Update unique-element-ids.md (#33)

docs/faq/unique-element-ids.md

@@ -4,7 +4,9 @@ The `id` attribute for an element needs to be unique across the entire page. Har
 
 But there are legitimate use cases for an `id`, such as ARIA attributes or tying together a `<label>` and `<input>`. In those scenarios you would need to generate a dynamic value for the `id`.
 
-While it is possible to use a library to generate a UUID, that is usually unnecessary. A counter is usually sufficient.
+## Using a counter
+
+In many cases, it's sufficient to use a simple counter to generate a suitable `id`.
 
 Put something like this in a `.js` file:
 
@@ -30,3 +32,33 @@ const id = newId()
 See it on the [SFC Playground](https://play.vuejs.org/#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).
 
 An element `id` beginning with a digit [can cause problems](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/id), so we put a prefix before the count.
+
+## Using `useId()`
+
+The simple counter outlined above is typically sufficient for client-side applications, but it doesn't work well with SSR (server-side rendering).
+
+With SSR, the `id` attributes in the server-generated HTML need to match the equivalent client-side attributes. There are several reasons this is difficult, but the two biggest problems are:
+
+1. If the counter is global on the server, it will be shared between requests. It needs to be scoped to the application instance instead, to ensure it starts back at 0 when a new request comes in.
+2. The calls to the counter must happen in exactly the same order on both the server and client. While this is usually the case, it breaks down for async components.
+
+Vue 3.5 introduced the [`useId()` helper](https://vuejs.org/api/composition-api-helpers.html#useid), which aims to solve this problem. Prior to that, Nuxt and Quasar both implemented their own `useId()` helpers. The approaches they took were very different, so it can be interesting to compare those alternatives:
+
+* The Quasar [`useId()` helper](https://quasar.dev/vue-composables/use-id) returns a ref. That ref initially has a value of `null`. The value is only changed to a meaningful ID on the client-side after hydration has completed. Vue won't render attributes with `null` values, so any attribute bound to the ref won't be rendered until after hydration.
+* The Nuxt [`useId()` helper](https://nuxt.com/docs/api/composables/use-id) is now just an alias for Vue's own `useId()`, but prior to Vue 3.5 it took a very different approach. Nuxt would generate an ID on the server and then pass that to the client using a special `data-n-ids` attribute in the generated HTML. This could then be checked immediately prior to hydration, allowing the client to use the same ID. A major downside of this approach was that it relied on having a suitable element node to hold the `data-n-ids` attribute.
+
+If you're using Vue 3.5+ then you'll likely want to use the built-in `useId()` helper to generate `id` attributes for SSR. It uses multiple counters, taking account of the component tree and the positions of async components. These counters are concatenated to generate the final value.
+
+```vue
+<script setup>
+import { useId } from 'vue'
+
+const id = useId()
+</script>
+```
+
+See it on the [SFC Playground](https://play.vuejs.org/#__SSR__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).
+
+## Other types of ID
+
+The `useId()` helper is only intended to solve the problem of generating attribute values, as described above. It is not a general-purpose way of generating IDs for other uses. For example, if you need to generate unique IDs for use in your data, then `useId()` likely isn't what you want. In many cases you'll be able to use the counter approach [described earlier](#using-a-counter). If you want a UUID instead then consider using the native [`crypto.randomUUID()`](https://developer.mozilla.org/en-US/docs/Web/API/Crypto/randomUUID) method or the [`uuid` package](https://www.npmjs.com/package/uuid) available on npm.