A lowercase tag, like `<div>`, denotes a regular HTML element. A capitalised tag, such as `<Widget>`, indicates a *component*.
```html
<script>
import Widget from './Widget.svelte';
</script>
<div>
<Widget/>
</div>
```
### Attributes
---
By default, attributes work exactly like their HTML counterparts.
```html
<divclass="foo">
<buttondisabled>can't touch this</button>
</div>
```
---
As in HTML, values may be unquoted.
```html
<inputtype=checkbox>
```
---
Attribute values can contain JavaScript expressions.
```html
<ahref="page/{p}">page {p}</a>
```
---
Or they can *be* JavaScript expressions.
```html
<buttondisabled={!clickable}>...</button>
```
---
An expression might include characters that would cause syntax highlighting to fail in regular HTML, so quoting the value is permitted. The quotes do not affect how the value is parsed:
```html
<buttondisabled="{number !== 42}">...</button>
```
---
When the attribute name and value match (`name={name}`), they can be replaced with `{name}`.
```html
<!-- These are equivalent -->
<buttondisabled={disabled}>...</button>
<button{disabled}>...</button>
```
---
*Spread attributes* allow many attributes or properties to be passed to an element or component at once.
An element or component can have multiple spread attributes, interspersed with regular ones.
In a text expression, characters like `<` and `>` are escaped. With HTML expressions, they're not.
> Svelte does not sanitize expressions before injecting HTML. If the data comes from an untrusted source, you must sanitize it, or you are exposing your users to an XSS vulnerability.
Iterating over lists of values can be done with an each block.
```html
<h1>Shopping list</h1>
<ul>
{#each items as item}
<li>{item.name} x {item.qty}</li>
{/each}
</ul>
```
---
An each block can also specify an *index*, equivalent to the second argument in an `array.map(...)` callback:
```html
{#each items as item, i}
<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}
```
---
If a *key* expression is provided — which must uniquely identify each list item — Svelte will use it to diff the list when data changes, rather than adding or removing items at the end.
```html
{#each items as item, i (item.id)}
<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}
```
---
You can freely use destructuring patterns in each blocks.
```html
{#each items as { id, name, qty }, i (id)}
<li>{i + 1}: {name} x {qty}</li>
{/each}
```
---
An each block can also have an `{:else}` clause, which is rendered if the list is empty.
Components can emit events using [createEventDispatcher](docs#createeventdispatcher), or by forwarding DOM events. Listening for component events looks the same as listening for DOM events:
Data ordinarily flows down, from parent to child. The `bind:` directive allows data to flow the other way, from child to parent. Most bindings are specific to particular elements.
The simplest bindings reflect the value of a property, such as `input.value`.
```html
<inputbind:value={name}>
<textareabind:value={text}></textarea>
<inputtype="checkbox"bind:checked={yes}>
```
---
If the name matches the value, you can use a shorthand.
```html
<!-- These are equivalent -->
<inputbind:value={value}>
<inputbind:value>
```
---
Numeric input values are coerced; even though `input.value` is a string as far as the DOM is concerned, Svelte will treat it as a number. If the input is empty or invalid (in the case of `type="number"`), the value is `undefined`.
A `<select>` value binding corresponds to the `value` property on the selected `<option>`, which can be any value (not just strings, as is normally the case in the DOM).
```html
<selectbind:value={selected}>
<optionvalue={a}>a</option>
<optionvalue={b}>b</option>
<optionvalue={c}>c</option>
</select>
```
---
A `<select multiple>` element behaves similarly to a checkbox group.
```html
<selectmultiplebind:value={fillings}>
<optionvalue="Rice">Rice</option>
<optionvalue="Beans">Beans</option>
<optionvalue="Cheese">Cheese</option>
<optionvalue="Guac (extra)">Guac (extra)</option>
</select>
```
---
When the value of an `<option>` matches its text content, the attribute can be omitted.
Block-level elements have 4 readonly bindings, measured using a technique similar to [this one](http://www.backalleycoder.com/2013/03/18/cross-browser-event-based-element-resize-detection/):
You can bind to component props using the same mechanism.
```html
<Keypadbind:value={pin}/>
```
---
Components also support `bind:this`, allowing you to interact with component instances programmatically.
> Note that we can do `{cart.empty}` rather than `{() => cart.empty()}`, since component methods are closures. You don't need to worry about the value of `this` when calling them.
Actions are functions that are called when an element is created. They can return an object with a `destroy` method that is called after the element is unmounted:
```html
<script>
function foo(node) {
// the node has been mounted in the DOM
return {
destroy() {
// the node has been removed from the DOM
}
};
}
</script>
<divuse:foo></div>
```
---
An action can have parameters. If the returned value has an `update` method, it will be called whenever those parameters change, immediately after Svelte has applied updates to the markup.
> Don't worry about the fact that we're redeclaring the `foo` function for every component instance — Svelte will hoist any functions that don't depend on local state out of the component definition.
A transition is triggered by an element entering or leaving the DOM as a result of a state change. Transitions do not run when a component is first mounted, but only on subsequent updates.
Elements inside an *outroing* block are kept in the DOM until all current transitions have completed.
The `transition:` directive indicates a *bidirectional* transition, which means it can be smoothly reversed while the transition is in progress.
```html
{#if visible}
<divtransition:fade>
fades in and out
</div>
{/if}
```
---
The `in:` and `out:` directives are not bidirectional. An in transition will continue to 'play' alongside the out transition, if the block is outroed while the transition is in progress. If an out transition is aborted, transitions will restart from scratch.
Transitions can use custom functions. If the returned object has a `css` function, Svelte will create a CSS animation that plays on the element.
The `t` argument passed to `css` is a value between `0` and `1` after the `easing` function has been applied. *In* transitions run from `0` to `1`, *out* transitions run from `1` to `0` —in other words `1` is the element's natural state, as though no transition had been applied. The `u` argument is equal to `1 - t`.
The function is called repeatedly *before* the transition begins, with different `t` and `u` arguments.
If a transition returns a function instead of a transition object, the function will be called in the next microtask. This allows multiple transitions to coordinate, making [crossfade effects](tutorial/deferred-transitions) possible.
Components can have child content, in the same way that elements can.
The content is exposed in the child component using the `<slot>` element, which can contain fallback content that is rendered if no children are provided.
```html
<!-- App.svelte -->
<Widget>
<p>this is some child content</p>
</Widget>
<!-- Widget.svelte -->
<div>
<slot>
this will be rendered if someone does <Widget/>
</slot>
</div>
```
---
Named slots allow consumers to target specific areas. They can also have fallback content.
Slots can be rendered zero or more times, and can pass values *back* to the parent using props. The parent exposes the values to the slot template using the `let:` directive.
The usual shorthand rules apply — `let:item` is equivalent to `let:item={item}`, and `<slot {item}>` is equivalent to `<slot item={item}>`.
```html
<!-- App.svelte -->
<FancyList{items}let:item={item}>
<div>{item.text}</div>
</FancyList>
<!-- FancyList.svelte -->
<ul>
{#each items as item}
<liclass="fancy">
<slotitem={item}></slot>
</li>
{/each}
</ul>
```
---
Named slots can also expose values. The `let:` directive goes on the element with the `slot` attribute.
The `<svelte:component>` element renders a component dynamically, using the component constructor specified as the `this` property. When the property changes, the component is destroyed and recreated.
The `<svelte:window>` element allows you to add event listeners to the `window` object without worrying about removing them when the component is destroyed, or checking for the existence of `window` when server-side rendering.
As with `<svelte:window>`, this element allows you to add listeners to events on `document.body`, such as `mouseenter` and `mouseleave` which don't fire on `window`.
This element makes it possible to insert elements into `document.head`. During server-side rendering, `head` content exposed separately to the main `html` content.
The `<svelte:options>` element provides a place to specify per-component compiler options, which are detailed in the [compiler section](docs#compile). The possible options are:
