mirror of
https://github.com/reactjs/react.dev.git
synced 2026-02-24 20:53:08 +00:00
96 lines
3.4 KiB
Markdown
96 lines
3.4 KiB
Markdown
---
|
|
id: rendering-elements
|
|
title: Rendering Elements
|
|
permalink: docs/rendering-elements.html
|
|
redirect_from: "docs/displaying-data.html"
|
|
prev: introducing-jsx.html
|
|
next: components-and-props.html
|
|
---
|
|
|
|
Elements are the smallest building blocks of React apps.
|
|
|
|
An element describes what you want to see on the screen:
|
|
|
|
```js
|
|
const element = <h1>Hello, world</h1>;
|
|
```
|
|
|
|
Unlike browser DOM elements, React elements are plain objects, and are cheap to create. React DOM takes care of updating the DOM to match the React elements.
|
|
|
|
>**Note:**
|
|
>
|
|
>One might confuse elements with a more widely known concept of "components". We will introduce components in the [next section](/docs/components-and-props.html). Elements are what components are "made of", and we encourage you to read this section before jumping ahead.
|
|
|
|
## Rendering an Element into the DOM
|
|
|
|
Let's say there is a `<div>` somewhere in your HTML file:
|
|
|
|
```html
|
|
<div id="root"></div>
|
|
```
|
|
|
|
We call this a "root" DOM node because everything inside it will be managed by React DOM.
|
|
|
|
Applications built with just React usually have a single root DOM node. If you are integrating React into an existing app, you may have as many isolated root DOM nodes as you like.
|
|
|
|
To render a React element into a root DOM node, pass both to `ReactDOM.render()`:
|
|
|
|
```js
|
|
const element = <h1>Hello, world</h1>;
|
|
ReactDOM.render(
|
|
element,
|
|
document.getElementById('root')
|
|
);
|
|
```
|
|
|
|
[Try it on CodePen.](http://codepen.io/gaearon/pen/rrpgNB?editors=1010)
|
|
|
|
It displays "Hello, world" on the page.
|
|
|
|
## Updating the Rendered Element
|
|
|
|
React elements are [immutable](https://en.wikipedia.org/wiki/Immutable_object). Once you create an element, you can't change its children or attributes. An element is like a single frame in a movie: it represents the UI at a certain point in time.
|
|
|
|
With our knowledge so far, the only way to update the UI is to create a new element, and pass it to `ReactDOM.render()`.
|
|
|
|
Consider this ticking clock example:
|
|
|
|
```js{8-11}
|
|
function tick() {
|
|
const element = (
|
|
<div>
|
|
<h1>Hello, world!</h1>
|
|
<h2>It is {new Date().toLocaleTimeString()}.</h2>
|
|
</div>
|
|
);
|
|
ReactDOM.render(
|
|
element,
|
|
document.getElementById('root')
|
|
);
|
|
}
|
|
|
|
setInterval(tick, 1000);
|
|
```
|
|
|
|
[Try it on CodePen.](http://codepen.io/gaearon/pen/gwoJZk?editors=0010)
|
|
|
|
It calls `ReactDOM.render()` every second from a [`setInterval()`](https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/setInterval) callback.
|
|
|
|
>**Note:**
|
|
>
|
|
>In practice, most React apps only call `ReactDOM.render()` once. In the next sections we will learn how such code gets encapsulated into [stateful components](/docs/state-and-lifecycle.html).
|
|
>
|
|
>We recommend that you don't skip topics because they build on each other.
|
|
|
|
## React Only Updates What's Necessary
|
|
|
|
React DOM compares the element and its children to the previous one, and only applies the DOM updates necessary to bring the DOM to the desired state.
|
|
|
|
You can verify by inspecting the [last example](http://codepen.io/gaearon/pen/gwoJZk?editors=0010) with the browser tools:
|
|
|
|
|
|
|
|
Even though we create an element describing the whole UI tree on every tick, only the text node whose contents has changed gets updated by React DOM.
|
|
|
|
In our experience, thinking about how the UI should look at any given moment rather than how to change it over time eliminates a whole class of bugs.
|