Svelte reactivity

Svelte automatically updates the DOM when the state of a component changes. Many times, some parts of a component’s state need to be computed from other parts and recomputed whenever they change. For example, in a weather app, we have the temperature in Celsius, but we want to display it in Fahrenheit. So, we need to compute Fahrenheit from Celsius whenever the Celsius temperature changes. This is why we have reactivity that keeps the DOM in sync with our application state.

In Svelte, reactivity is built into the language itself. This means we don’t need special syntax or methods to make our components reactive. The key concepts in Svelte’s reactivity are reactive assignments, reactive declarations, and reactive statements.

Reactive assignments

Reactive assignments in Svelte empower developers to create dynamic interfaces by seamlessly updating variables and recalculating expressions in real-time. 

Unlike conventional frameworks that require specific methods or syntax for reactivity, Svelte leverages standard JavaScript assignments. When a variable changes, Svelte detects these changes and automatically updates the UI, ensuring it remains in sync with the underlying data state. 

Let's explore this concept with a practical example to see how straightforward and efficient reactivity in Svelte can be:

# This repo is no longer maintained. Consider using `npm init vite` and selecting the `svelte` option or — if you want a full-fledged app framework — use [SvelteKit](https://kit.svelte.dev), the official application framework for Svelte.

---

# svelte app

This is a project template for [Svelte](https://svelte.dev) apps. It lives at https://github.com/sveltejs/template.

To create a new project based on this template using [degit](https://github.com/Rich-Harris/degit):

```bash
npx degit sveltejs/template svelte-app
cd svelte-app
```

*Note that you will need to have [Node.js](https://nodejs.org) installed.*


## Get started

Install the dependencies...

```bash
cd svelte-app
npm install
```

...then start [Rollup](https://rollupjs.org):

```bash
npm run dev
```

Navigate to [localhost:8080](http://localhost:8080). You should see your app running. Edit a component file in `src`, save it, and reload the page to see your changes.

By default, the server will only respond to requests from localhost. To allow connections from other computers, edit the `sirv` commands in package.json to include the option `--host 0.0.0.0`.

If you're using [Visual Studio Code](https://code.visualstudio.com/) we recommend installing the official extension [Svelte for VS Code](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode). If you are using other editors you may need to install a plugin in order to get syntax highlighting and intellisense.

## Building and running in production mode

To create an optimised version of the app:

```bash
npm run build
```

You can run the newly built app with `npm run start`. This uses [sirv](https://github.com/lukeed/sirv), which is included in your package.json's `dependencies` so that the app will work when you deploy to platforms like [Heroku](https://heroku.com).


## Single-page app mode

By default, sirv will only respond to requests that match files in `public`. This is to maximise compatibility with static fileservers, allowing you to deploy your app anywhere.

If you're building a single-page app (SPA) with multiple routes, sirv needs to be able to respond to requests for *any* path. You can make it so by editing the `"start"` command in package.json:

```js
"start": "sirv public --single"
```

## Using TypeScript

This template comes with a script to set up a TypeScript development environment, you can run it immediately after cloning the template with:

```bash
node scripts/setupTypeScript.js
```

Or remove the script via:

```bash
rm scripts/setupTypeScript.js
```

If you want to use `baseUrl` or `path` aliases within your `tsconfig`, you need to set up `@rollup/plugin-alias` to tell Rollup to resolve the aliases. For more info, see [this StackOverflow question](https://stackoverflow.com/questions/63427935/setup-tsconfig-path-in-svelte).

## Deploying to the web

### With [Vercel](https://vercel.com)

Install `vercel` if you haven't already:

```bash
npm install -g vercel
```

Then, from within your project folder:

```bash
cd public
vercel deploy --name my-project
```

### With [surge](https://surge.sh/)

Install `surge` if you haven't already:

```bash
npm install -g surge
```

Then, from within your project folder:

```bash
npm run build
surge public my-project.surge.sh
```
Reactive assignments

Code explanation

  • Line 1: We define the script block, which contains the JavaScript code for our component.

