## Svelte cheatsheet

[Doc examples](https://svelte.dev/examples)

Your components are inside `.svelte` files.

### Structure

Your component use few tags to be operating:
- `<script>`
- `<style>`

`<script>` will describe how to animate the component.
`<style>` how to render it


### Imports
You can include a component inside another one by importing it:
```svelte
<script>
  import Nested from './Nested.svelte'
</script>

<Nested/>
```

### Variables

You can define variables in your `<script>` tag and then use it with brackets:
```svelte
<script>
  let foo = "bar"
</script>

<p class={bar}>Some text</p>
```

> **Tip**: It also works inside double-quotes: `<p class="primary-{bar}">Some text</p>`

Moreover, when var = attribute, (`src={src}`) you can use it this way: `{src}`.

### Reactivity

**Reactivity is triggered by assignments**
What does it mean? Every change will be triggered after an assignment has been done.

More on that later.

##### Variables
Sometimes a variable is result from a computation, you'll have to use a *reactive declaration*:
```javascript
let count = 0
$: doubled = count * 2
```

##### Statements
We're not limited to variable declaration, we can also use reactive statements:
```javascript
$: console.log(`the count is ${count}`);
```
Here `console.log` is called whenever `count` value changes.
Notice the usage of *`* (backquotes).

> **Tip**: You can group more statements by using `$: { ... }`.

##### Updating objects

Because Svelte's reactivity is **triggered** by **assignments** following code won't do anything when called:
```javascript
function addNumber() {
  numbers.push(numbers.length + 1);
}
```
Unless you add it this assignment:
```javascript
function addNumber() {
  numbers.push(numbers.length + 1);
  numbers = numbers;
}
```

### Props

There's an easy way to pass props to a child component. Just `export` it !
```svelte
// Nested.svelte
<script>
  export let answer = 42
</script>
<p>The answer is {answer}</p>

// App.svelte
<script>
  import Nested from './Nested.svelte'
</script>

<Nested/> // Will display: The answer is 42
```

> **Tip**: You can also use `<Nested answer={42}/>` to set a default value to a prop that hasn't been initialized into it's own component.


When passing multiple **props** typically as an object of properties, you can **spread** them to a component instead of specifying each one with `...`:

```svelte
<script>
  import Info from './Info.svelte';

  const pkg = {
    name: 'svelte',
    version: 3,
    speed: 'blazing',
    website: 'https://svelte.dev'
  };
</script>

<Info name={pkg.name} version={pkg.version} speed={pkg.speed} website={pkg.website}/>
<Info {...pkg}/>
```

Assuming we have `export`ed `name`, `version`, and so on in the `Info` component.

### Logic

#### Conditions

To conditionally render some markup, we wrap it in an if block:
```svelte
{#if user.loggedIn}
  <button on:click={toggle}>
    Log out
  </button>
{/if}

{#if !user.loggedIn}
  <button on:click={toggle}>
    Log in
  </button>
{/if}
```

You can also make use of :
- `{:else}`
- `{:else if .. }`

#### Loops

Expressed this way:

```svelte
{#each cats as cat, i}
  <li><a target="_blank" href="https://www.youtube.com/watch?v={cat.id}">
    {i}: {cat.name}
  </a></li>
{/each}
```
> **Tip**: You can also de-structure as: `{#each cats as { id, name }}` and then use `{name}` instead of `{cat.name}`

When a value isn't **exported** but is computed by a **prop**, you can go into problems when updating values since it's a *copy* and not a *reference*.
Thus, be sure to **identify** the right object when updating its value(s):

```svelte
{#each cats as cat, i (cat)} // Notice the (cat)
  <li><a target="_blank" href="https://www.youtube.com/watch?v={cat.id}">
    {i}: {cat.name}
  </a></li>
{/each}
```

#### Promises
You can deal with **promises** and display at each of the promise lifecycle:

```svelte
{#await promise}
  <p>Loading...</p>
{:then result}
  <p>It returns {result}</p>
{:catch error}
  <p>An error occurred: {error.message}</p>
{/await}
```

> **Tip**: You can also skip loading step by using: `{#await promise then result}`

### Events
You can bind functions / events to your tags:

```svelte
<script>
  let count = 0;

  function handleClick() {
    count += 1;
  }
</script>

<button on:click={handleClick}>
  Clicked {count} {count === 1 ? 'time' : 'times'}
</button>
```

You can use all javascript event (click, mousemove, ...).
You can also *modify* event by *piping* **modifiers** to it:
`<button on:click|once={handleClick}>`
You can *chain* them by adding pipes.

