Write docs for React.PureComponent

[zpao]

Need to at least update the top level api page, but should consider writing something more about it.

cc @spicyj

[kevinSuttle]

The most confusing thing for me:

"Pure component" = functional stateless component, doesn't extend a class

[xiaofan2406]

@kevinSuttle wait... PureComponet === functional component ???

[albertoblaz]

Yeah, it'd be great to have some docs about this topic. Do Functional Components implement by default the same logic as React.PureComponent? If so, developers could enforce the use of:

1. Functional Components
2. If you need lifecycle hooks, then PureComponent
3. If you need local state, then just a normal Component

[zpao]

"Pure component" = functional stateless component, doesn't extend a class

That's not the case. It's effectively just a base class to extend with PureRenderMixin built in.

This is why we need docs 😉

[raphaguasta]

Hello Guys!

I think we don't have a docs, so... I have one question!

If I have a dump component, that only render something. It's better to use the stateless component, or a class that extends PureComponent?

Thanks ;)

[cbengtson85]

@raphaguasta
I have the same question!

[frandsaw]

@raphaguasta @cbengtson85 according to this code the main difference between PureComponent and StatelessComponent:

• StatelessComponent's shouldUpdate value will be always true
• PureComponent's shouldUpdate value is equal to the result of shallow equality check

So in your case shouldUpdate value will be false for PureComponent and true for StatelessComponent. I think it's cheaper to make shallow equality check than making a Virtual DOM comparison, but I may be wrong here.

[gaearon]

It is in the docs now.

[webdif]

Thanks a lot for adding some docs about React.PureComponent.

However, it's still obscur to me when use or not use it. Most of components are tied to their props and state, thus the majority of Component should be extending PureComponent? When using React component with Redux for example.

Also, I always wrote my component as StatelessComponent. But it seems that PureComponent can also be a more efficient than a StatelessComponent, as @frandsaw stated, a shallow equality check may be cheaper than a virtual dom comparison.

So my conclusion right now is "PureComponent all the things!", but I fear I may be missing something here… If you have some good examples where Component or StatelessComponent is better than a PureComponent, it would be great! Thanks a lot for your help 👍

[gaearon]

If something is slow then try using PureComponent and see if it helps.
If nothing is slow then don’t bother optimizing it.

There is no sure way to tell which is going to be slower: reconciliation or shallow comparisons everywhere. In pathological cases (shallow equality checks often failing) PureComponents can be slower. So the only rule of thumb is to only add one when you know you have a perf problem and you can verify adding it solves that problem. Does this help?

[webdif]

Thank you @gaearon for your response, it helps!
Build first and optimize later when needed seems like the most pragmatic way.

[JLarky]

I wish there would be at least one example somewhere in docs that you can copy-paste instead of historical perspective on what was changed and what add-ons were deprecated :) I can still copy-paste code for old add-ons from the docs! why can't I get new stuff?

[gaearon]

I'm not sure what you mean. If I open "Optimizing Performance", it has a code example. Are you looking at some different page?

[tocttou]

I wrote a simple blog post explaining PureComponent. Hope it helps!

https://ashishchaudhary.in/using-purecomponent-to-prevent-wasted-renders-in-react

[yesmeck]

@gaearon

In pathological cases (shallow equality checks often failing) PureComponents can be slower. So the only rule of thumb is to only add one when you know you have a perf problem and you can verify adding it solves that problem. Does this help?

I saw many people misusing PureComponent in the project. Can we add this suggestion to the docs?