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Using Svelte you can create a two-way binding between data and the UI.

Many other Web frameworks can provide two-way bindings, it’s a very common pattern.

They are especially useful with forms.

bind:value

Let’s start with the most common form of binding you’ll often use, which you can apply using bind:value. You take a variable from the component state, and you bind it to a form field:

<script>
let name = ''
</script>

<input bind:value={name}>

Now if name changes the input field will update its value. And the opposite is true, as well: if the form is updated by the user, the name variable value changes.

Just be aware that the variable must be defined using let/var and not const, otherwise it can’t be updated by Svelte, as const defines a variable with a value that can’t be reassigned.

bind:value works on all flavors of input fields (type="number", type="email" and so on), but it also works for other kind of fields, like textarea and select (more on select later).

Checkboxes and radio buttons

Checkboxes and radio inputs (input elements with type="checkbox" or type="radio") allow these 3 bindings:

  • bind:checked
  • bind:group
  • bind:indeterminate

bind:checked allows us to bind a value to the checked state of the element:

<script>
let isChecked
</script>

<input type=checkbox bind:checked={isChecked}>

bind:group is handy with checkboxes and radio inputs because those are very often used in groups. Using bind:group you can associate a JavaScript array to a list of checkboxes and have it populated based on the choices made by the user.

Here’s an example. The goodDogs array populates based on the checkboxes I tick:

<script>
let goodDogs = []
let dogs = ['Roger', 'Syd']
</script>

<h2>
  Who's a good dog?
</h2>

<ul>
  {#each dogs as dog}
    <li>{dog} <input type=checkbox bind:group={goodDogs} value={dog}></li>
  {/each}
</ul>

<h2>
  Good dogs according to me:
</h2>

<ul>
  {#each goodDogs as dog}
    <li>{dog}</li>
  {/each}
</ul>

See the example on https://svelte.dev/repl/059c1b5edffc4b058ad36301dd7a1a58

bind:indeterminate allows us to bind to the indeterminate state of an element (if you want to learn more head to https://css-tricks.com/indeterminate-checkboxes/)

Select fields

bind:value also works for the select form field to get the selected value automatically assigned to the value of a variable:

<script>
let selected
</script>

<select bind:value={selected}>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
</select>

{selected}

The cool thing is that if you generate options dynamically from an array of objects, the selected option is now an object not a string:

<script>
let selected

const goodDogs = [
  { name: 'Roger' },
  { name: 'Syd' }
]
</script>

<h2>List of possible good dogs:</h2>
<select bind:value={selected}>
  {#each goodDogs as goodDog}
    <option value={goodDog}>{goodDog.name}</option>
  {/each}
</select>

{#if selected}
<h2>
  Good dog selected: {selected.name}
</h2>
{/if}

See example: https://svelte.dev/repl/7e06f9b7becd4c57880db5ed184ea0f3

select also allows the multiple attribute:

<script>
let selected = []

const goodDogs = [
  { name: 'Roger' },
  { name: 'Syd' }
]
</script>

<h2>List of possible good dogs:</h2>
<select multiple bind:value={selected}>
  {#each goodDogs as goodDog}
    <option value={goodDog}>{goodDog.name}</option>
  {/each}
</select>

{#if selected.length}
<h2>Good dog selected:</h2>
<ul>
  {#each selected as dog}
    <li>{dog.name}</li>
  {/each}
</ul>
{/if}

See example: https://svelte.dev/repl/b003248e87f04919a2f9fed63dbdab8c

Other bindings

Depending on the HTML tag you are working on, you can apply different kinds of bindings.

bind:files is a binding valid on type="file" input elements to bind the list of selected files.

The details HTML element allows the use of bind:open to bind its open/close value.

The audio and video media HTML tags allow you to bind several of their properties: currentTime, duration, paused, buffered, seekable, played, volume, playbackRate.

textContent and innerHTML can be bound on contenteditable fields.

All things very useful for those specific HTML elements.

Read-only bindings

offsetWidth, offsetHeight, clientWidth, clientHeight can be bound read only on any block level HTML element, excluding void tags (like br) and elements that are set to be inline (display: inline).

Get a reference to the HTML element in JavaScript

bind:this is a special kind of binding that allows you to get a reference to an HTML element and bind it to a JavaScript variable:

<script>
let myInputField
</script>

<input bind:this={myInputField} />

This is handy when you need to apply logic to elements after you mount them, for example, using the onMount() lifecycle event callback.

Binding components props

Using bind: you can bind a value to any prop that a component exposes.

Say you have a Car.svelte component:

<script>
export let inMovement = false
</script>

<button on:click={() => inMovement = true }>Start car</button>

You can import the component and bind the inMovement prop:

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

  let carInMovement;
</script>

<Car bind:inMovement={carInMovement} />

{carInMovement}

This can allow for interesting scenarios.

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