Difference between $derived() and $derived.by()?

[frederikhors]

Since in Svelte 5 docs are present two similar runes: $derived() and $derived.by() I'm wondering what is the difference between:

let test = $derived(() => {});

and

let test = $derived.by(() => {});

[brunnerh]

One takes a function, useful for utility functions that return a closure or for more complex logic that requires a { ... } block.
These are equivalent:

const a = $derived(x * 2);

const b = $derived.by(() => x * 2);
const c = $derived.by(() => {
  return x * 2;
});

[brunnerh]

Yes, that's just how arrow functions work in JS.
(Did not mean to self-reference the variable, so I renamed them to a, b and c.)

[brunnerh]

Note that this makes x a function, not the value returned by the function.

[7nik]

There also is a crucial difference when using a factory. Example

[frederikhors]

There also is a crucial difference when using a factory.

I don't understand. They work all good, right?

[7nik]

der1 re-calls the factory, which leads to changing rand each time. This behaviour can often be undesired.