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Hydration in Astro

Hydration is always a tricky part in SSR. Here is a quick guide with Astro 3!

Did you find yourself in a situation where you needed to hydrate a global store, or pass arguments to a bundled script? Well, many SSR frameworks kinda take care of this for you but not in Astro.

If you’re facing this issue, then this article is for you!

Found an issue, a better way or something else? Open an issue on my portfolio!

What is hydration?

I’ll borrow this really good explanation from Vike docs :

When doing SSR our pages are rendered to HTML. But HTML alone isn’t sufficient to make a page interactive. For example, a page with zero browser-side JavaScript cannot be interactive (there are no JavaScript event handlers to react to user actions such as a click on a button).

To make our page interactive, in addition to render our page to HTML in Node.js, our UI framework (Vue/React/…) also loads and renders the page in the browser. (It creates an internal representation of the page, and then maps the internal representation to the DOM elements of the HTML we rendered in Node.js.)

This process is called hydration. Informally speaking: it makes our page interactive/alive/hydrated.

Usage in Astro

We’ll look at 2 ways of achieving this!

Using @ayco/astro-resume

Ayo Ayco has created this package for this exact situation. Have a look at the docs !

You can use it like so:

import Serialize from '@ayco/astro-resume'

const data = {
    hello: 'world',
    isOkay: true,
// define the type of data to be serialized
export type Data = typeof data

<Serialize id="my-data" {data} />

    import { deserialize } from '@ayco/astro-resume'
    import type { Data } from './ThisComponent.astro'

    const data = deserialize<Data>('my-data')
    console.log(data) // { hello: 'world', isOkay: true }

While this works, I prefer a more manual approach that I have full control over.

Using scripts and data attributes

That approach is actually inspired by how both Astro and Nuxt work! Let’s have a look at the code first:

const someData = { foo: 'bar' }

<script is:inline data-state={JSON.stringify(someData)}>
    const state = JSON.parse(
        document.currentScript?.getAttribute('data-state') ?? null

    window.__state = state

    // script has no `is:inline`!
    const someData = window.__state

So here is how it works:

  1. You pass your serializable data as data attribute to an inline script
  2. You retrieve this value from within the script and deserialize it
  3. You add that data to the global window object. Because the script is inlined, it happends (almost?) immediatly
  4. You can safely access your data in bundled scripts!

A few important points there:

  • Make sure the property you’re setting on the window object is not already taken. Prefixing by 2 underscores seems pretty safe though but always check!
  • Your data needs to be serializable

If you’re using TypeScript (you really should!), it will complain about your property not existing on the window object. You need to augment the global type:

// src/env.d.ts

/// <reference types="astro/client" />

interface Window {
  __state: { foo: "bar" };

export {};

You’ll likely be using a more complex type, and therefore want to import it. But beware! You can’t import a type in a d.ts file as you would do in a ts file. You need to use the following syntax:

// src/env.d.ts

/// <reference types="astro/client" />

interface Window {
  __state: import("./my-file").State;

export {};


This approach probably has flaws I’m not aware of (although I hope that’s not the case) and I’d like to know about it! Reach out and tell me about it!

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