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.
@ -340,3 +340,134 @@ In that case, the `<style>` tag will be inserted as-is into the DOM, no scoping
</style>
</style>
</div>
</div>
```
```
## Tags
A lowercase tag, like `<div>`, denotes a regular HTML element. A capitalised tag, such as `<Widget>` or `<Namespace.Widget>`, indicates a _component_.
```svelte
<script>
import Widget from './Widget.svelte';
</script>
<div>
<Widget/>
</div>
```
## Attributes and props
By default, attributes work exactly like their HTML counterparts.
```svelte
<divclass="foo">
<buttondisabled>can't touch this</button>
</div>
```
As in HTML, values may be unquoted.
```svelte
<inputtype="checkbox"/>
```
Attribute values can contain JavaScript expressions.
```svelte
<ahref="page/{p}">page {p}</a>
```
Or they can _be_ JavaScript expressions.
```svelte
<buttondisabled={!clickable}>...</button>
```
Boolean attributes are included on the element if their value is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy) and excluded if it's [falsy](https://developer.mozilla.org/en-US/docs/Glossary/Falsy).
All other attributes are included unless their value is [nullish](https://developer.mozilla.org/en-US/docs/Glossary/Nullish) (`null` or `undefined`).
```svelte
<inputrequired={false}placeholder="This input field is not required"/>
<divtitle={null}>This div has no title attribute</div>
```
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:
```svelte
<buttondisabled={number!==42}>...</button>
```
When the attribute name and value match (`name={name}`), they can be replaced with `{name}`.
```svelte
<!-- These are equivalent -->
<button{disabled}>...</button>
<button{disabled}>...</button>
```
By convention, values passed to components are referred to as _properties_ or _props_ rather than _attributes_, which are a feature of the DOM.
As with elements, `name={name}` can be replaced with the `{name}` shorthand.
```svelte
<Widgetfoo={bar}answer={42}text="hello"/>
```
_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.
```svelte
<Widget{...things}/>
```
_`$$props`_ references all props that are passed to a component, including ones that are not declared with `export`. It is not generally recommended, as it is difficult for Svelte to optimise. But it can be useful in rare cases – for example, when you don't know at compile time what props might be passed to a component.
```svelte
<Widget{...$$props}/>
```
_`$$restProps`_ contains only the props which are _not_ declared with `export`. It can be used to pass down other unknown attributes to an element in a component. It shares the same optimisation problems as _`$$props`_, and is likewise not recommended.
```svelte
<input{...$$restProps}/>
```
> The `value` attribute of an `input` element or its children `option` elements must not be set with spread attributes when using `bind:group` or `bind:checked`. Svelte needs to be able to see the element's `value` directly in the markup in these cases so that it can link it to the bound variable.
> Sometimes, the attribute order matters as Svelte sets attributes sequentially in JavaScript. For example, `<input type="range" min="0" max="1" value={0.5} step="0.1"/>`, Svelte will attempt to set the value to `1` (rounding up from 0.5 as the step by default is 1), and then set the step to `0.1`. To fix this, change it to `<input type="range" min="0" max="1" step="0.1" value={0.5}/>`.
> Another example is `<img src="..." loading="lazy" />`. Svelte will set the img `src` before making the img element `loading="lazy"`, which is probably too late. Change this to `<img loading="lazy" src="...">` to make the image lazily loaded.
## Text expressions
```svelte
{expression}
```
Text can also contain JavaScript expressions:
> If you're using a regular expression (`RegExp`) [literal notation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#literal_notation_and_constructor), you'll need to wrap it in parentheses.
```svelte
<h1>Hello {name}!</h1>
<p>{a} + {b} = {a + b}.</p>
<div>{/^[A-Za-z ]+$/.test(value) ? x : y}</div>
```
## Comments
You can use HTML comments inside components.
```svelte
<!-- this is a comment! --><h1>Hello world</h1>
```
Comments beginning with `svelte-ignore` disable warnings for the next block of markup. Usually, these are accessibility warnings; make sure that you're disabling them for a good reason.
A lowercase tag, like `<div>`, denotes a regular HTML element. A capitalised tag, such as `<Widget>` or `<Namespace.Widget>`, indicates a _component_.
