function includes<T, U extends T>(arr: readonly U[], elem: T): elem is U { return arr.includes(elem as any); }

See #14520 for discussion of upper-bounded generics.

the generic means "give me one of each function a -> Boolean" so if any of those functions doesn't exist, then the generic doesn't exist.

it is inconvenient to write specific implementations for each datatype contained, especially if the code for each datatype is virtually identical. For example, in C++, this duplication of code can be circumvented by defining a class template

Note that you can also use dynamic partials, that conditionally select the partial to render based on the value in the JSON.

we update the validation schema on the fly (we had a similar case with a validation that needs to be included whenever some fetch operation was completed)

What would happen if we get the list from a rest api when the form component is mounted? That's an interesting topic, you can add a rule once a component has been mounted and update the associated validation schema.

Ok, I have seen that there are lot of built-in and third party validations, but sooner or later I will face a validation rule not covered by this buffet. Can I build a custom one? Of course you can!

Svelte will not offer a generic way to support style customizing via contextual class overrides (as we'd do it in plain HTML). Instead we'll invent something new that is entirely different. If a child component is provided and does not anticipate some contextual usage scenario (style wise) you'd need to copy it or hack around that via :global hacks.

Explicit interfaces are preferable, even if it places greater demand on library authors to design both their components and their style interfaces with these things in mind.

The point of the feature is to not rely on the third-party author of the child component to add a prop for every action under the sun. Rather, they could just mark a recipient for actions on the component (assuming there is a viable target element), and then consumers of the library could extend the component using whatever actions they desire.

Your LazyLoad image is now inextensible. What if you want to add a class? Perhaps the author of LazyLoad thought of that and sets className onto the <img>. But will the author consider everything? Perhaps if we get {...state} attributes.

You'll have to create a new component that brings in the functionality of both. TooltipButton, TooltipLink, Link, and TooltipRoutedLink. We're starting to get a lot of components to handle a bit of added functionality.

For the tooltip example, if you had a whole bunch of tooltips on different elements, it would be annoying to have different event listeners and "should it be shown" variables for each one.

Perhaps at that point we're better off settling on a way to pass components through as parameters? <!-- App.html --> <Outer contents={Inner}/> <!-- Outer.html --> <div> <div>Something</div> <[contents] foo='bar'/> </div>

I would hope for it to come with React-like behavior where I could pass in a string (like div or a) and have it show up as a normal div/a element when the child component used it.

Use case: Wrapper components that need to render an element (e.g. because they attach event listeners). You'd probably use a <div> there by default but there may be places where this is not desirable for semantic reasons (e.g. in lists).

const components = { Label, Tree, Menu };

The lack of spread continues to be a big pain for me, adding lots of difficult-to-maintain cruft in my components. Having to maintain a list of all possible attributes that I might ever need to pass through a component is causing me a lot of friction in my most composable components.

In other words you cannot choose the definition of a value based on its type (for now).

Using Type Parameters in Generic Constraints