In runes mode, reactions to state updates are handled with the [`$derived`]($derived) and [`$effect`]($effect) runes.
In legacy mode, any top-level statement (i.e. not inside a block or a function) can be made reactive by prefixing it with a `$:` [label](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label). These statements run after other code in the `<script>` and before the component markup is rendered, then whenever the values that they depend on change.
```svelte
<script>
let a = 1;
let b = 2;
// this is a 'reactive statement', and it will re-run
Statements are ordered _topologically_ by their dependencies and their assignments: since the `console.log` statement depends on `sum`, `sum` is calculated first even though it appears later in the source.
Multiple statements can be combined by putting them in a block:
```js
// @noErrors
$: {
// recalculate `total` when `items` changes
total = 0;
for (const item of items) {
total += item.value;
}
}
```
The left-hand side of a reactive assignments can be an identifier, or it can be a destructuring assignment:
```js
// @noErrors
$: ({ larry, moe, curly } = stooges);
```
## Understanding dependencies
The dependencies of a `$:` statement are determined at compile time — they are whichever variables are referenced (but not assigned to) inside the statement.
In other words, a statement like this will _not_ re-run when `count` changes, because the compiler cannot 'see' the dependency:
```js
// @noErrors
let count = 0;
let double = () => count * 2;
$: doubled = double();
```
Similarly, topological ordering will fail if dependencies are referenced indirectly: `z` will never update, because `y` is not considered 'dirty' when the update occurs. Moving `$: z = y` below `$: setY(x)` will fix it:
```svelte
<script>
let x = 0;
let y = 0;
$: z = y;
$: setY(x);
function setY(value) {
y = value;
}
</script>
```
## Browser-only code
Reactive statements run during server-side rendering as well as in the browser. This means that any code that should only run in the browser must be wrapped in an `if` block:
