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Incorrect. This idea was a result of some early React users misunderstanding the purpose of the virtual DOM, and has unfortunately been thoughtlessly repeated e

Incorrect. This idea was a result of some early React users misunderstanding the purpose of the virtual DOM, and has unfortunately been thoughtlessly repeated ever since. In reality, browser engineers are not that stupid. Mutating the DOM will queue a layout operation, but it will not actually occur until the current JS task has finished executing. Within a JS task, such as an event handler, an XHR completion, or a setTimeout, you can mutate the DOM as many times as you like, and it will only result in a single layout pass. (The exception is if you try to read back some measurement from the DOM, such as an element's bounding box, after mutating it. In this case, the browser does have to block while it performs layout, but that is not something can be solved with virtual DOM). So, what is the purpose of the virtual DOM? It was invented to allow React to provide the illusion of full re-rendering. React's authors wanted to a provide an experience similar to that found on the server, where an entire HTML page is re-rendered from scratch for every load. In that way, there is never any possibility of part of the HTML becoming stale, as it all gets recreated from scratch each time. However, the browser DOM is not designed to be blown away and recreated from scratch all the time. Nodes are expensive objects, spanning the JS and C++ worlds. Recreating the whole DOM tree each time any part of it needed updating would be too slow. So, instead, they created the virtual DOM as an intermediate data structure. React renders the virtual DOM. The virtual DOM is diffed against its previous state, and then the changes are applied to the actual DOM tree. In that way, every component's render() method can be executed, but only those parts of the DOM that have actually changed will be updated. It's a nifty optimisation, but it's not about avoiding reflow, it's just another method of dirty checking, similar to that done in other frameworks like Angular or Ember. It's just that React chooses to diff the data structure produced by render(), rather than diffing the model data that is later used for rendering.

Felz 8y ago
It seems odd to characterize a declarative approach to rendering as an optimization. The draw for me is that I don't need to manage the state of a bunch of DOM objects anymore. I wouldn't even mind if React were slower- the performance isn't really the point. I don't and will never want web components for this reason.
- Touche 8y ago
  You can use vdom in web components. This article, that you didn't read, talks about using one.
- Rusky 8y ago
  Virtual DOM isn't what makes things declarative. You can solve the same problem by diffing the model instead of the render output- the important part is that you have a comprehensive way to tell what parts of the view need to change.
dmitriid 8y ago
Please read this blog post titled "How to win in Web Framework Benchmarks" https://medium.com/@localvoid/how-to-win-in-web-framework-benchmarks-8bc31af76ce7 https://medium.com/@localvoid/how-to-win-in-web-framework-be... It goes into some details how different approaches in different frameworks work, and why. Moreover, it shows how you basically need to re-implement Virtual DOM and other tricks in vanilla JS code to approach the same speed. Yes, browser engineers are not that stupid. But they don't batch operations as efficiently as a proper Virtual DOM implementation would (including efficiently handling event listeners, looking for clues like `keys` on repeating DOM elements etc. etc.).
- stupidcar 8y ago
  That article doesn't quite support what you're claiming. The Vanilla JS implementation uses optimised DOM mutation techniques that are inspired by how the Virtual DOM libraries do things, but it doesn't re-implement a Virtual DOM. Yet it's faster than any of the Virtual DOM implementations its benchmarked against. So the takeaway is that DOM mutation can be slower or faster depending on the techniques you use. Using a Virtual DOM library can help you improve performance, because these techniques are often baked in, but they don't intrinsically result from using a Virtual DOM: You could write a V-DOM library that didn't use them, and was slow, and you could write, for example, a static template library that did, and was fast. In theory, a compiled, static template approach should be faster than V-DOM (or anything other than a set of totally bespoke and optimised vanilla JS functions for each app operation). Because while a compiled template does not have the turing-complete flexibility of something like JSX, it has the advantage of knowing in advance exactly which parts of the DOM can change and how, which eliminates the need to build and diff the Virtual DOM tree.
  - localvoid 8y ago
    > In theory, a compiled, static template approach should be faster than V-DOM Maybe faster in basic microbenchmarks. As soon as you start optimizing UI library for complex applications, there are many other optimization goals you should focus on, like the size of the generated code per component type, the amount of different code paths(increase probability that executed code is JITed), etc. Virtual DOM solves many issues, it is not just diffing, it is also a lightweight component model, possibility to add lightweight synthetic events, single code path for creating/updating/destroying DOM nodes, the simplest algo that handles complex tree transformations, etc.
Jasper_ 8y ago
> It's a nifty optimisation, but it's not about avoiding reflow, it's just another method of dirty checking, similar to that done in other frameworks like Angular or Ember. There's also a lot of "implicit state" not captured in such a virtual DOM, like the user's current selection, the scroll position, form values, and so on. If these are blown away, it creates a really poor experience. The virtual DOM isn't just an optimization, it is actually a correctness fix.

