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svelte

The svelte package exposes lifecycle functions and the context API.

onMount

onMount(callback: () => void)
onMount(callback: () => () => void)

The onMount function schedules a callback to run as soon as the component has been mounted to the DOM. It must be called during the component's initialisation (but doesn't need to live inside the component; it can be called from an external module).

onMount does not run inside a server-side component.

<script>
  import { onMount } from 'svelte';

  onMount(() => {
    console.log('the component has mounted');
  });
</script>

If a function is returned from onMount, it will be called when the component is unmounted.

<script>
  import { onMount } from 'svelte';

  onMount(() => {
    const interval = setInterval(() => {
      console.log('beep');
    }, 1000);

    return () => clearInterval(interval);
  });
</script>

This behaviour will only work when the function passed to onMount synchronously returns a value. async functions always return a Promise, and as such cannot synchronously return a function.

beforeUpdate

beforeUpdate(callback: () => void)

Schedules a callback to run immediately before the component is updated after any state change.

The first time the callback runs will be before the initial onMount

<script>
  import { beforeUpdate } from 'svelte';

  beforeUpdate(() => {
    console.log('the component is about to update');
  });
</script>

afterUpdate

afterUpdate(callback: () => void)

Schedules a callback to run immediately after the component has been updated.

The first time the callback runs will be after the initial onMount

<script>
  import { afterUpdate } from 'svelte';

  afterUpdate(() => {
    console.log('the component just updated');
  });
</script>

onDestroy

onDestroy(callback: () => void)

Schedules a callback to run immediately before the component is unmounted.

Out of onMount, beforeUpdate, afterUpdate and onDestroy, this is the only one that runs inside a server-side component.

<script>
  import { onDestroy } from 'svelte';

  onDestroy(() => {
    console.log('the component is being destroyed');
  });
</script>

tick

promise: Promise = tick()

Returns a promise that resolves once any pending state changes have been applied, or in the next microtask if there are none.

<script>
  import { beforeUpdate, tick } from 'svelte';

  beforeUpdate(async () => {
    console.log('the component is about to update');
    await tick();
    console.log('the component just updated');
  });
</script>

setContext

setContext(key: any, context: any)

Associates an arbitrary context object with the current component and the specified key and returns that object. The context is then available to children of the component (including slotted content) with getContext.

Like lifecycle functions, this must be called during component initialisation.

<script>
  import { setContext } from 'svelte';

  setContext('answer', 42);
</script>

Context is not inherently reactive. If you need reactive values in context then you can pass a store into context, which will be reactive.

getContext

context: any = getContext(key: any)

Retrieves the context that belongs to the closest parent component with the specified key. Must be called during component initialisation.

<script>
  import { getContext } from 'svelte';

  const answer = getContext('answer');
</script>

hasContext

hasContext: boolean = hasContext(key: any)

Checks whether a given key has been set in the context of a parent component. Must be called during component initialisation.

<script>
  import { hasContext } from 'svelte';

  if (hasContext('answer')) {
    // do something
  }
</script>

getAllContexts

contexts: Map<any, any> = getAllContexts()

Retrieves the whole context map that belongs to the closest parent component. Must be called during component initialisation. Useful, for example, if you programmatically create a component and want to pass the existing context to it.

<script>
  import { getAllContexts } from 'svelte';

  const contexts = getAllContexts();
</script>

createEventDispatcher

dispatch: ((name: string, detail?: any, options?: DispatchOptions) => boolean) = createEventDispatcher();

Creates an event dispatcher that can be used to dispatch component events. Event dispatchers are functions that can take two arguments: name and detail.

Component events created with createEventDispatcher create a CustomEvent. These events do not bubble. The detail argument corresponds to the CustomEvent.detail property and can contain any type of data.

<script>
  import { createEventDispatcher } from 'svelte';

  const dispatch = createEventDispatcher();
</script>

<button on:click="{() => dispatch('notify', 'detail value')}">Fire Event</button>

Events dispatched from child components can be listened to in their parent. Any data provided when the event was dispatched is available on the detail property of the event object.

<script>
  function callbackFunction(event) {
    console.log(`Notify fired! Detail: ${event.detail}`)
  }
</script>

<Child on:notify="{callbackFunction}"/>

Events can be cancelable by passing a third parameter to the dispatch function. The function returns false if the event is cancelled with event.preventDefault(), otherwise it returns true.

