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306 lines
8.7 KiB
306 lines
8.7 KiB
---
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title: Bindings
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---
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- how for dom elements
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- list of all bindings
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- how for components
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Most of the time a clear separation between data flowing down and events going up is worthwhile and results in more robust apps. But in some cases - especially when interacting with form elements - it's more ergonomic to declare a two way binding. Svelte provides many element bindings out of the box, and also allows component bindings.
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## bind:_property_ for elements
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```svelte
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<!--- copy: false --->
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bind:property={variable}
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```
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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.
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The simplest bindings reflect the value of a property, such as `input.value`.
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```svelte
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<input bind:value={name} />
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<textarea bind:value={text} />
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<input type="checkbox" bind:checked={yes} />
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```
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If the name matches the value, you can use a shorthand.
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```svelte
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<input bind:value />
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<!-- equivalent to
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<input bind:value={value} />
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-->
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```
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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`.
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```svelte
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<input type="number" bind:value={num} />
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<input type="range" bind:value={num} />
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```
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On `<input>` elements with `type="file"`, you can use `bind:files` to get the [`FileList` of selected files](https://developer.mozilla.org/en-US/docs/Web/API/FileList). It is readonly.
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```svelte
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<label for="avatar">Upload a picture:</label>
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<input accept="image/png, image/jpeg" bind:files id="avatar" name="avatar" type="file" />
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```
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If you're using `bind:` directives together with `on:` directives, the order that they're defined in affects the value of the bound variable when the event handler is called.
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```svelte
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<script>
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let value = 'Hello World';
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</script>
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<input
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on:input={() => console.log('Old value:', value)}
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bind:value
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on:input={() => console.log('New value:', value)}
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/>
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```
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Here we were binding to the value of a text input, which uses the `input` event. Bindings on other elements may use different events such as `change`.
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## Binding `<select>` value
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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).
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```svelte
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<select bind:value={selected}>
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<option value={a}>a</option>
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<option value={b}>b</option>
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<option value={c}>c</option>
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</select>
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```
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A `<select multiple>` element behaves similarly to a checkbox group. The bound variable is an array with an entry corresponding to the `value` property of each selected `<option>`.
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```svelte
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<select multiple bind:value={fillings}>
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<option value="Rice">Rice</option>
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<option value="Beans">Beans</option>
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<option value="Cheese">Cheese</option>
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<option value="Guac (extra)">Guac (extra)</option>
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</select>
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```
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When the value of an `<option>` matches its text content, the attribute can be omitted.
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```svelte
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<select multiple bind:value={fillings}>
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<option>Rice</option>
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<option>Beans</option>
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<option>Cheese</option>
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<option>Guac (extra)</option>
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</select>
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```
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Elements with the `contenteditable` attribute support the following bindings:
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- [`innerHTML`](https://developer.mozilla.org/en-US/docs/Web/API/Element/innerHTML)
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- [`innerText`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/innerText)
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- [`textContent`](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent)
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There are slight differences between each of these, read more about them [here](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent#Differences_from_innerText).
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<!-- for some reason puts the comment and html on same line -->
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<!-- prettier-ignore -->
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```svelte
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<div contenteditable="true" bind:innerHTML={html} />
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```
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`<details>` elements support binding to the `open` property.
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```svelte
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<details bind:open={isOpen}>
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<summary>Details</summary>
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<p>Something small enough to escape casual notice.</p>
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</details>
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```
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## Media element bindings
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Media elements (`<audio>` and `<video>`) have their own set of bindings — seven _readonly_ ones...
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- `duration` (readonly) — the total duration of the video, in seconds
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- `buffered` (readonly) — an array of `{start, end}` objects
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- `played` (readonly) — ditto
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- `seekable` (readonly) — ditto
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- `seeking` (readonly) — boolean
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- `ended` (readonly) — boolean
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- `readyState` (readonly) — number between (and including) 0 and 4
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...and five _two-way_ bindings:
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- `currentTime` — the current playback time in the video, in seconds
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- `playbackRate` — how fast or slow to play the video, where 1 is 'normal'
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- `paused` — this one should be self-explanatory
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- `volume` — a value between 0 and 1
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- `muted` — a boolean value indicating whether the player is muted
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Videos additionally have readonly `videoWidth` and `videoHeight` bindings.
