Components are the building blocks of Svelte applications. They are written into `.svelte` files, using a superset of HTML:
---
Components are the building blocks of Svelte applications. They are written into `.svelte` files, using a superset of HTML.
All three sections — script, styles and markup — are optional.
```html
<script>
@ -16,11 +20,10 @@ Components are the building blocks of Svelte applications. They are written into
<!-- markup (zero or more items) goes here -->
```
All three sections — script, styles and markup — are optional.
### Script
---
A `<script>` block contains JavaScript that runs when a component instance is created. Variables declared (or imported) at the top level are 'visible' from the component's markup. There are four additional rules:
#### 1. `export` creates a component prop
@ -45,10 +48,13 @@ Svelte uses the `export` keyword to mark a variable declaration as a *property*
</script>
```
#### 2. Assignments are 'reactive'
To change component state and trigger a re-render, just assign to a locally declared variable:
---
To change component state and trigger a re-render, just assign to a locally declared variable.
Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.
```html
<script>
@ -62,11 +68,10 @@ To change component state and trigger a re-render, just assign to a locally decl
</script>
```
Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.
#### 3. `$:` marks a statement as reactive
---
Any top-level statement (i.e. not inside a block or a function) can be made reactive by prefixing it with the `$:` label. Reactive statements run immediately before the component updates, whenever the values that they depend on have changed:
```html
@ -84,6 +89,8 @@ Any top-level statement (i.e. not inside a block or a function) can be made reac
</script>
```
---
If a statement consists entirely of an assignment to an undeclared variable, Svelte will inject a `let` declaration on your behalf:
```html
@ -94,10 +101,13 @@ If a statement consists entirely of an assignment to an undeclared variable, Sve
</script>
```
#### 4. Prefix stores with `$` to access their values
Any time you have a reference to a store, you can access its value inside a component by prefixing it with the `$` character. This causes Svelte to declare the prefixed variable, and set up a store subscription that will be unsubscribed when appropriate:
---
Any time you have a reference to a store, you can access its value inside a component by prefixing it with the `$` character. This causes Svelte to declare the prefixed variable, and set up a store subscription that will be unsubscribed when appropriate.
Local variables (that do not represent store values) must *not* have a `$` prefix.
```html
<script>
@ -111,8 +121,6 @@ Any time you have a reference to a store, you can access its value inside a comp
</script>
```
Local variables (that do not represent store values) must *not* have a `$` prefix.
Rich Harris
7 years ago