Comment by rglover
5 years ago
Surgical = "It checks a diff of what the component rendered before an update vs. after and then only patches or updates the parts of the DOM that need to be changed (as opposed to replacing the entire DOM, which is slow)." Similar to React. Svelte may not be using the exact same mechanisms but this is in the ballpark.
Virtual DOM is just an in-memory representation of the DOM that describes which child elements (e.g., a <p /> nested within a <div />) belong to which parents, and which attributes/values those elements need at render time. When a change happens, you can generate an updated virtual DOM and diff it against the previous (or current) version and pick out what specifically needs to be changed in the DOM (read: surgically).
There is no diff going on with Svelte. And there doesn't need to be an in-memory representation of anything. Because it is compiling the code, it knows exactly what will change and in response to what.
https://svelte.dev/repl/7161f10f8eb645d8984fb231a0bc3982?ver...
For example in that repl, you can see that it makes a call to `$$invalidate` when the input value changes (see `input_input_handler()`) which triggers a `set_data` call (see `p()`) to update just the textNode that contains the name. This is what is meant by surgical. No other work is done other than what is required to update the specific places where a variable is used.
This is a cool approach, thank you for sharing that link.