<script>
  import { createEventDispatcher } from 'svelte';

  const dispatch = createEventDispatcher();

  function notify() {
    const shouldContinue = dispatch('notify', 'detail value', { cancelable: true });
    if (shouldContinue) {
      // no one called preventDefault
    } else {
      // a listener called preventDefault
    }
  }
</script>

svelte/store

The svelte/store module exports functions for creating readable, writable and derived stores.

Keep in mind that you don't have to use these functions to enjoy the reactive $store syntax in your components. Any object that correctly implements .subscribe, unsubscribe, and (optionally) .set is a valid store, and will work both with the special syntax, and with Svelte's built-in derived stores.

This makes it possible to wrap almost any other reactive state handling library for use in Svelte. Read more about the store contract to see what a correct implementation looks like.

writable

store = writable(value?: any)
store = writable(value?: any, start?: (set: (value: any) => void) => () => void)

Function that creates a store which has values that can be set from 'outside' components. It gets created as an object with additional set and update methods.

set is a method that takes one argument which is the value to be set. The store value gets set to the value of the argument if the store value is not already equal to it.

update is a method that takes one argument which is a callback. The callback takes the existing store value as its argument and returns the new value to be set to the store.

import { writable } from 'svelte/store';

const count = writable(0);

count.subscribe(value => {
  console.log(value);
}); // logs '0'

count.set(1); // logs '1'

count.update(n => n + 1); // logs '2'

If a function is passed as the second argument, it will be called when the number of subscribers goes from zero to one (but not from one to two, etc). That function will be passed a set function which changes the value of the store. It must return a stop function that is called when the subscriber count goes from one to zero.

import { writable } from 'svelte/store';

const count = writable(0, () => {
  console.log('got a subscriber');
  return () => console.log('no more subscribers');
});

count.set(1); // does nothing

const unsubscribe = count.subscribe(value => {
  console.log(value);
}); // logs 'got a subscriber', then '1'

unsubscribe(); // logs 'no more subscribers'

Note that the value of a writable is lost when it is destroyed, for example when the page is refreshed. However, you can write your own logic to sync the value to for example the localStorage.

readable

store = readable(value?: any, start?: (set: (value: any) => void) => () => void)

Creates a store whose value cannot be set from 'outside', the first argument is the store's initial value, and the second argument to readable is the same as the second argument to writable.

import { readable } from 'svelte/store';

const time = readable(null, set => {
  set(new Date());

  const interval = setInterval(() => {
    set(new Date());
  }, 1000);

  return () => clearInterval(interval);
});

derived

store = derived(a, callback: (a: any) => any)
store = derived(a, callback: (a: any, set: (value: any) => void) => void | () => void, initial_value: any)
store = derived([a, ...b], callback: ([a: any, ...b: any[]]) => any)
store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) => void) => void | () => void, initial_value: any)

Derives a store from one or more other stores. The callback runs initially when the first subscriber subscribes and then whenever the store dependencies change.

In the simplest version, derived takes a single store, and the callback returns a derived value.

import { derived } from 'svelte/store';

const doubled = derived(a, $a => $a * 2);

The callback can set a value asynchronously by accepting a second argument, set, and calling it when appropriate.

In this case, you can also pass a third argument to derived — the initial value of the derived store before set is first called.

import { derived } from 'svelte/store';

const delayed = derived(a, ($a, set) => {
  setTimeout(() => set($a), 1000);
}, 'one moment...');

If you return a function from the callback, it will be called when a) the callback runs again, or b) the last subscriber unsubscribes.

import { derived } from 'svelte/store';

const tick = derived(frequency, ($frequency, set) => {
  const interval = setInterval(() => {
    set(Date.now());
  }, 1000 / $frequency);

  return () => {
    clearInterval(interval);
  };
}, 'one moment...');

In both cases, an array of arguments can be passed as the first argument instead of a single store.

import { derived } from 'svelte/store';

const summed = derived([a, b], ([$a, $b]) => $a + $b);

const delayed = derived([a, b], ([$a, $b], set) => {
  setTimeout(() => set($a + $b), 1000);
});

get

value: any = get(store)

Generally, you should read the value of a store by subscribing to it and using the value as it changes over time. Occasionally, you may need to retrieve the value of a store to which you're not subscribed. get allows you to do so.

