Why Did You Render

why-did-you-render by Welldone Software monkey patches React to notify you about potentially avoidable re-renders. (Works with React Native as well.)

For example, if you pass style={{width: '100%'}} to a big pure component it would always re-render on every element creation:

<BigListPureComponent style={{width: '100%'}}/>

It can also help you to simply track when and why a certain component re-renders.

The latest version of the library was tested (unit tests and E2E) with React@18 only. For React 17 and 16, please use version @^7.

npm install @welldone-software/why-did-you-render --save-dev

or

yarn add --dev @welldone-software/why-did-you-render

If you use the automatic JSX transformation, set the library to be the import source, and make sure preset-react is in development mode.

['@babel/preset-react', {
  runtime: 'automatic',
  development: process.env.NODE_ENV === 'development',
  importSource: '@welldone-software/why-did-you-render',
}]

Unfortunately, the metro-react-native-babel-preset that comes with react-native out of the box does not allow you to change the options of the babel/plugin-transform-react-jsx plugin. Just add the plugin with options as listed below and start react-native packager as usual. Default env for babel is "development". If you do not use expo when working with react-native, the following method will help you.

module.exports = {
  presets: ['module:metro-react-native-babel-preset'],

  env: {
    development: {
      plugins: [['@babel/plugin-transform-react-jsx', { runtime: 'classic' }]],
    },
  },
}

You can pass params to @babel/preset-react through babel-preset-expo

// babel.config.js
module.exports = function (api) {
  api.cache(true);
  return {
    presets: [
      [
        "babel-preset-expo",
        {
          jsxImportSource: "@welldone-software/why-did-you-render",
        },
      ],
    ],
  };
};

Notice: Create React App (CRA) ^4 does use the automatic JSX transformation. See the following comment on how to do this step with CRA

Create a wdyr.js file and import it as the first import in your application.

wdyr.js:

import React from 'react';

if (process.env.NODE_ENV === 'development') {
  const whyDidYouRender = require('@welldone-software/why-did-you-render');
  whyDidYouRender(React, {
    trackAllPureComponents: true,
  });
}

Notice: The library should NEVER be used in production because it slows down React

In Typescript, call the file wdyr.ts and add the following line to the top of the file to import the package's types:

/// <reference types="@welldone-software/why-did-you-render" />

Import wdyr as the first import (even before react-hot-loader):

index.js:

import './wdyr'; // <--- first import

import 'react-hot-loader';
import {hot} from 'react-hot-loader/root';

import React from 'react';
import ReactDOM from 'react-dom';
// ...
import {App} from './app';
// ...
const HotApp = hot(App);
// ...
ReactDOM.render(<HotApp/>, document.getElementById('root'));

If you use trackAllPureComponents like we suggest, all pure components (React.PureComponent or React.memo) will be tracked.

Otherwise, add whyDidYouRender = true to component classes/functions you want to track. (f.e Component.whyDidYouRender = true)

More information about what is tracked can be found in Tracking Components.

Can't see any WDYR logs? Check out the troubleshooting section or search in the issues.

Also, tracking custom hooks is possible by using trackExtraHooks. For example if you want to track useSelector from React Redux:

wdyr.js:

import React from 'react';

// For react-native you might want to use 
// the __DEV__ flag instead of process.env.NODE_ENV === 'development'
if (process.env.NODE_ENV === 'development') {
  const whyDidYouRender = require('@welldone-software/why-did-you-render');
  const ReactRedux = require('react-redux');
  whyDidYouRender(React, {
    trackAllPureComponents: true,
    trackExtraHooks: [
      [ReactRedux, 'useSelector']
    ]
  });
}

Notice that there's currently a problem with rewriting exports of imported files in webpack. A quick workaround can help with it: #85 - trackExtraHooks cannot set property.

• Why Did You Render Mr. Big Pure React Component???
• Common fixing scenarios this library can help with
• React Hooks - Understand and fix hooks issues
• Why Did You Render v4 Released! - TypeScript support, Custom hooks tracking (like React-Redux’s useSelector), Tracking of all pure components.

• Next.js example
• React-Redux With Hooks
• Mobx is currently not supported
• React-Native flipper plugin made by @allen-hsu

You can test the library in the official sandbox.

And another official sandbox with hooks tracking

You can track all pure components (React.PureComponent or React.memo) using the trackAllPureComponents: true option.

