Why Did You Render

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

For example, when you pass style={{width: '100%'}} to a big pure component and make it always re-render:

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

Setup

The last version of the library has been tested with React@16.13.1 but it is expected to work with all React@16 versions.

For versions before 16.8 try turning off hooks support by using trackHooks: false in whyDidYouRender's init options.*

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

Installation

Execute whyDidYouRender as the first thing that happens in your application (even before react-hot-loader).

The best way of doing this would be to create a file (lets say wdyr.js) near the entrypoint of 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,
  });
}

And then import wdyr.js (even before react-hot-loader):

index.js:

import './wdyr';

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 the latest react-redux with hooks (or any other custom library), you can also patch it like this:

import React from 'react';

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']
    ]
  });
}

But there is currently a problem with rewriting exports of imported files in webpack and a small workaround should be applied to support this feature #85 - trackExtraHooks cannot set property

Read More

• Why Did You Render Mr. Big Pure React Component???
• Common fixing scenarios this library can helps 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.

Integration With Other Libraries

• Next.js example
• React-Redux With Hooks

Sandbox

You can test the library in the official sandbox.

And another official sandbox with hooks tracking

Tracking Components

You can track all pure components (components that are extending React.PureComponent, or function components that are wrapped with 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
    )
  }
}

And 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 very inconvenient. For example:

  const EnhancedMenu = withPropsOnChange(withPropsOnChange(withStateHandlers(withPropsOnChange(withState(withPropsOnChange(lifecycle(withPropsOnChange(withPropsOnChange(onlyUpdateForKeys(LoadNamespace(Connect(withState(withState(withPropsOnChange(lifecycle(withPropsOnChange(withHandlers(withHandlers(withHandlers(withHandlers(Connect(lifecycle(Menu)))))))))))))))))))))))

  will have the display name:

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

  To prevent polluting the console, and any other reason, you can change it using customName.

Options

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: []
• logOnDifferentValues: false
• logOwnerReasons: false
• hotReloadBufferMs: 500
• onlyLogs: false
• collapseGroups: false
• titleColor
• diffNameColor
• diffPathColor
• notifier: ({Component, displayName, prevProps, prevState, nextProps, nextState, reason, options}) => void

include / exclude

You can include or exclude tracking for re-renders for components by their displayName with the include and exclude options.

Notice: exclude takes priority over both include and whyDidYouRender statics on components.

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

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

trackAllPureComponents

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.

trackHooks

You can turn off tracking of hooks changes.

Understand and fix hook issues.

trackExtraHooks

Adding extra hooks to track for "redundant" results:

whyDidYouRender(React, {
  trackExtraHooks: [
    [ReactRedux, 'useSelector']
  ]
});

There is currently a problem with rewriting exports of imported files in webpack.

To see available workarounds check out the discussion at bug #85 - trackExtraHooks cannot set property

logOnDifferentValues

Normally, you only want notifications about component re-renders when their props and state are the same, because it means these re-renders could have been avoided. But you can also track all re-renders, even on different state/props.

render(<BigListPureComponent a={1}/>)
render(<BigListPureComponent a={2}/>)
// this will only cause whyDidYouRender notifications for {logOnDifferentValues: true}

logOwnerReasons

One way of fixing re-render issues is preventing the component's owner from re-rendering. To make that easier, you can use logOwnerReasons: true to view the reasons why owner component re-renders.

hotReloadBufferMs

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

We can't currently know exactly if a render was triggered by hot reload, so instead, we ignore all updates for hotReloadBufferMs (default: 500) after a hot reload.

onlyLogs

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

collapseGroups

Grouped logs can start collapsed:

titleColor / diffNameColor / diffPathColor

Controls the colors used in the console notifications

notifier

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

Troubleshooting

Class constructors must be invoked with 'new'.

If you are building for latest browsers (or using es6 classes without building) you don't transpile the "class" keyword.

This causes an error because the library uses transpiled classes, and transpiled classes currently can't extend native classes.

To fix this, use the "no-classes-transpile" dist:

import React from 'react';

if (process.env.NODE_ENV === 'development') {
  const whyDidYouRender = require('@welldone-software/why-did-you-render/dist/no-classes-transpile/umd/whyDidYouRender.min.js');
  whyDidYouRender(React);
}

React-Redux connect HOC is spamming the console

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

Credit

Inspired by the following previous work:

https://github.com/maicki/why-did-you-update 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.

License

This library is MIT licensed.