This works by creating a subscription, reading the value, then unsubscribing. It's therefore not recommended in hot code paths.

import { get } from 'svelte/store';

const value = get(store);

readonly

readableStore = readonly(writableStore);

This simple helper function makes a store readonly. You can still subscribe to the changes from the original one using this new readable store.

import { readonly } from 'svelte/store';

const writableStore = writable(1);
const readableStore = readonly(writableStore);

readableStore.subscribe(console.log);

writableStore.set(2); // console: 2
readableStore.set(2); // ERROR

svelte/motion

The svelte/motion module exports two functions, tweened and spring, for creating writable stores whose values change over time after set and update, rather than immediately.

tweened

store = tweened(value: any, options)

Tweened stores update their values over a fixed duration. The following options are available:

  • delay (number, default 0) — milliseconds before starting
  • duration (number | function, default 400) — milliseconds the tween lasts
  • easing (function, default t => t) — an easing function
  • interpolate (function) — see below

store.set and store.update can accept a second options argument that will override the options passed in upon instantiation.

Both functions return a Promise that resolves when the tween completes. If the tween is interrupted, the promise will never resolve.

Out of the box, Svelte will interpolate between two numbers, two arrays or two objects (as long as the arrays and objects are the same 'shape', and their 'leaf' properties are also numbers).

<script>
  import { tweened } from 'svelte/motion';
  import { cubicOut } from 'svelte/easing';

  const size = tweened(1, {
    duration: 300,
    easing: cubicOut
  });

  function handleClick() {
    // this is equivalent to size.update(n => n + 1)
    $size += 1;
  }
</script>

<button
  on:click={handleClick}
  style="transform: scale({$size}); transform-origin: 0 0"
>embiggen</button>

If the initial value is undefined or null, the first value change will take effect immediately. This is useful when you have tweened values that are based on props, and don't want any motion when the component first renders.

const size = tweened(undefined, {
  duration: 300,
  easing: cubicOut
});

$: $size = big ? 100 : 10;

The interpolate option allows you to tween between any arbitrary values. It must be an (a, b) => t => value function, where a is the starting value, b is the target value, t is a number between 0 and 1, and value is the result. For example, we can use the d3-interpolate package to smoothly interpolate between two colours.

<script>
  import { interpolateLab } from 'd3-interpolate';
  import { tweened } from 'svelte/motion';

  const colors = [
    'rgb(255, 62, 0)',
    'rgb(64, 179, 255)',
    'rgb(103, 103, 120)'
  ];

  const color = tweened(colors[0], {
    duration: 800,
    interpolate: interpolateLab
  });
</script>

{#each colors as c}
  <button
    style="background-color: {c}; color: white; border: none;"
    on:click="{e => color.set(c)}"
  >{c}</button>
{/each}

<h1 style="color: {$color}">{$color}</h1>

spring

store = spring(value: any, options)

A spring store gradually changes to its target value based on its stiffness and damping parameters. Whereas tweened stores change their values over a fixed duration, spring stores change over a duration that is determined by their existing velocity, allowing for more natural-seeming motion in many situations. The following options are available:

  • stiffness (number, default 0.15) — a value between 0 and 1 where higher means a 'tighter' spring
  • damping (number, default 0.8) — a value between 0 and 1 where lower means a 'springier' spring
  • precision (number, default 0.01) — determines the threshold at which the spring is considered to have 'settled', where lower means more precise

All of the options above can be changed while the spring is in motion, and will take immediate effect.

const size = spring(100);
size.stiffness = 0.3;
size.damping = 0.4;
size.precision = 0.005;

As with tweened stores, set and update return a Promise that resolves if the spring settles.

Both set and update can take a second argument — an object with hard or soft properties. { hard: true } sets the target value immediately; { soft: n } preserves existing momentum for n seconds before settling. { soft: true } is equivalent to { soft: 0.5 }.

const coords = spring({ x: 50, y: 50 });
// updates the value immediately
coords.set({ x: 100, y: 200 }, { hard: true });
// preserves existing momentum for 1s
coords.update(
  (target_coords, coords) => {
    return { x: target_coords.x, y: coords.y };
  },
  { soft: 1 }
);

See a full example on the spring tutorial.

