types

packages/svelte/types/index.d.ts

@@ -596,10 +596,19 @@ declare module 'svelte' {
 		[K in keyof T]: () => T[K];
 	};
 
+	/** Decode a value. The value will be whatever the evaluated JavaScript emitted by the corresponding {@link Encode} function evaluates to. */
 	type Decode<T> = (value: any) => T;
 
+	/** Encode a value as a string. The string should be _valid JavaScript code_ -- for example, the output of `devalue`'s `uneval` function. */
 	type Encode<T> = (value: T) => string;
 
+	/**
+	 * Custom encode and decode options. This must be used in combination with an environment variable to enable treeshaking, eg:
+	 * ```ts
+	 * import { BROWSER } from 'esm-env';
+	 * const transport: Transport<MyType> = BROWSER ? { decode: myDecodeFunction } : { encode: myEncodeFunction };
+	 * ```
+	 */
 	type Transport<T> =
 		| {
 				encode: Encode<T>;
@@ -610,10 +619,26 @@ declare module 'svelte' {
 				decode: Decode<T>;
 		  };
 
+	/** Make the result of a function hydratable. This means it will be serialized on the server and available synchronously during hydration on the client. */
 	type Hydratable = {
-		<T>(key: string, fn: () => T, options?: { transport?: Transport<T> }): T;
+		<T>(
+			/**
+			 * A key to identify this hydratable value. Each hydratable value must have a unique key.
+			 * If writing a library that utilizes `hydratable`, prefix your keys with your library name to prevent naming collisions.
+			 */
+			key: string,
+			/**
+			 * A function that returns the value to be hydrated. On the server, this value will be stashed and serialized.
+			 * On the client during hydration, the value will be used synchronously instead of invoking the function.
+			 */
+			fn: () => T,
+			options?: { transport?: Transport<T> }
+		): T;
+		/** Get a hydratable value from the server-rendered store. If used after hydration, will always return `undefined`. Only works on the client. */
 		get: <T>(key: string, options?: { decode?: Decode<T> }) => T | undefined;
+		/** Check if a hydratable value exists in the server-rendered store. */
 		has: (key: string) => boolean;
+		/** Set a hydratable value. Only works on the server during `render`. */
 		set: <T>(key: string, value: T, options?: { encode?: Encode<T> }) => void;
 	};