You can also manually track any component you want by setting whyDidYouRender on them like this:

class BigList extends React.Component {
  static whyDidYouRender = true
  render(){
    return (
      //some heavy render you want to ensure doesn't happen if its not necessary
    )
  }
}

Or for functional components:

const BigListPureComponent = props => (
  <div>
    //some heavy component you want to ensure doesn't happen if its not necessary
  </div>
)
BigListPureComponent.whyDidYouRender = true

You can also pass an object to specify more advanced tracking settings:

EnhancedMenu.whyDidYouRender = {
  logOnDifferentValues: true,
  customName: 'Menu'
}

• logOnDifferentValues:

  Normally, only re-renders that are caused by equal values in props / state trigger notifications:

  render(<Menu a={1}/>)
  render(<Menu a={1}/>)

  This option will trigger notifications even if they occurred because of different props / state (Thus, because of "legit" re-renders):

  render(<Menu a={1}/>)
  render(<Menu a={2}/>)

• customName:

  Sometimes the name of the component can be missing or very inconvenient. For example:

  withPropsOnChange(withPropsOnChange(withStateHandlers(withPropsOnChange(withState(withPropsOnChange(lifecycle(withPropsOnChange(withPropsOnChange(onlyUpdateForKeys(LoadNamespace(Connect(withState(withState(withPropsOnChange(lifecycle(withPropsOnChange(withHandlers(withHandlers(withHandlers(withHandlers(Connect(lifecycle(Menu)))))))))))))))))))))))

Optionally you can pass in options as the second parameter. The following options are available:

• include: [RegExp, ...] (null by default)
• exclude: [RegExp, ...] (null by default)
• trackAllPureComponents: false
• trackHooks: true
• trackExtraHooks: []
• logOwnerReasons: true
• logOnDifferentValues: false
• hotReloadBufferMs: 500
• onlyLogs: false
• collapseGroups: false
• titleColor
• diffNameColor
• diffPathColor
• notifier: ({Component, displayName, hookName, prevProps, prevState, prevHook, nextProps, nextState, nextHook, reason, options, ownerDataMap}) => void
• getAdditionalOwnerData: (element) => {...}

You can include or exclude tracking of components by their displayName using the include and exclude options.

For example, the following code is used to track all redundant re-renders that are caused by older React-Redux:

whyDidYouRender(React, { include: [/^ConnectFunction/] });

Notice: exclude takes priority over both include and manually set whyDidYouRender =

You can track all pure components (both React.memo and React.PureComponent components)

Notice: You can exclude the tracking of any specific component with whyDidYouRender = false

You can turn off tracking of hooks changes.

Understand and fix hook issues.

Track custom hooks:

whyDidYouRender(React, {
  trackExtraHooks: [
    // notice that 'useSelector' is a named export
    [ReactRedux, 'useSelector'],
  ]
});

There is currently a problem with rewriting exports of imported files in webpack. A workaround is available here: #85 - trackExtraHooks cannot set property

One way of fixing re-render issues is preventing the component's owner from re-rendering.

This option is true by default and it lets you view the reasons why an owner component re-renders.

Normally, you only want logs about component re-renders when they could have been avoided.

With this option, it is possible to track all re-renders.

For example:

render(<BigListPureComponent a={1}/>)
render(<BigListPureComponent a={2}/>)
// will only log if you use {logOnDifferentValues: true}

Time in milliseconds to ignore updates after a hot reload is detected.

When a hot reload is detected, we ignore all updates for hotReloadBufferMs to not spam the console.

If you don't want to use console.group to group logs you can print them as simple logs.

Grouped logs can be collapsed.

Controls the colors used in the console notifications

You can create a custom notifier if the default one does not suite your needs.

You can provide a function that harvests additional data from the original react element. The object returned from this function will be added to the ownerDataMap which can be accessed later within your notifier function override.

• If you are in production, WDYR is probably disabled.
• Maybe no component is tracked
  • Check out Tracking Components once again.
• If you only track pure components using trackAllPureComponents: true then you would only track either (React.PureComponent or React.memo), maybe none of your components are pure so none of them will get tracked.
• Maybe you have no issues

  • Try causing an issue by temporary rendering the whole app twice in it's entry point:

    index.js:

    const HotApp = hot(App);
    HotApp.whyDidYouRender = true;
    ReactDOM.render(<HotApp/>, document.getElementById('root'));
    ReactDOM.render(<HotApp/>, document.getElementById('root'));

There's currently a problem with rewriting exports of imported files in webpack. A quick workaround can help with it: #85 - trackExtraHooks cannot set property.

Since connect hoists statics, if you add WDYR to the inner component, it is also added to the HOC component where complex hooks are running.

To fix this, add the whyDidYouRender = true static to a component after the connect:

  const SimpleComponent = ({a}) => <div data-testid="foo">{a.b}</div>)
  // not before the connect:
  // SimpleComponent.whyDidYouRender = true
  const ConnectedSimpleComponent = connect(
    state => ({a: state.a})
  )(SimpleComponent)
  // after the connect:
  SimpleComponent.whyDidYouRender = true

To see the library's sourcemaps use the source-map-loader.

Inspired by the following previous work:

• github.com/maicki/why-did-you-update (no longer public) which I had the chance to maintain for some time.
• https://github.com/garbles/why-did-you-update where A deep dive into React perf debugging is credited for the idea.

This library is MIT licensed.