Internally, React uses several clever techniques to minimize the number of costly DOM operations required to update the UI. For many applications, using React will lead to a fast user interface without doing much work to specifically optimize for performance. Nevertheless, there are several ways you can speed up your React application.
By default, React includes many helpful warnings. These warnings are very useful in development. However, they make React larger and slower so you should make sure to use the production version when you deploy the app.
If you aren't sure whether your build process is set up correctly, you can check it by installing [React Developer Tools for Chrome](https://chrome.google.com/webstore/detail/react-developer-tools/fmkadmapgofadopljbjfkapdkoienihi). If you visit a site with React in production mode, the icon will have a dark background:
Remember that you only need to do this for production builds. You shouldn't pass the `-p` flag or apply this plugin in development, because it will hide useful React warnings and make the builds much slower.
* Finally, the resulting bundle is piped to [`terser`](https://github.com/terser-js/terser) for mangling ([read why](https://github.com/hughsk/uglifyify#motivationusage)).
Remember that you only need to do this for production builds. You shouldn't apply these plugins in development because they will hide useful React warnings, and make the builds much slower.
Remember that you only need to do this for production builds. You shouldn't apply the `terser` plugin or the `replace` plugin with `'production'` value in development because they will hide useful React warnings, and make the builds much slower.
Remember that you only need to do this for production builds. You shouldn't apply `TerserPlugin` in development because it will hide useful React warnings, and make the builds much slower.
In the **development** mode, you can visualize how components mount, update, and unmount, using the performance tools in supported browsers. For example:
3. Open the Chrome DevTools **[Performance](https://developers.google.com/web/tools/chrome-devtools/evaluate-performance/timeline-tool)** tab and press **Record**.
Note that **the numbers are relative so components will render faster in production**. Still, this should help you realize when unrelated UI gets updated by mistake, and how deep and how often your UI updates occur.
Currently Chrome, Edge, and IE are the only browsers supporting this feature, but we use the standard [User Timing API](https://developer.mozilla.org/en-US/docs/Web/API/User_Timing_API) so we expect more browsers to add support for it.
If your application renders long lists of data (hundreds or thousands of rows), we recommended using a technique known as "windowing". This technique only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created.
[react-window](https://react-window.now.sh/) and [react-virtualized](https://bvaughn.github.io/react-virtualized/) are popular windowing libraries. They provide several reusable components for displaying lists, grids, and tabular data. You can also create your own windowing component, like [Twitter did](https://medium.com/@paularmstrong/twitter-lite-and-high-performance-react-progressive-web-apps-at-scale-d28a00e780a3), if you want something more tailored to your application's specific use case.
React builds and maintains an internal representation of the rendered UI. It includes the React elements you return from your components. This representation lets React avoid creating DOM nodes and accessing existing ones beyond necessity, as that can be slower than operations on JavaScript objects. Sometimes it is referred to as a "virtual DOM", but it works the same way on React Native.
When a component's props or state change, React decides whether an actual DOM update is necessary by comparing the newly returned element with the previously rendered one. When they are not equal, React will update the DOM.
Even though React only updates the changed DOM nodes, re-rendering still takes some time. In many cases it's not a problem, but if the slowdown is noticeable, you can speed all of this up by overriding the lifecycle function `shouldComponentUpdate`, which is triggered before the re-rendering process starts. The default implementation of this function returns `true`, leaving React to perform the update:
If you know that in some situations your component doesn't need to update, you can return `false` from `shouldComponentUpdate` instead, to skip the whole rendering process, including calling `render()` on this component and below.
In most cases, instead of writing `shouldComponentUpdate()` by hand, you can inherit from [`React.PureComponent`](/docs/react-api.html#reactpurecomponent). It is equivalent to implementing `shouldComponentUpdate()` with a shallow comparison of current and previous props and state.
Here's a subtree of components. For each one, `SCU` indicates what `shouldComponentUpdate` returned, and `vDOMEq` indicates whether the rendered React elements were equivalent. Finally, the circle's color indicates whether the component had to be reconciled or not.
Since `shouldComponentUpdate` returned `false` for the subtree rooted at C2, React did not attempt to render C2, and thus didn't even have to invoke `shouldComponentUpdate` on C4 and C5.
For C1 and C3, `shouldComponentUpdate` returned `true`, so React had to go down to the leaves and check them. For C6 `shouldComponentUpdate` returned `true`, and since the rendered elements weren't equivalent React had to update the DOM.
The last interesting case is C8. React had to render this component, but since the React elements it returned were equal to the previously rendered ones, it didn't have to update the DOM.
Note that React only had to do DOM mutations for C6, which was inevitable. For C8, it bailed out by comparing the rendered React elements, and for C2's subtree and C7, it didn't even have to compare the elements as we bailed out on `shouldComponentUpdate`, and `render` was not called.
If the only way your component ever changes is when the `props.color` or the `state.count` variable changes, you could have `shouldComponentUpdate` check that:
In this code, `shouldComponentUpdate` is just checking if there is any change in `props.color` or `state.count`. If those values don't change, the component doesn't update. If your component got more complex, you could use a similar pattern of doing a "shallow comparison" between all the fields of `props` and `state` to determine if the component should update. This pattern is common enough that React provides a helper to use this logic - just inherit from `React.PureComponent`. So this code is a simpler way to achieve the same thing:
Most of the time, you can use `React.PureComponent` instead of writing your own `shouldComponentUpdate`. It only does a shallow comparison, so you can't use it if the props or state may have been mutated in a way that a shallow comparison would miss.
This can be a problem with more complex data structures. For example, let's say you want a `ListOfWords` component to render a comma-separated list of words, with a parent `WordAdder` component that lets you click a button to add a word to the list. This code does *not* work correctly:
The problem is that `PureComponent` will do a simple comparison between the old and new values of `this.props.words`. Since this code mutates the `words` array in the `handleClick` method of `WordAdder`, the old and new values of `this.props.words` will compare as equal, even though the actual words in the array have changed. The `ListOfWords` will thus not update even though it has new words that should be rendered.
The simplest way to avoid this problem is to avoid mutating values that you are using as props or state. For example, the `handleClick` method above could be rewritten using `concat` as:
ES6 supports a [spread syntax](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator) for arrays which can make this easier. If you're using Create React App, this syntax is available by default.
You can also rewrite code that mutates objects to avoid mutation, in a similar way. For example, let's say we have an object named `colormap` and we want to write a function that changes `colormap.right` to be `'blue'`. We could write:
```js
function updateColorMap(colormap) {
colormap.right = 'blue';
}
```
To write this without mutating the original object, we can use [Object.assign](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) method:
`updateColorMap` now returns a new object, rather than mutating the old one. `Object.assign` is in ES6 and requires a polyfill.
There is a JavaScript proposal to add [object spread properties](https://github.com/sebmarkbage/ecmascript-rest-spread) to make it easier to update objects without mutation as well:
```js
function updateColorMap(colormap) {
return {...colormap, right: 'blue'};
}
```
If you're using Create React App, both `Object.assign` and the object spread syntax are available by default.
When you deal with deeply nested objects, updating them in an immutable way can feel convoluted. If you run into this problem, check out [Immer](https://github.com/mweststrate/immer) or [immutability-helper](https://github.com/kolodny/immutability-helper). These libraries let you write highly readable code without losing the benefits of immutability.
