Basic markup

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Tagspermalink

A lowercase tag, like <div>, denotes a regular HTML element. A capitalised tag, such as <Widget> or <Namespace.Widget>, indicates a component.

<script>
  import Widget from './Widget.svelte';
</script>

<div>
  <Widget />
</div>

Attributes and propspermalink

By default, attributes work exactly like their HTML counterparts.

<div class="foo">
  <button disabled>can't touch this</button>
</div>

As in HTML, values may be unquoted.

<input type=checkbox />

Attribute values can contain JavaScript expressions.

<a href="page/{p}">page {p}</a>

Or they can be JavaScript expressions.

<button disabled={!clickable}>...</button>

Boolean attributes are included on the element if their value is truthy and excluded if it's falsy.

All other attributes are included unless their value is nullish (null or undefined).

<input required={false} placeholder="This input field is not required" />
<div title={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:

<button disabled="{number !== 42}">...</button>

When the attribute name and value match (name={name}), they can be replaced with {name}.

<button {disabled}>...</button>
<!-- equivalent to
<button disabled={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.

<Widget foo={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.

<Widget {...things} />

$$props references all props that are passed to a component, including ones that are not declared with export. Using $$props will not perform as well as references to a specific prop because changes to any prop will cause Svelte to recheck all usages of $$props. But it can be useful in some cases – for example, when you don't know at compile time what props might be passed to a component.

<Widget {...$$props} />

$$restProps contains only the props which are not declared with export. It can be used to pass down other unknown attributes to an element in a component. It shares the same performance characteristics compared to specific property access as $$props.

<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 expressionspermalink

{expression}

Text can also contain JavaScript expressions:

If you're using a regular expression (RegExp) literal notation, you'll need to wrap it in parentheses.

<h1>Hello {name}!</h1>
<p>{a} + {b} = {a + b}.</p>

<div>{(/^[A-Za-z ]+$/).test(value) ? x : y}</div>

Commentspermalink

You can use HTML comments inside components.

<!-- 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-ignore a11y-autofocus -->
<input bind:value={name} autofocus />