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```svelte
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<video
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src={clip}
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bind:duration
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bind:buffered
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bind:played
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bind:seekable
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bind:seeking
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bind:ended
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bind:readyState
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bind:currentTime
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bind:playbackRate
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bind:paused
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bind:volume
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bind:muted
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bind:videoWidth
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bind:videoHeight
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/>
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```
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## Image element bindings
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Image elements (`<img>`) have two readonly bindings:
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- `naturalWidth` (readonly) — the original width of the image, available after the image has loaded
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- `naturalHeight` (readonly) — the original height of the image, available after the image has loaded
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```svelte
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<img
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bind:naturalWidth
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bind:naturalHeight
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></img>
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```
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## Block-level element bindings
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Block-level elements have 4 read-only bindings, measured using a technique similar to [this one](http://www.backalleycoder.com/2013/03/18/cross-browser-event-based-element-resize-detection/):
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- `clientWidth`
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- `clientHeight`
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- `offsetWidth`
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- `offsetHeight`
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```svelte
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<div bind:offsetWidth={width} bind:offsetHeight={height}>
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<Chart {width} {height} />
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</div>
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```
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## bind:group
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```svelte
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<!--- copy: false --->
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bind:group={variable}
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```
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Inputs that work together can use `bind:group`.
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```svelte
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<script>
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let tortilla = 'Plain';
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/** @type {Array<string>} */
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let fillings = [];
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</script>
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<!-- grouped radio inputs are mutually exclusive -->
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<input type="radio" bind:group={tortilla} value="Plain" />
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<input type="radio" bind:group={tortilla} value="Whole wheat" />
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<input type="radio" bind:group={tortilla} value="Spinach" />
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<!-- grouped checkbox inputs populate an array -->
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<input type="checkbox" bind:group={fillings} value="Rice" />
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<input type="checkbox" bind:group={fillings} value="Beans" />
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<input type="checkbox" bind:group={fillings} value="Cheese" />
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<input type="checkbox" bind:group={fillings} value="Guac (extra)" />
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```
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> `bind:group` only works if the inputs are in the same Svelte component.
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## bind:this
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```svelte
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<!--- copy: false --->
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bind:this={dom_node}
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```
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To get a reference to a DOM node, use `bind:this`.
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```svelte
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<script>
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import { onMount } from 'svelte';
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/** @type {HTMLCanvasElement} */
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let canvasElement;
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onMount(() => {
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const ctx = canvasElement.getContext('2d');
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drawStuff(ctx);
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});
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</script>
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<canvas bind:this={canvasElement} />
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```
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Components also support `bind:this`, allowing you to interact with component instances programmatically.
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```svelte
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<!--- App.svelte --->
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<ShoppingCart bind:this={cart} />
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<button onclick={() => cart.empty()}> Empty shopping cart </button>
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```
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```svelte
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<!--- ShoppingCart.svelte --->
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<script>
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// All instance exports are available on the instance object
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export function empty() {
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// ...
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}
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</script>
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```
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> Note that we can't do `{cart.empty}` since `cart` is `undefined` when the button is first rendered and throws an error.
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## bind:_property_ for components
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```svelte
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bind:property={variable}
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```
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You can bind to component props using the same syntax as for elements.
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```svelte
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<Keypad bind:value={pin} />
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```
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While Svelte props are reactive without binding, that reactivity only flows downward into the component by default. Using `bind:property` allows changes to the property from within the component to flow back up out of the component.
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To mark a property as bindable, use the `$bindable` rune:
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```svelte
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<script>
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let { readonlyProperty, bindableProperty = $bindable() } = $props();
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</script>
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```
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Declaring a property as bindable means it _can_ be used using `bind:`, not that it _must_ be used using `bind:`.
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Bindable properties can have a fallback value:
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```svelte
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<script>
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let { bindableProperty = $bindable('fallback value') } = $props();
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</script>
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```
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This fallback value _only_ applies when the property is _not_ bound. When the property is bound and a fallback value is present, the parent is expected to provide a value other than `undefined`, else a runtime error is thrown. This prevents hard-to-reason-about situations where it's unclear which value should apply.
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