0.2.3 • Public • Published

React Wastage Monitor

How pure are your components, really? Just throwing on shallowCompare doesn't actually get you performant components, you depend on the parents just as much, only sending down the props you actually need, ensuring they cache object literals rather than rebuild inside render functions, and it's so easy to make a critical performance mistake and not notice it.

If we're to rely on Redux managing the state for our entire app, it's imperative that we can count on our components to only render when we need them to. React Wastage Monitor is a little tool that attaches to React and ensures your components are being all they can be, never wasting computation on renders that aren't required.


npm install react-wastage-monitor


React Wastage Monitor attaches to React by making a component, and then using that opportunity to monkey patch React's CompositeComponent. This allows us to hook into every component as it goes through the Component Lifecycle to verify your prop and state usage. All you need to do is run this code early in your codebase before mounting the rest of the app:

import React from 'react'
import ReactDOM from 'react-dom'
import ReactWastageMonitor from 'react-wastage-monitor'
ReactWastageMonitor(React, ReactDOM)

Ideally you should always run this code in development so you're constantly kept up to date on poorly performing components. Have a strategy to strip this code in production such as using environment variables and a minifier:

if (process.env.NODE_ENV !== 'production') {
  ReactWastageMonitor(React, ReactDOM)

While care has been taken to avoid breaking the internals of React, this kind of monkey patching does come with risks especially as new versions are released. Please let us know if React Wastage Monitor breaks React for you. You should not use React Wastage Monitor in production.


Pass options to React Wastage Monitor as the third argument when calling it, ie:

ReactWastageMonitor(React, ReactDOM, {exclude: ['SomeComponent']})


Exclude specific components by name from being checked. No logging will be generated by excluded components. Provide exclusions as a list of strings or regular expressions, ie:

ReactWastageMonitor(React, ReactDOM, {exclude: [

React Wastage Monitor refers to components by their displayName if set, otherwise just their constructor name. Exclude will test the regex or string against that component name.

What does it check?

Components that are not pure, and don't implement shouldComponentUpdate

Any components that are impure, ie they don't inherit from PureComponent and they don't implement shouldComponentUpdate, will always update regardless of the equality of their props. This should be avoided wherever possible and will be brought to your attention when they rerender:

<ParentComponent> > <Component> is impure and will ALWAYS update when a component above it does

Components that return true on shouldComponentUpdate even when their props/state are deeply equal

Usually this is a side effect of reusing object literal inside the component parent's render function. Because it's a new object every time, shallow compare will incorrectly believe it has new props, wasting render time. Components that render whilst having deeply equal changes will be brought to your attention along with what deeply equal yet referentially unequal values were found:

<ParentComponent> > <Component> updated when it shouldn't need to
  ref inequality:propName <fromValue> -> <toValue>

Components where their props genuinely change yet the HTML didn't

This is usually the result of passing down more props than you need to, or functions that are continually recreated anonymously or through binding. The monitor will compare a component's HTML before and after an update, and if it's identical, will check to see what props/state have changed to trigger this re-render.

<Component> props/state changed and updated but the HTML didn't
  changed:onClick <fromValue> -> <toValue>

I think it should test...

Sure thing, let us know in the issues and we'll see if it makes sense and is doable.


Copyright (c) 2017 James Rakich, used under The MIT License (MIT)

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