<script>
  import { spring } from 'svelte/motion';

  const coords = spring({ x: 50, y: 50 }, {
    stiffness: 0.1,
    damping: 0.25
  });
</script>

If the initial value is undefined or null, the first value change will take effect immediately, just as with tweened values (see above).

const size = spring();
$: $size = big ? 100 : 10;

svelte/transition

The svelte/transition module exports seven functions: fade, blur, fly, slide, scale, draw and crossfade. They are for use with Svelte transitions.

fade

transition:fade={params}
in:fade={params}
out:fade={params}

Animates the opacity of an element from 0 to the current opacity for in transitions and from the current opacity to 0 for out transitions.

fade accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • duration (number, default 400) — milliseconds the transition lasts
  • easing (function, default linear) — an easing function

You can see the fade transition in action in the transition tutorial.

<script>
  import { fade } from 'svelte/transition';
</script>

{#if condition}
  <div transition:fade="{{delay: 250, duration: 300}}">
    fades in and out
  </div>
{/if}

blur

transition:blur={params}
in:blur={params}
out:blur={params}

Animates a blur filter alongside an element's opacity.

blur accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • duration (number, default 400) — milliseconds the transition lasts
  • easing (function, default cubicInOut) — an easing function
  • opacity (number, default 0) - the opacity value to animate out to and in from
  • amount (number | string, default 5) - the size of the blur. Supports css units (for example: "4rem"). The default unit is px
<script>
  import { blur } from 'svelte/transition';
</script>

{#if condition}
  <div transition:blur="{{amount: 10}}">
    fades in and out
  </div>
{/if}

fly

transition:fly={params}
in:fly={params}
out:fly={params}

Animates the x and y positions and the opacity of an element. in transitions animate from the provided values, passed as parameters to the element's default values. out transitions animate from the element's default values to the provided values.

fly accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • duration (number, default 400) — milliseconds the transition lasts
  • easing (function, default cubicOut) — an easing function
  • x (number | string, default 0) - the x offset to animate out to and in from
  • y (number | string, default 0) - the y offset to animate out to and in from
  • opacity (number, default 0) - the opacity value to animate out to and in from

x and y use px by default but support css units, for example x: '100vw' or y: '50%'. You can see the fly transition in action in the transition tutorial.

<script>
  import { fly } from 'svelte/transition';
  import { quintOut } from 'svelte/easing';
</script>

{#if condition}
  <div transition:fly="{{delay: 250, duration: 300, x: 100, y: 500, opacity: 0.5, easing: quintOut}}">
    flies in and out
  </div>
{/if}

slide

transition:slide={params}
in:slide={params}
out:slide={params}

Slides an element in and out.

slide accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • duration (number, default 400) — milliseconds the transition lasts
  • easing (function, default cubicOut) — an easing function
  • axis (x | y, default y) — the axis of motion along which the transition occurs
<script>
  import { slide } from 'svelte/transition';
  import { quintOut } from 'svelte/easing';
</script>

{#if condition}
  <div transition:slide="{{delay: 250, duration: 300, easing: quintOut, axis: 'x'}}">
    slides in and out horizontally
  </div>
{/if}

scale

transition:scale={params}
in:scale={params}
out:scale={params}

Animates the opacity and scale of an element. in transitions animate from an element's current (default) values to the provided values, passed as parameters. out transitions animate from the provided values to an element's default values.

scale accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • duration (number, default 400) — milliseconds the transition lasts
  • easing (function, default cubicOut) — an easing function
  • start (number, default 0) - the scale value to animate out to and in from
  • opacity (number, default 0) - the opacity value to animate out to and in from
<script>
  import { scale } from 'svelte/transition';
  import { quintOut } from 'svelte/easing';
</script>

{#if condition}
  <div transition:scale="{{duration: 500, delay: 500, opacity: 0.5, start: 0.5, easing: quintOut}}">
    scales in and out
  </div>
{/if}

draw

transition:draw={params}
in:draw={params}
out:draw={params}

Animates the stroke of an SVG element, like a snake in a tube. in transitions begin with the path invisible and draw the path to the screen over time. out transitions start in a visible state and gradually erase the path. draw only works with elements that have a getTotalLength method, like <path> and <polyline>.

draw accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • speed (number, default undefined) - the speed of the animation, see below.
  • duration (number | function, default 800) — milliseconds the transition lasts
  • easing (function, default cubicInOut) — an easing function

The speed parameter is a means of setting the duration of the transition relative to the path's length. It is a modifier that is applied to the length of the path: duration = length / speed. A path that is 1000 pixels with a speed of 1 will have a duration of 1000ms, setting the speed to 0.5 will double that duration and setting it to 2 will halve it.

