Metadata
So far, everything we’ve rendered has been inside the <body> of the HTML document. And this makes sense. After all, everything you can see on a web page lives inside the <body>.
However, there are plenty of occasions where you might want to update something inside the <head> of the document using the same reactive primitives and component patterns you use for your UI.
That’s where the leptos_meta package comes in.
Metadata Components
leptos_meta provides special components that let you inject data from inside components anywhere in your application into the <head>:
<Title/> allows you to set the document’s title from any component. It also takes a formatter function that can be used to apply the same format to the title set by other pages. So, for example, if you put <Title formatter=|text| format!("{text} — My Awesome Site")/> in your <App/> component, and then <Title text="Page 1"/> and <Title text="Page 2"/> on your routes, you’ll get Page 1 — My Awesome Site and Page 2 — My Awesome Site.
<Link/> takes the standard attributes of the <link> element.
<Stylesheet/> creates a <link rel="stylesheet"> with the href you give.
<Style/> creates a <style> with the children you pass in (usually a string). You can use this to import some custom CSS from another file at compile time <Style>{include_str!("my_route.css")}</Style>.
<Meta/> lets you set <meta> tags with descriptions and other metadata.
<Script/> and <script>
leptos_meta also provides a <Script/> component, and it’s worth pausing here for a second. All of the other components we’ve considered inject <head>-only elements in the <head>. But a <script> can also be included in the body.
There’s a very simple way to determine whether you should use a capital-S <Script/> component or a lowercase-s <script> element: the <Script/> component will be rendered in the <head>, and the <script> element will be rendered wherever in the <body> of your user interface you put it in, alongside other normal HTML elements. These cause JavaScript to load and run at different times, so use whichever is appropriate to your needs.
<Body/> and <Html/>
There are even a couple elements designed to make semantic HTML and styling easier. <Html/> lets you set the lang and dir on your <html> tag from your application code. <Html/> and <Body/> both have class props that let you set their respective class attributes, which is sometimes needed by CSS frameworks for styling.
<Body/> and <Html/> both also have attributes props which can be used to set any number of additional attributes on them via the attr: syntax:
<Html
lang="he"
dir="rtl"
attr:data-theme="dark"
/>Metadata and Server Rendering
Now, some of this is useful in any scenario, but some of it is especially important for search-engine optimization (SEO). Making sure you have things like appropriate <title> and <meta> tags is crucial. Modern search engine crawlers do handle client-side rendering, i.e., apps that are shipped as an empty index.html and rendered entirely in JS/WASM. But they prefer to receive pages in which your app has been rendered to actual HTML, with metadata in the <head>.
This is exactly what leptos_meta is for. And in fact, during server rendering, this is exactly what it does: collect all the <head> content you’ve declared by using its components throughout your application, and then inject it into the actual <head>.
But I’m getting ahead of myself. We haven’t actually talked about server-side rendering yet. The next chapter will talk about integrating with JavaScript libraries. Then we’ll wrap up the discussion of the client side, and move onto server side rendering.