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---
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title: Advanced
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---
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### Keyed each blocks
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Associating a *key* with a block allows Svelte to be smarter about how it adds and removes items to and from a list. To do so, add an `(expression)` that uniquely identifies each member of the list:
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```html
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<!-- { repl: false } -->
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{#each people as person (person.name)}
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<div>{person.name}</div>
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{/each}
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```
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It's easier to show the effect of this than to describe it. Open the following example in the REPL:
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```html
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<!-- { title: 'Keyed each blocks' } -->
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<button on:click="{update}">update</button>
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<section>
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<h2>Keyed</h2>
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{#each people as person (person.name)}
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<div transition:slide>{person.name}</div>
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{/each}
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</section>
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<section>
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<h2>Non-keyed</h2>
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{#each people as person}
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<div transition:slide>{person.name}</div>
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{/each}
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</section>
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<style>
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button {
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display: block;
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}
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section {
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width: 10em;
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float: left;
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}
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</style>
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<script>
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import { slide } from 'svelte/transition';
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const names = ['Alice', 'Barry', 'Cecilia', 'Douglas', 'Eleanor', 'Felix', 'Grace', 'Horatio', 'Isabelle'];
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function random() {
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return names
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.filter(() => Math.random() < 0.5)
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.map(name => ({ name }));
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}
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let people = random();
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function update() {
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people = random();
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}
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</script>
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```
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### Hydration
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If you're using [server-side rendering](guide#server-side-rendering), it's likely that you'll need to create a client-side version of your app *on top of* the server-rendered version. A naive way to do that would involve removing all the existing DOM and rendering the client-side app in its place:
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```js
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import App from './App.html';
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const target = document.querySelector('#element-with-server-rendered-html');
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// avoid doing this!
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target.innerHTML = '';
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new App({
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target
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});
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```
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Ideally, we want to reuse the existing DOM instead. This process is called *hydration*. First, we need to tell the compiler to include the code necessary for hydration to work by passing the `hydratable: true` option:
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```js
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const { js } = svelte.compile(source, {
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hydratable: true
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});
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```
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(Most likely, you'll be passing this option to [rollup-plugin-svelte](https://github.com/rollup/rollup-plugin-svelte) or [svelte-loader](https://github.com/sveltejs/svelte-loader).)
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Then, when we instantiate the client-side component, we tell it to use the existing DOM with `hydrate: true`:
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```js
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import App from './App.html';
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const target = document.querySelector('#element-with-server-rendered-html');
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new App({
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target,
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hydrate: true
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});
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```
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> It doesn't matter if the client-side app doesn't perfectly match the server-rendered HTML — Svelte will repair the DOM as it goes.
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### Immutable
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Because arrays and objects are *mutable*, Svelte must err on the side of caution when deciding whether or not to update things that refer to them.
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But if all your data is [immutable](https://en.wikipedia.org/wiki/Immutable_object), you can use the `{ immutable: true }` compiler option to use strict object comparison (using `===`) everywhere in your app. If you have one component that uses immutable data you can set it to use the strict comparison for just that component.
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In the example below, `searchResults` would normally be recalculated whenever `items` *might* have changed, but with `immutable: true` it will only update when `items` has *definitely* changed. This can improve the performance of your app.
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```html
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<!-- { repl: false } -->
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{#each searchResults as item}
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<div>{item.name}</div>
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{/each}
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<script>
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import FuzzySearch from 'fuzzy-search';
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export default {
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immutable: true,
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computed: {
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searchResults: ({ searchString, items }) => {
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if (!searchString) return items;
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const searcher = new FuzzySearch(items, ['name', 'location']);
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return searcher.search(searchString);
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}
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}
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}
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</script>
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```
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[Here's a live example](repl?demo=immutable) showing the effect of `immutable: true`.
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