling the relationship between JavaScript modules is another useful way to understand your app. We refer to it as a module dependency tree.

A dependency tree is often used by build tools to bundle all the relevant JavaScript code for the client to download and render. A large bundle size regresses user experience for React apps. Understanding the module dependency tree is helpful to debug such issues.

Ready to learn this topic?

Read Your UI as a Tree to learn how to create a render and module dependency trees for a React app and how they’re useful mental models for improving user experience and performance.

Read More

---

Что дальше?

Перейдите к статье Ваш первый компонент, чтобы начать чтение этой главы статья за статьёй!

Или, если эти темы вам знакомы, почему бы не прочитать про Добавление интерактивности?