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Interactive islands

Islands enable client side interactivity in Fresh. Islands are isolated Preact components that are rendered on the client. This is different from all other components in Fresh, as they are usually just rendered on the server.

Islands are defined by creating a file in the islands/ folder in a Fresh project. The name of this file must be a PascalCase or kebab-case name of the island. The file must have a default export that is a regular Preact component.

// islands/MyIsland.tsx

import { useState } from "preact/hooks";

export default function MyIsland() {
  const [count, setCount] = useState(0);

  return (
      Counter is at {count}.{" "}
      <button onClick={() => setCount(count + 1)}>+</button>

An island can be used in a page like a regular Preact component. Fresh will take care of automatically re-hydrating the island on the client.

Passing props to islands is supported, but only if the props are JSON serializable. This means that you can only pass:

  • Primitive types string, boolean, and null
  • Most numbers (Infinity, -Infinity, and NaN are silently converted to null, and bigints are not supported)
  • Plain objects with string keys and primitive, plain object, or array values
  • Arrays containing primitives, plain objects, or other arrays

It is currently not possible to pass complex objects like Date, custom classes, or functions. This means that it is not possible to pass children to an island, as children are VNodes, which are not serializable. To find out more about JSON serializable values, check out how JSON.stringify() works.

It is also not supported to nest islands within other islands.