  • Line 2: We declare a reactive variable named likesCount and initialize it with a value of 0. Any changes to the likesCount variable will trigger updates in the UI.

  • Lines 4–6: We define the likePost function that increments the likesCount variable. Since likesCount is reactive, any part of the template that references it will be automatically updated to reflect the new value.

  • Line 9: We define a simple template that displays the value of the reactive likesCount variable.

  • Line 10: We define a button with an on:click directive that attaches a click event listener to the button, so when it is clicked, the likePost function is called, updating the reactive likesCount variable and triggering a UI update.

Reactive declarations

Reactive declarations in Svelte introduce a powerful paradigm where variables can be automatically recalculated based on changes to their dependent values. This feature allows developers to define complex relationships between data points, simplifying state management and enhancing the reactivity of the application.

In Svelte, this is achieved using a simple, intuitive syntax that marks a variable as reactive. As a result, whenever a dependency changes, Svelte efficiently updates the reactive variable to reflect the new state.

Below, we'll dive into an example to illustrate how reactive declarations streamline the process of creating dynamic and interactive UIs, demonstrating the ease with which developers can implement sophisticated data interactions.

# This repo is no longer maintained. Consider using `npm init vite` and selecting the `svelte` option or — if you want a full-fledged app framework — use [SvelteKit](https://kit.svelte.dev), the official application framework for Svelte.

---

# svelte app

This is a project template for [Svelte](https://svelte.dev) apps. It lives at https://github.com/sveltejs/template.

To create a new project based on this template using [degit](https://github.com/Rich-Harris/degit):

```bash
npx degit sveltejs/template svelte-app
cd svelte-app
```

*Note that you will need to have [Node.js](https://nodejs.org) installed.*


## Get started

Install the dependencies...

```bash
cd svelte-app
npm install
```

...then start [Rollup](https://rollupjs.org):

```bash
npm run dev
```

Navigate to [localhost:8080](http://localhost:8080). You should see your app running. Edit a component file in `src`, save it, and reload the page to see your changes.

By default, the server will only respond to requests from localhost. To allow connections from other computers, edit the `sirv` commands in package.json to include the option `--host 0.0.0.0`.

If you're using [Visual Studio Code](https://code.visualstudio.com/) we recommend installing the official extension [Svelte for VS Code](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode). If you are using other editors you may need to install a plugin in order to get syntax highlighting and intellisense.

## Building and running in production mode

To create an optimised version of the app:

```bash
npm run build
```

You can run the newly built app with `npm run start`. This uses [sirv](https://github.com/lukeed/sirv), which is included in your package.json's `dependencies` so that the app will work when you deploy to platforms like [Heroku](https://heroku.com).


## Single-page app mode

By default, sirv will only respond to requests that match files in `public`. This is to maximise compatibility with static fileservers, allowing you to deploy your app anywhere.

If you're building a single-page app (SPA) with multiple routes, sirv needs to be able to respond to requests for *any* path. You can make it so by editing the `"start"` command in package.json:

```js
"start": "sirv public --single"
```

## Using TypeScript

This template comes with a script to set up a TypeScript development environment, you can run it immediately after cloning the template with:

```bash
node scripts/setupTypeScript.js
```

Or remove the script via:

```bash
rm scripts/setupTypeScript.js
```

If you want to use `baseUrl` or `path` aliases within your `tsconfig`, you need to set up `@rollup/plugin-alias` to tell Rollup to resolve the aliases. For more info, see [this StackOverflow question](https://stackoverflow.com/questions/63427935/setup-tsconfig-path-in-svelte).

## Deploying to the web

### With [Vercel](https://vercel.com)

Install `vercel` if you haven't already:

```bash
npm install -g vercel
```

Then, from within your project folder:

```bash
cd public
vercel deploy --name my-project
```

### With [surge](https://surge.sh/)

Install `surge` if you haven't already:

```bash
npm install -g surge
```

Then, from within your project folder:

```bash
npm run build
surge public my-project.surge.sh
```
Reactive declarations

Code explanation

  • Line 1: We again define the script block, which contains the JavaScript code for our component.