Felz 8y ago

It seems odd to characterize a declarative approach to rendering as an optimization. The draw for me is that I don't need to manage the state of a bunch of DOM objects anymore. I wouldn't even mind if React were slower- the performance isn't really the point. I don't and will never want web components for this reason.

Touche 8y ago
You can use vdom in web components. This article, that you didn't read, talks about using one.
Rusky 8y ago
Virtual DOM isn't what makes things declarative. You can solve the same problem by diffing the model instead of the render output- the important part is that you have a comprehensive way to tell what parts of the view need to change.

dmitriid 8y ago

Please read this blog post titled "How to win in Web Framework Benchmarks" https://medium.com/@localvoid/how-to-win-in-web-framework-benchmarks-8bc31af76ce7 https://medium.com/@localvoid/how-to-win-in-web-framework-be... It goes into some details how different approaches in different frameworks work, and why. Moreover, it shows how you basically need to re-implement Virtual DOM and other tricks in vanilla JS code to approach the same speed. Yes, browser engineers are not that stupid. But they don't batch operations as efficiently as a proper Virtual DOM implementation would (including efficiently handling event listeners, looking for clues like `keys` on repeating DOM elements etc. etc.).

stupidcar 8y ago
That article doesn't quite support what you're claiming. The Vanilla JS implementation uses optimised DOM mutation techniques that are inspired by how the Virtual DOM libraries do things, but it doesn't re-implement a Virtual DOM. Yet it's faster than any of the Virtual DOM implementations its benchmarked against. So the takeaway is that DOM mutation can be slower or faster depending on the techniques you use. Using a Virtual DOM library can help you improve performance, because these techniques are often baked in, but they don't intrinsically result from using a Virtual DOM: You could write a V-DOM library that didn't use them, and was slow, and you could write, for example, a static template library that did, and was fast. In theory, a compiled, static template approach should be faster than V-DOM (or anything other than a set of totally bespoke and optimised vanilla JS functions for each app operation). Because while a compiled template does not have the turing-complete flexibility of something like JSX, it has the advantage of knowing in advance exactly which parts of the DOM can change and how, which eliminates the need to build and diff the Virtual DOM tree.
- localvoid 8y ago
  > In theory, a compiled, static template approach should be faster than V-DOM Maybe faster in basic microbenchmarks. As soon as you start optimizing UI library for complex applications, there are many other optimization goals you should focus on, like the size of the generated code per component type, the amount of different code paths(increase probability that executed code is JITed), etc. Virtual DOM solves many issues, it is not just diffing, it is also a lightweight component model, possibility to add lightweight synthetic events, single code path for creating/updating/destroying DOM nodes, the simplest algo that handles complex tree transformations, etc.

Jasper_ 8y ago

> It's a nifty optimisation, but it's not about avoiding reflow, it's just another method of dirty checking, similar to that done in other frameworks like Angular or Ember. There's also a lot of "implicit state" not captured in such a virtual DOM, like the user's current selection, the scroll position, form values, and so on. If these are blown away, it creates a really poor experience. The virtual DOM isn't just an optimization, it is actually a correctness fix.