If it's behaviour that you can imagine needing to reuse among multiple components, or if it's something that you can imagine applying to an element inside an {#if ...} block (for example), then it probably belongs in an action. It it's something that 'belongs' to the component itself, rather than a specific element, then it's probably more of a lifecycle thing.

A simple rule of thumb: the name of the updated variable must appear on the left hand side of the assignment. For example this... const foo = obj.foo; foo.bar = 'baz';...won't update references to obj.foo.bar, unless you follow it up with obj = obj.

Could I get your intuition for why that rule of thumb applies to svelte components but not Javascript functions? I tend to make heavy use of let x = e when writing normal Javascript, as I do in most other languages (though unlambda is a notable exception). How is svelte different?

Most copyright experts recommend this rule of thumb -- when in doubt, assume a work is copyrighted and ask permission to use it.