#### Event dispatcher

You can create an event dispatcher inside a component.
It must be called when component is first instantiated.
```svelte
<script>
  import { createEventDispatcher } from 'svelte';

  const dispatch = createEventDispatcher();

  function sayHello() {
    dispatch('message', {
      text: 'Hello!'
    });
  }
</script>
```
On the other side:

```svelte
<script>
  import Inner from './Inner.svelte';

  function handleMessage(event) {
    alert(event.detail.text);
  }
</script>

<Inner on:message={handleMessage}/> // on:eventname
```

#### Event forwarding

You can handle events from any other component by calling them with their own event:
```svelte
// Outer.svelte
<script>
  import Inner from './Inner.svelte';
</script>

<Inner on:message/>
```
Here we can intercept event `message` from `Inner`.
Then, **when called**, we can define how the component **reacts**:

```svelte
<Outer on:message={handleMessage}/>
```

### Bindings

Data flow is *top down*, that means a **parent** component can **set props on a child** component but **not the other way** around.

**Bindings** can break that rule:

```svelte
<script>
  let name = 'world';
</script>

<input bind:value={name}>  // name and value are tied together, they are bind

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

You can also use a **group** of bindings:
```svelte
<script>
  let flavourList = [
    'Cookies and cream',
    'Mint choc chip',
    'Raspberry ripple'
  ];
</script>

<h2>Flavours</h2>

{#each flavourList as flavour}
  <label>
    <input type=checkbox bind:group={flavourList} value={flavour}>
    {flavour}
  </label>
{/each}
```

### Lifecycle

Is ruled by a bunch of functions:

- `onMount`

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

  let photos = [];

  onMount(async () => {
    const res = await fetch(`https://jsonplaceholder.typicode.com/photos?_limit=20`);
    photos = await res.json();
  });
</script>
```

- `onDestroy`
- `beforeUpdate`
- `afterUpdate`

### Store

A store is simply an object with a **subscribe** method that allows interested parties to be **notified** whenever the **store value** changes:
```svelte
<script>
  let countValue;
  const unsubscribe = count.subscribe(value => {
    count_value = value;
  });
  onDestroy(unsubscribe);
</script>

<h1>The count is {count_value}</h1>
```
Then, you can **update** the stored value:
```svelte
<script>
  function increment() {
    count.update(n => n + 1)
  }
</script>
```
Or set a value:
```svelte
<script>
  function reset() {
    count.set(0);
  }
</script>
```

> **Best practice**: Auto-subscription

#### Usage

You can use auto-subscription to get rid of `subscribe` and `onDestroy` methods:

```svelte
<script>
  import { storeValue } from './AnotherComponent'
  // ...other imports come after
</script>

<h1>The count is {$storeValue}</h1>
```

> **Warning**: The store value must be imported or declared at the top-level scope of a component !

#### Readable store (read-only store)

```svelte
<script>
  export const time = readable(new Date(), function start(set) {
    // Implementation
    set()

    return function stop() {
      // Implementation
    }
  })
</script>
```
The **first argument** can be null, it represents an **initial value**.
You then have to define a `set` callback which is called when the store gets its **first subscriber** and a `stop` callback when the last subscriber unsubscribes.

#### Writable store
```svelte
<script>
  import { writable } from 'svelte/store';

  const count = writable(0, () => {
    console.log('got a subscriber');
    return () => console.log('no more subscribers');
  });
</script>
```
It can be altered with `set` and `update`. It can also be `bind` to a tag.

#### Derived store (from another store)
```svelte
<script>
  import { derived } from 'svelte/store';

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

  // or (read-only derived):

  const elapsed = derived(time, $time =>
    Math.round(($time - start) / 1000)
  );

</script>
```
First argument is the original store. **Second** one is **facultative** and added if you want to **write** on the store.