```svelte
<script>
import Widget from './Widget.svelte';
</script>
<div>
<Widget/>
</div>
```
## Attributes and props
By default, attributes work exactly like their HTML counterparts.
```svelte
<divclass="foo">
<buttondisabled>can't touch this</button>
</div>
```
As in HTML, values may be unquoted.
```svelte
<inputtype="checkbox"/>
```
Attribute values can contain JavaScript expressions.
```svelte
<ahref="page/{p}">page {p}</a>
```
Or they can _be_ JavaScript expressions.
```svelte
<buttondisabled={!clickable}>...</button>
```
Boolean attributes are included on the element if their value is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy) and excluded if it's [falsy](https://developer.mozilla.org/en-US/docs/Glossary/Falsy).
All other attributes are included unless their value is [nullish](https://developer.mozilla.org/en-US/docs/Glossary/Nullish) (`null` or `undefined`).
```svelte
<inputrequired={false}placeholder="This input field is not required"/>
<divtitle={null}>This div has no title attribute</div>
```
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:
```svelte
<buttondisabled={number!==42}>...</button>
```
When the attribute name and value match (`name={name}`), they can be replaced with `{name}`.
```svelte
<!-- These are equivalent -->
<button{disabled}>...</button>
<button{disabled}>...</button>
```
By convention, values passed to components are referred to as _properties_ or _props_ rather than _attributes_, which are a feature of the DOM.
As with elements, `name={name}` can be replaced with the `{name}` shorthand.
```svelte
<Widgetfoo={bar}answer={42}text="hello"/>
```
_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.
```svelte
<Widget{...things}/>
```
_`$$props`_ references all props that are passed to a component, including ones that are not declared with `export`. It is not generally recommended, as it is difficult for Svelte to optimise. But it can be useful in rare cases – for example, when you don't know at compile time what props might be passed to a component.
```svelte
<Widget{...$$props}/>
```
_`$$restProps`_ contains only the props which are _not_ declared with `export`. It can be used to pass down other unknown attributes to an element in a component. It shares the same optimisation problems as _`$$props`_, and is likewise not recommended.
```svelte
<input{...$$restProps}/>
```
> The `value` attribute of an `input` element or its children `option` elements must not be set with spread attributes when using `bind:group` or `bind:checked`. Svelte needs to be able to see the element's `value` directly in the markup in these cases so that it can link it to the bound variable.
> Sometimes, the attribute order matters as Svelte sets attributes sequentially in JavaScript. For example, `<input type="range" min="0" max="1" value={0.5} step="0.1"/>`, Svelte will attempt to set the value to `1` (rounding up from 0.5 as the step by default is 1), and then set the step to `0.1`. To fix this, change it to `<input type="range" min="0" max="1" step="0.1" value={0.5}/>`.
> Another example is `<img src="..." loading="lazy" />`. Svelte will set the img `src` before making the img element `loading="lazy"`, which is probably too late. Change this to `<img loading="lazy" src="...">` to make the image lazily loaded.
## Text expressions
```svelte
{expression}
```
Text can also contain JavaScript expressions:
> If you're using a regular expression (`RegExp`) [literal notation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#literal_notation_and_constructor), you'll need to wrap it in parentheses.
```svelte
<h1>Hello {name}!</h1>
<p>{a} + {b} = {a + b}.</p>
<div>{/^[A-Za-z ]+$/.test(value) ? x : y}</div>
```
## Comments
You can use HTML comments inside components.
```svelte
<!-- this is a comment! --><h1>Hello world</h1>
```
Comments beginning with `svelte-ignore` disable warnings for the next block of markup. Usually, these are accessibility warnings; make sure that you're disabling them for a good reason.
Svelte components can also be compiled to custom elements (aka web components) using the `customElement: true` compiler option. You should specify a tag name for the component using the `<svelte:options>` [element](/docs/special-elements#svelte-options).
Svelte components can also be compiled to custom elements (aka web components) using the `customElement: true` compiler option. You should specify a tag name for the component using the `<svelte:options>` [element](/docs/special-elements#svelte-options).
Puru Vijay
3 years ago