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React performance code tutorial

Objectives:

1. Explain React performance pitfalls
2. Implement shouldComponentUpdate
3. Implement PureComponent
4. Understand why component methods should be bound in constructor
5. Utilize why-did-you-update
6. Understand the use and necessity of immutability

The code is commented out, refer there for explanations. Feel free to open a pull request to add additional comments, etc.

Notice right now that if you toggle the pointless state, List.jsx is re-rendered unnecessarily. why-did-you-update warns us about this in the console. The point of this tutorial is to implement either shouldComponentUpdate or PureComponent to avoid that re-render. Immutability is an important component of that.