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Svelte automatically updates the DOM when your component's state changes. Often, some parts of a component's state need to be computed from other parts (such as a fullname derived from a firstname and a lastname), and recomputed whenever they change.

For these, we have reactive declarations. They look like this:

let count = 0;
$: doubled = count * 2;

Don't worry if this looks a little alien. It's valid (if unconventional) JavaScript, which Svelte interprets to mean 're-run this code whenever any of the referenced values change'. Once you get used to it, there's no going back.

Let's use doubled in our markup:

<p>{count} doubled is {doubled}</p>

Of course, you could just write {count * 2} in the markup instead — you don't have to use reactive values. Reactive values become particularly valuable when you need to reference them multiple times, or you have values that depend on other reactive values.

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