#### Custom store
```javascript
// store.js

import { writable } from 'svelte/store';
function createCount() {
  const { subscribe, set, update } = writable(0);

  return {
    subscribe,
    increment: () => update(n => n + 1),
    decrement: () => update(n => n - 1),
    reset: () => set(0)
  };
}
export const count = createCount();
```

In the component:
```svelte
<script>
  import { count } from './store.js';
</script>

<h1>The count is {$count}</h1>

<button on:click={count.increment}>+</button>
<button on:click={count.decrement}>-</button>
<button on:click={count.reset}>reset</button>
```

### Motion

Stores are cool but you can make them even smoother by adding animation when transposed to UI.

#### Tweened

It's usable like an ordinary store:

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

  const progress = tweened(0, {
      duration: 400,
      easing: cubicOut
  });
</script>

<progress value={$progress}></progress>

<button on:click="{() => progress.set(0)}">0%</button>
```

#### Spring (works better with frequently changing value)

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

  let coords = spring({ x: 50, y: 50 }, {
      stiffness: 0.1,
      damping: 0.25
  });
  let size = spring(10);
</script>
```

### Transitions

There're a lot:
- Fade in/out: `transition:fade` (ie: `<p transition:fade>Some text</p>`)
- Fly (disappear in a given direction): `transition:fly`
- ...

### Dynamic classes

To affect a `class` to a tag dynamically when condition are met:
```javascript
<button
  class:active="{current === 'foo'}"
>
  Text
</button>

<style>
.active {
  background-color: #ff3e00;
  color: white;
}
</style>
```
Here we affect the class `.active` when condition is true.

### Slots

Slots give you the opportunity to shape a component inside another one.
```svelte
<script>
  import Box from './Box.svelte';
</script>

<Box>
  <h2>Hello!</h2>
  <p>This is a box. It can contain anything.</p>
</Box>

// Box.svelte
<div class="box">
  <slot>A default value here</slot>
</div>
```
Slots can be named:
```svelte
<script>
  import ContactCard from './ContactCard.svelte';
</script>

<ContactCard>
  <span slot="name">T. Pellegatta</span>
</ContactCard>

// ContactCard.svelte
<article class="contact-card">
  <h2>
    <slot name="name">
      <span class="missing">Unknown name</span>    // Will display T. Pellegatta because it has been defined
    </slot>
  </h2>

  <div class="address">
    <slot name="address">
      <span class="missing">Unknown address</span>
    </slot>
  </div>
</article>
```

You can pass props to slots:
```svelte
<!-- App.svelte -->
<FancyList {items}>
  <div slot="item" let:item={item}>{item.text}</div>
  <p slot="footer">Copyright (c) 2019 Svelte Industries</p>
</FancyList>

<!-- FancyList.svelte -->
<ul>
  {#each items as item}
    <li class="fancy">
        <slot name="item" item={item}></slot>
    </li>
  {/each}
</ul>

<slot name="footer"></slot>
```
The `let:` directive goes on the element with the slot attribute.

### Contexts

Almost the same as **stores** but **less reactive**.
[Documentation](https://svelte.dev/docs#setContext)

### Debugging

Instead of using `debugger;` use `{@debug variable}`

## Integrations
### GraphQL integration (with Apollo)

You'll have to import some plugins:

- `apollo-boost`
- `graphql`
- `svelte-apollo`

Then, in the main component instantiate a client:
```svelte
<script>
  import ApolloClient from 'apollo-boost'
  import { setClient } from 'svelte-apollo'
  const client = new ApolloClient({
      uri: 'http://site/graphqlApi/endpoint'
  })
  setClient(client)
</script>
```

In another one (or the same) use it to fetch/push data:
```svelte
<script>
  import { getClient, query } from 'svelte-apollo'
  import { gql } from 'apollo-boost'

  const GET_TODOS = gql`
    {
      getTodos {
        id
        text
      }
    }
  `

  const client = getClient()
  const todos = query(client, { query: GET_TODOS })
</script>


{#await $todos}
  Loading...
{:then result}
  <p>Total todos: {result.data.getTodos.length}</p>
  <ul>
    {each result.data.getTodos as todo}
      <li>{todo.text}</li>
    {/each}
  </ul>
{:catch error}
  <p>{error.message}</p>
{/await}
```