The react's algorithm of detecting changes is based on rebuilding a Virtual DOM tree (i.e. recalculating all components) and comparing that tree to the previous one. It is done, as I understand, every time someone calls update() or similar method on a component.
So React won't cause performance issues only if you have few model variables or manually optimize the code by using immutable data structures (which have their own performance issues) and pure components.
For comparison, Vue detects changing by wrapping all model variables with proxies and linking them to UI components that use them. This has an advantage of not having to recalculate everything and disadvantage of having an overhead for using proxies and creating a dependency graph. Also, having to deal with proxies adds more pain for the programmer.
This is the virtual DOM mental model of react, and it is pretty much entirely wrong.
- React doesn't really distinguish between DOM components and your own components in how it evaluates. It's all part of the same "VDOM" tree. Creating and updating HTML tags doesn't flow differently from updating the props on your own components.
- React does sparse updates, starting at the topmost component(s) whose state changed. Frequently this is just one widget or a button. Full tree re-evaluation is rare.
- If a component has the _exact_ same `children` prop as before (`===`), as is often the case with e.g. context providers, because it was assigned by a parent, then React will skip re-rendering the children entirely with no effort from the developer.
- If a component is memoized with `memo(...)`, then a re-render will be stopped if it has the same props as before. This means even if your state lives high up in the tree, judicious use of memo can make it zippy af.
TLDR: If your react app is re-calculating the entire tree, you suck at react and you never bothered to learn it. Skill issue, git gud, etc. You're welcome.
> If a component is memoized with `memo(...)`, then a re-render will be stopped if it has the same props as before
This works only with immutable data structures. Am I wrong? Because a modified array will pass the identity test (=== operator). If you are using mutable data structures, you cannot use identity operator to detect changes.
Also if "memo" optimization is that good why does one have to add it manually?
> If your react app is re-calculating the entire tree, you suck at react
The point of UI framework is to do the optimization for me and not require me to use some special style of coding like immutable structures or manually write functions like componentShouldUpdate. What I write is how UI variables depend on model variables and the rest is framework's job. I don't want to use weird patterns like a function with a giant switch that processes update events and clones the whole model graph to change a single variable.
Vue of course has its own set of issues, caused by using proxies (for example when adding on object to the set you must "unwrap" it manually to prevent adding a proxy to the set).
Yes, immutability is a requirement. And if you think that's a "special style of coding" then you probably haven't run into situations where that is a necessity, like optimistic updates with rollback, or undo/redo systems.
Mutable code is code that destroys the paper trail of what happened.
>The point of UI framework is to do the optimization for me
No, the point of the UI framework is to allow you to build applications that work correctly and perform well. The React style of coding is designed to scale up to large applications of e.g. a Figma or Excel caliber, while eliminating entire categories of subtle bugs. If you've never coded something like that, React will seem like it is getting in your way.
>Also if "memo" optimization is that good why does one have to add it manually?
Because the compute vs memory trade-off isn't always free, especially in a world where CPU computations are much faster than accessing uncached memory.
That comparison is apples to oranges, because React components are typically much smaller than Vue components (think 20 components in one file). So while its true that a state update will rebuild an entire component, that diff might still only impact 2-3 dom nodes.
For high frequency updates such as reacting to mouse interactions, you can compose components in such a way that only one small component handles the high-frequency update, while it's siblings and children remain static.
In this way, React-components are closer to Vue's computed properties than Vue-components.
> So while its true that a state update will rebuild an entire component
What if the component does some long computations, for example, calculating a sum of 10000 elements of an array? React will do the computation only to find that nothing changed.
> The react's algorithm of detecting changes is based on rebuilding a Virtual DOM tree
Not exactly. This is the "mental model" but not how the algorithm works internally. There are a lot of optimizations to do as little work as possible while "appearing" to rerender everything.
This is not to say one is better than the other - each has its own benefits. But that's not a reason to choose Vue over React.
React can be very performant with little optimization if you apply FP principals to it, which makes sense since it was built with the goal of using FP to do frontend work.
If your whole tree is recalculating on every mousemove event, that is a giant code smell to say the least. You’d have to architect the app to work that way.
FP principle is that functions are first-class objects and one can pass a function to a function. I don't see how it helps programming the UI. Maybe you meant using immutable data structures? I mentioned that and noted that they have their own issues and generally are a pain to use.
> If your whole tree is recalculating on every mousemove event
If you update a variable in the root component, that will happen. Am I wrong?
Thats the 1-sentence summary of FP, but there’s a lot more to it. Immutable data structures are part of it, although less important because of how props are scoped.
Composition, my friend. That’s the key. If your whole tree is rerendering whenever your state changes, it’s a failure to compose your components properly.
FP includes strategies for limiting side effects, relying on architectural patterns that push side effects to the edges of your application. This is a very sane way of working in react.
And a lot of prop drilling can be reduced by passing down closure functions with curried values in them.
So React won't cause performance issues only if you have few model variables or manually optimize the code by using immutable data structures (which have their own performance issues) and pure components.
For comparison, Vue detects changing by wrapping all model variables with proxies and linking them to UI components that use them. This has an advantage of not having to recalculate everything and disadvantage of having an overhead for using proxies and creating a dependency graph. Also, having to deal with proxies adds more pain for the programmer.