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Summary

An island is a component that is interactive and will be hydrated on the client side. The server sends all the data from the island’s props for hydration, and the browser needs time to process and render these islands. Therefore, it is important to take some precautions when using islands:
  1. Minimize the amount of props to be sent/used for an island
  2. Make an island only what is necessary, remembering to use children for internal elements that do not need hydration.

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Reducing the size of the props JSON sent to islands

When loading data from external APIs using Loaders and sending them to the Section, it is possible that the size of the payload negatively impacts the performance of the site. The impact occurs both in the initial loading time and in the hydration, where the page is “initialized” in the browser to make it interactive (using useEffect, useSignal, etc…). You can view the size of the final JSON through the Performance tab of one of your site pages in the Deco CMS.
Performance information of a page
When the JSON size exceeds ~500kb, it is likely that the UI does not need the complete data, but rather some part of it (or a computation based on other values). To reduce this size and improve page performance, it is possible to filter the data in the Loader so that only the necessary data is passed to the UI.

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Reducing data sent to islands

In this first example, we will show how to avoid sending too much data to an island. Let’s say there is a component called ProductCard that receives the entire JSON of a product. 
import Image from "apps/website/components/Image.tsx";

export default function ProductCard({ product }: Props) {
  return (
    <div>
      <Image src={product.image} width="100" height="100" />
    </div>
  );
}
In it, you want to include an Island to create the buy button. 
import BuyButton from "$store/components/ui";
import Image from "apps/website/components/Image.tsx";

export default function ProductCard({ product }: Props) {
  return (
    <div>
      <Image src={product.image} width="100" height="100" />
      <BuyButton />
    </div>
  );
}
It is possible that this BuyButton needs some product information in order to add it to the cart. This is where we need to be careful about the amount of data sent to the Island. For example, it is very likely that the buy button does not need to receive image data. The ideal approach is to send only the necessary data.
❌ Inappropriate approach 
import BuyButton from "$store/components/ui";
import Image from "apps/website/components/Image.tsx";

export default function ProductCard({ product }: Props) {
  return (
    <div>
      <Image src={product.image} width="100" height="100" />
      <BuyButton product={product} />
    </div>
  );
}
✅ Correct approach 
import BuyButton from "$store/components/ui";
import Image from "apps/website/components/Image.tsx";

export default function ProductCard({ product }: Props) {
  return (
    <div>
      <Image src={product.image} width="100" height="100" />
      <BuyButton id={product.id} seller={product.seller} />
    </div>
  );
}
The correct approach sends only the ID and Seller data, which in the example are the only ones needed in the Island. Thus, during hydration, the JSON that the Island will load will not be as large.

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Reducing the scope of an island

An island and its components will all be hydrated on the client side in order to operate. This means that for all defined elements of the island, they will be recursively hydrated. It is possible to reduce the scope of the island by passing any internal elements as children of the island.
❌ Inappropriate approach
In the example below, we create an island that interacts with localStorage to set a title for a gallery of items. In the example below, both the gallery props will be passed to hydrate the TitleContainer and will also be passed to hydrate the Gallery. 
import { computed } from "@preact/signals";
import { IS_BROWSER } from "$fresh/runtime.ts";
import type { GalleryProps } from "../components/Gallery.tsx";
import { Gallery } from "../components/Gallery.tsx";

export default function TitleContainer(
  { galleryProps }: { galleryProps: GalleryProps },
) {
  const title = computed(() => {
    IS_BROWSER ? localStorage.getItem("title") : "Loading...";
  });

  return (
    <div>
      <h1>{title}</h1>
      <Gallery {...galleryProps} />
    </div>
  );
}
✅ Correct approach
However, if the Gallery is passed as children to the island, it will be rendered, serialized, and not hydrated! For the TitleContainer, the children is pre-rendered HTML ready to be displayed, and therefore it is not an island itself. 
import { computed } from "@preact/signals";
import type { ComponentChildren } from "preact";
import { IS_BROWSER } from "$fresh/runtime.ts";

export default function TitleContainer(
  { children }: { children: ComponentChildren },
) {
  const title = computed(() => {
    IS_BROWSER ? localStorage.getItem("title") : "Loading...";
  });

  return (
    <div>
      <h1>{title}</h1>
      {children}
    </div>
  );
}
Uso do title container (em uma section, por exemplo): 
//...
<TitleContainer>
    <Gallery {...galleryProps}>
</TitleContainer>
//...