<script>
  import { draw } from 'svelte/transition';
  import { quintOut } from 'svelte/easing';
</script>

<svg viewBox="0 0 5 5" xmlns="http://www.w3.org/2000/svg">
  {#if condition}
    <path transition:draw="{{duration: 5000, delay: 500, easing: quintOut}}"
          d="M2 1 h1 v1 h1 v1 h-1 v1 h-1 v-1 h-1 v-1 h1 z"
          fill="none"
          stroke="cornflowerblue"
          stroke-width="0.1px"
          stroke-linejoin="round"
    />
  {/if}
</svg>

crossfade

The crossfade function creates a pair of transitions called send and receive. When an element is 'sent', it looks for a corresponding element being 'received', and generates a transition that transforms the element to its counterpart's position and fades it out. When an element is 'received', the reverse happens. If there is no counterpart, the fallback transition is used.

crossfade accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • duration (number | function, default 800) — milliseconds the transition lasts
  • easing (function, default cubicOut) — an easing function
  • fallback (function) — A fallback transition to use for send when there is no matching element being received, and for receive when there is no element being sent.
<script>
  import { crossfade } from 'svelte/transition';
  import { quintOut } from 'svelte/easing';

  const [send, receive] = crossfade({
    duration:1500,
    easing: quintOut
  });
</script>

{#if condition}
  <h1 in:send={{key}} out:receive={{key}}>BIG ELEM</h1>
{:else}
  <small in:send={{key}} out:receive={{key}}>small elem</small>
{/if}

svelte/animate

The svelte/animate module exports one function for use with Svelte animations.

flip

animate:flip={params}

The flip function calculates the start and end position of an element and animates between them, translating the x and y values. flip stands for First, Last, Invert, Play.

flip accepts the following parameters:

  • delay (number, default 0) — milliseconds before starting
  • duration (number | function, default d => Math.sqrt(d) * 120) — see below
  • easing (function, default cubicOut) — an easing function

duration can be provided as either:

  • a number, in milliseconds.
  • a function, distance: number => duration: number, receiving the distance the element will travel in pixels and returning the duration in milliseconds. This allows you to assign a duration that is relative to the distance travelled by each element.

You can see a full example on the animations tutorial

<script>
  import { flip } from 'svelte/animate';
  import { quintOut } from 'svelte/easing';

  let list = [1, 2, 3];
</script>

{#each list as n (n)}
  <div animate:flip="{{delay: 250, duration: 250, easing: quintOut}}">
    {n}
  </div>
{/each}

svelte/easing

Easing functions specify the rate of change over time and are useful when working with Svelte's built-in transitions and animations as well as the tweened and spring utilities. svelte/easing contains 31 named exports, a linear ease and 3 variants of 10 different easing functions: in, out and inOut.

You can explore the various eases using the ease visualiser in the examples section.

ease in out inOut
back backIn backOut backInOut
bounce bounceIn bounceOut bounceInOut
circ circIn circOut circInOut
cubic cubicIn cubicOut cubicInOut
elastic elasticIn elasticOut elasticInOut
expo expoIn expoOut expoInOut
quad quadIn quadOut quadInOut
quart quartIn quartOut quartInOut
quint quintIn quintOut quintInOut
sine sineIn sineOut sineInOut

svelte/register

To render Svelte components in Node.js without bundling, use require('svelte/register'). After that, you can use require to include any .svelte file.

require('svelte/register');

const App = require('./App.svelte').default;

...

const { html, css, head } = App.render({ answer: 42 });

The .default is necessary because we're converting from native JavaScript modules to the CommonJS modules recognised by Node. Note that if your component imports JavaScript modules, they will fail to load in Node and you will need to use a bundler instead.

To set compile options, or to use a custom file extension, call the register hook as a function:

require('svelte/register')({
  extensions: ['.customextension'], // defaults to ['.html', '.svelte']
  preserveComments: true
});

Client-side component API

Creating a component

const component = new Component(options)

A client-side component — that is, a component compiled with generate: 'dom' (or the generate option left unspecified) is a JavaScript class.

import App from './App.svelte';

const app = new App({
  target: document.body,
  props: {
    // assuming App.svelte contains something like
    // `export let answer`:
    answer: 42
  }
});

The following initialisation options can be provided:

option default description
target none An HTMLElement or ShadowRoot to render to. This option is required
anchor null A child of target to render the component immediately before
props {} An object of properties to supply to the component
context new Map() A Map of root-level context key-value pairs to supply to the component
hydrate false See below
intro false If true, will play transitions on initial render, rather than waiting for subsequent state changes

Existing children of target are left where they are.

