I'm not saying never mark methods private. I'm saying the better rule of thumb is to "make methods protected unless there's a good reason not to".

A rule of thumb is that data should just be data - it is not recommended to observe objects with their own stateful behavior.

The most important guideline to give is the following: Write clean unit tests if there is actual value in testing a complex piece of logic in isolation to prevent it from breaking in the future Otherwise, try to write your specs as close to the user’s flow as possible

A litmus test on whether an option belongs to adapter config or kit config, would be to ask whether the option becomes irrelevant when you switch the adapter to static.

The basic rule of thumb is: "I'm not aware of all types of security exploits. I must protect against those I do know of and then I must be proactive!".

Now you can easily spot them! The rule is: if you see a Result it means that this function can throw an exception. And you even know its type in advance.

In other words: the controllers usually contain only routing and rendering code and dispatch instantly to a particular operation/activity class.

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.