[genezeta]

No, not at all.

On the one hand it is rare that you'd need to update a thousand elements at once one by one.

On the other hand, and more importantly, the problem that Virtual DOM aims to solve is not that one. Instead, what it wants to avoid is not "updating a thousand elements" but "replacing large sub-trees in the DOM unnecessarily".

That is, Virtual DOM competes, mostly, against doing something like...

    someElement.innerHTML = '...<a large piece of HTML>...'

The main concern of doing this is not so much that the performance is bad or not, but that it is intrinsically wasteful. That is, it [almost always] unnecessarily forces the browser to rebuild a part of the DOM and re-render a part of the page... just to change but a few values.

This is written in that way because it's easy to write it like that. But, being such a wasteful approach, it does end causing performance concerns.

Going from this, you can use various approaches. A Virtual DOM basically let's you manage your [virtual] DOM fragments as you were already doing but then inserts itself as a middleman to avoid doing those unnecessary DOM replacements and only modify what needs to be modified.

There are two things you need to notice here: First it does add some additional processing an calculation -"the diff"-. Second it still, because there is no other way to do it, uses the same DOM API calls you would use if "manually updating things".

So now you can balance this to see if it's worth it: On the one hand there's a cost -the added calculations-, on the other hand there is a gain -modifying only smaller parts of the DOM-. But be aware that this gain is compared not against "updating many elements" but against "rebuilding parts of the DOM when you don't need to". The distinction is rather important because if, suppose, you actually need to update the content of say a thousand <td>s in a table, your virtual DOM library will still need to do exactly that, it will do so by using the DOM API -because there's no other way-, and any additional processing it does will be added on top of doing that.

So... bad option: build a whole...

    <table><tr><td>A</td><td>B</td></tr><tr><td>C</td><td>D</td></tr>...</table>

...every time and dump it all into the DOM element; let the browser do all the work and live with the bad performance you get from having easy to write but wasteful code.

Virtual DOM solution: Still write easy to write and wasteful code but apply something in between that reads your "whole block" and finds what actually needs to be done and then does only that.

But it's not the only solution. Another solution is to just don't write that wasteful code in the first place. Instead of building large but mostly unchanged blocks and dump them, only modify what needs to be modified. Of course, it usually means that your code has to keep track of an additional bunch of things -i.e. where in the DOM does each piece of information go-, but this is where those other alternatives mentioned, like Svelte or others, may come into play.

The conclusion from all this is that:

a. No, a virtual DOM is not inherently faster than doing updates manually

b. A virtual DOM provides some gain by not doing unneeded updates of DOM branches. If you were doing those, using a virtual DOM may speed things up.

c. A virtual DOM still needs to do the DOM modifications with exactly the same API you would/could use manually so there's no gain there.

[jcelerier]

> On the one hand it is rare that you'd need to update a thousand elements at once one by one.

Is it ? I come from the desktop app world and that would frankly barely register above "toy project" ; my experience is more around a few tens / hundred of thousands objects that can change at once (with of course the UI framework only redrawing what needs redrawing).

> There are two things you need to notice here: First it does add some additional processing an calculation -"the diff"-. Second it still, because there is no other way to do it, uses the same DOM API calls you would use if "manually updating things".

I have a hard time understanding how, say, calling 500 methods per second on a DOM object, e.g. `label.innerText = someSensorValueComingFromWebsocketsVeryFast;`, which needs to trigger various events, callbacks, etc... every time could possibly be faster than modifying a pure JS object at the "incoming message rate" and then blitting that object at the screen refresh rate or something similar ?

[genezeta]

> I have a hard time understanding how, say, calling 500 methods per second on a DOM object, e.g. `label.innerText = someSensorValueComingFromWebsocketsVeryFast;`, which needs to trigger various events, callbacks, etc... every time could possibly be faster than modifying a pure JS object at the "incoming message rate" and then blitting that object at the screen refresh rate or something similar ?

Updating a value on the DOM 500 times per second just because you can read it 500 times per second falls, again, in the category of writing wasteful code just because it's simpler. i.e. "don't care about performance, just let the browser work".

If you compare code which is initially bad, then sure, a lot of "solutions" will be better than that.

The appropriate comparison for evaluating the value of using a virtual DOM should not be about that, but only about the part you describe as "blitting the object to the screen". You have the data, no matter how, where, or even -to some extent- how often, and you want to put it on the screen.

[jcelerier]

> The appropriate comparison for evaluating the value of using a virtual DOM should not be about that, but only about the part you describe as "blitting the object to the screen". You have the data, no matter how, where, or even -to some extent- how often, and you want to put it on the screen.

But the solution to the problem which is

    how to go from 
      inputs (which you don't have any control over) 
    to 
      "You have the data, no matter how, where, or even -to some extent- how often, and you want to put it on the screen"

will pretty much look like a vdom, no ?

[genezeta]

> will pretty much look like a vdom, no ?

Not necessarily.

Maybe you can find some rendering comparisons from a few years ago. There was some "demo" called DBMonster or DBMON that showed the performance of updating a large amount of values on a table in a web page. The thing is a lot of different implementations were then written by many people using many different frameworks -including many using a virtual DOM and many not using one- and also even some "vanilla" implementations. I don't suggest it for the performance comparison -which you can of course look into- but, more in line with the question: it may be a good way to find a number of different approaches.

[zozbot234]

> That is, it [almost always] unnecessarily forces the browser to rebuild a part of the DOM and re-render a part of the page... just to change but a few values.

That's a mere implementation detail. In practice, letting the browser re-render a single page chunk using its internal implementation will almost always be faster than using JS code to serially update any number of DOM elements. VDOM diffing is a kludge to make DOM manipulation in pure JS usable, it's not actually going to be faster than relying on the browser's own rendering. The biggest problem with the innerHTML is going to be generating the actual HTML, but one can most likely use WASM to speed that up.