The hydrate option instructs Svelte to upgrade existing DOM (usually from server-side rendering) rather than creating new elements. It will only work if the component was compiled with the hydratable: true option. Hydration of <head> elements only works properly if the server-side rendering code was also compiled with hydratable: true, which adds a marker to each element in the <head> so that the component knows which elements it's responsible for removing during hydration.

Whereas children of target are normally left alone, hydrate: true will cause any children to be removed. For that reason, the anchor option cannot be used alongside hydrate: true.

The existing DOM doesn't need to match the component — Svelte will 'repair' the DOM as it goes.

import App from './App.svelte';

const app = new App({
  target: document.querySelector('#server-rendered-html'),
  hydrate: true
});

$set

component.$set(props)

Programmatically sets props on an instance. component.$set({ x: 1 }) is equivalent to x = 1 inside the component's <script> block.

Calling this method schedules an update for the next microtask — the DOM is not updated synchronously.

component.$set({ answer: 42 });

$on

component.$on(event, callback)

Causes the callback function to be called whenever the component dispatches an event.

A function is returned that will remove the event listener when called.

const off = app.$on('selected', event => {
  console.log(event.detail.selection);
});

off();

$destroy

component.$destroy()

Removes a component from the DOM and triggers any onDestroy handlers.

Component props

component.prop
component.prop = value

If a component is compiled with accessors: true, each instance will have getters and setters corresponding to each of the component's props. Setting a value will cause a synchronous update, rather than the default async update caused by component.$set(...).

By default, accessors is false, unless you're compiling as a custom element.

console.log(app.count);
app.count += 1;

Custom element API

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.

<svelte:options tag="my-element" />

<script>
  export let name = 'world';
</script>

<h1>Hello {name}!</h1>
<slot></slot>

Alternatively, use tag={null} to indicate that the consumer of the custom element should name it.

import MyElement from './MyElement.svelte';

customElements.define('my-element', MyElement);

Once a custom element has been defined, it can be used as a regular DOM element:

document.body.innerHTML = `
  <my-element>
    <p>This is some slotted content</p>
  </my-element>
`;

By default, custom elements are compiled with accessors: true, which means that any props are exposed as properties of the DOM element (as well as being readable/writable as attributes, where possible).

To prevent this, add accessors={false} to <svelte:options>.

const el = document.querySelector('my-element');

// get the current value of the 'name' prop
console.log(el.name);

// set a new value, updating the shadow DOM
el.name = 'everybody';

Custom elements can be a useful way to package components for consumption in a non-Svelte app, as they will work with vanilla HTML and JavaScript as well as most frameworks. There are, however, some important differences to be aware of:

  • Styles are encapsulated, rather than merely scoped. This means that any non-component styles (such as you might have in a global.css file) will not apply to the custom element, including styles with the :global(...) modifier
  • Instead of being extracted out as a separate .css file, styles are inlined into the component as a JavaScript string
  • Custom elements are not generally suitable for server-side rendering, as the shadow DOM is invisible until JavaScript loads
  • In Svelte, slotted content renders lazily. In the DOM, it renders eagerly. In other words, it will always be created even if the component's <slot> element is inside an {#if ...} block. Similarly, including a <slot> in an {#each ...} block will not cause the slotted content to be rendered multiple times
  • The let: directive has no effect
  • Polyfills are required to support older browsers

Server-side component API

const result = Component.render(...)

Unlike client-side components, server-side components don't have a lifespan after you render them — their whole job is to create some HTML and CSS. For that reason, the API is somewhat different.

A server-side component exposes a render method that can be called with optional props. It returns an object with head, html, and css properties, where head contains the contents of any <svelte:head> elements encountered.

You can import a Svelte component directly into Node using svelte/register.

require('svelte/register');

const App = require('./App.svelte').default;

const { head, html, css } = App.render({
  answer: 42
});

The .render() method accepts the following parameters:

parameter default description
props {} An object of properties to supply to the component
options {} An object of options

The options object takes in the following options:

option default description
context new Map() A Map of root-level context key-value pairs to supply to the component 
const { head, html, css } = App.render(
  // props
  { answer: 42 },
  // options
  {
    context: new Map([['context-key', 'context-value']])
  }
);