  • Line 2–8: We declare the same reactive variable named likesCount and the likePost function that increments the likesCount variable only if remainingLikes are greater than 0.

  • Line 10: We use a reactive declaration, marked using a $:, to declare a new reactive variable named remainingLikes. It will be recomputed anytime likesCount changes.

  • Line 13–14: We define a button to like the post and display the remaining likes.

Reactive statements

Reactive statements in Svelte automatically execute code in response to data changes. For example, in a user profile interface, a reactive statement can update a message based on the number of followers. When the follower count changes, the message dynamically adjusts, demonstrating real-time reactivity without manual DOM updates or state management. Let’s look at an example of it below:

# This repo is no longer maintained. Consider using `npm init vite` and selecting the `svelte` option or — if you want a full-fledged app framework — use [SvelteKit](https://kit.svelte.dev), the official application framework for Svelte.

---

# svelte app

This is a project template for [Svelte](https://svelte.dev) apps. It lives at https://github.com/sveltejs/template.

To create a new project based on this template using [degit](https://github.com/Rich-Harris/degit):

```bash
npx degit sveltejs/template svelte-app
cd svelte-app
```

*Note that you will need to have [Node.js](https://nodejs.org) installed.*


## Get started

Install the dependencies...

```bash
cd svelte-app
npm install
```

...then start [Rollup](https://rollupjs.org):

```bash
npm run dev
```

Navigate to [localhost:8080](http://localhost:8080). You should see your app running. Edit a component file in `src`, save it, and reload the page to see your changes.

By default, the server will only respond to requests from localhost. To allow connections from other computers, edit the `sirv` commands in package.json to include the option `--host 0.0.0.0`.

If you're using [Visual Studio Code](https://code.visualstudio.com/) we recommend installing the official extension [Svelte for VS Code](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode). If you are using other editors you may need to install a plugin in order to get syntax highlighting and intellisense.

## Building and running in production mode

To create an optimised version of the app:

```bash
npm run build
```

You can run the newly built app with `npm run start`. This uses [sirv](https://github.com/lukeed/sirv), which is included in your package.json's `dependencies` so that the app will work when you deploy to platforms like [Heroku](https://heroku.com).


## Single-page app mode

By default, sirv will only respond to requests that match files in `public`. This is to maximise compatibility with static fileservers, allowing you to deploy your app anywhere.

If you're building a single-page app (SPA) with multiple routes, sirv needs to be able to respond to requests for *any* path. You can make it so by editing the `"start"` command in package.json:

```js
"start": "sirv public --single"
```

## Using TypeScript

This template comes with a script to set up a TypeScript development environment, you can run it immediately after cloning the template with:

```bash
node scripts/setupTypeScript.js
```

Or remove the script via:

```bash
rm scripts/setupTypeScript.js
```

If you want to use `baseUrl` or `path` aliases within your `tsconfig`, you need to set up `@rollup/plugin-alias` to tell Rollup to resolve the aliases. For more info, see [this StackOverflow question](https://stackoverflow.com/questions/63427935/setup-tsconfig-path-in-svelte).

## Deploying to the web

### With [Vercel](https://vercel.com)

Install `vercel` if you haven't already:

```bash
npm install -g vercel
```

Then, from within your project folder:

```bash
cd public
vercel deploy --name my-project
```

### With [surge](https://surge.sh/)

Install `surge` if you haven't already:

```bash
npm install -g surge
```

Then, from within your project folder:

```bash
npm run build
surge public my-project.surge.sh
```
Reactive statements

Code explanation

  • Line 1: We again define the script block, which contains the JavaScript code for our component.

  • Line 2–8: We declare the same reactive variable named likesCount and the likePost function that increments the likesCount variable only if remainingLikes are greater than 0.

  • Line 10: We again use a reactive declaration to declare the reactive variable named remainingLikes.

  • Line 11–14: We define and group reactive statements to log messages to the console whenever a referenced value changes, respectively.

  • Line 17–18: We again define a button to like the post and display the remaining likes.

Svelte’s reactivity offers a refreshing approach to building reactive user interfaces. By incorporating reactivity directly into it, Svelte simplifies the development process.

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