Add Svelte notes

JavaScript/Svelte/Svelte.md

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+# [Svelte](https://svelte.dev/docs)
+
+```sh
+npx degit sveltejs/template <project name>
+
+# set project to use typescript
+npm install --save-dev @tsconfig/svelte typescript svelte-preprocess svelte-check
+node scripts/setupTypeScript.js
+```
+
+## App Entrypoint
+
+```js
+import App from "./App.svelte"; // import the component
+
+const app = new App({
+  target: document.body,
+  props: {
+    // props passed to the App component
+  },
+});
+
+export default app;
+```
+
+## Components (`.svelte`)
+
+### Basic Structure
+
+```html
+<!-- code for the component -->
+<script lang="ts">
+  import { Component } from "Component.svelte";
+
+  export let prop; // make a variable a prop
+</script>
+
+<!-- CSS for the component -->
+<style>
+  /* CSS rules */
+
+  /* target elements outside of the current component */
+  :global(selector) {
+  }
+</style>
+
+<!-- html of the component  -->
+
+<!-- dynamic expressions -->
+<div>{variable}</div>
+
+<!-- nested components -->
+<Component prop="{value}" />
+```
+
+### If-Else
+
+```js
+{#if <condition>}
+  // markup here
+{:else if <condition>}
+  // markup here
+{:else}
+  // markup here
+{/if}
+```
+
+### Loops
+
+```js
+{#each array as item, index}  // index is optional
+    // markup here
+{/each}
+
+{#each array as item (key)}  // usse key to determine changes
+    // markup here
+{/each}
+```
+
+### Await Blocks
+
+```js
+{#await promise}
+ <p>...waiting</p>
+{:then number}
+ <p>The number is {number}</p>
+{:catch error}
+ <p style="color: red">{error.message}</p>
+{/await}
+```
+
+### Event Handling
+
+The full list of modifiers:
+
+- `preventDefault` — calls `event.preventDefault()` before running the handler. Useful for client-side form handling, for example.
+- `stopPropagation` — calls `event.stopPropagation()`, preventing the event reaching the next element
+- `passive` — improves scrolling performance on touch/wheel events (Svelte will add it automatically where it's safe to do so)
+- `nonpassive` — explicitly set `passive: false`
+- `capture` — fires the handler during the capture phase instead of the bubbling phase
+- `once` — remove the handler after the first time it runs
+- `self` — only trigger handler if `event.target` is the element itself
+
+```js
+<script>
+  const eventHandler = () => {};
+</script>
+
+<button on:event={eventHandler}>
+// or
+<button on:event={() => eventHandler(args)}>
+
+<button on:event|modifier={eventHandler}>
+```
+
+**NOTE**: It's possible to chain modifiers together, e.g. `on:click|once|capture={...}`.
+
+## Binding
+
+```html
+<script>
+  let name = "Foo";
+</script>
+
+<!-- modify value in real time -->
+<input bind:value="{stringValue}" />
+<input type="checkbox" bind:checked={boolean}/ >
+<!-- ... -->
+```
+
+### Reactive declarations & Reactive Statements
+
+Svelte automatically updates the DOM when the component's state changes.  
+Often, some parts of a component's state need to be computed from other parts and recomputed whenever they change.
+
+For these, Svelte has reactive declarations. They look like this:
+
+```js
+let count = 0;
+$: double =  count * 2;  // recaclulatd when count changes
+// or
+$: { }
+$: <expression>
+```
+
+## Routing
+
+[Svelte Routing](https://github.com/EmilTholin/svelte-routing)
+
+```js
+<!-- App.svelte -->
+<script>
+  import { Router, Link, Route } from "svelte-routing";
+  import Home from "./routes/Home.svelte";
+  import About from "./routes/About.svelte";
+  import Blog from "./routes/Blog.svelte";
+
+  export let url = "";
+</script>
+
+<Router url="{url}">
+  <nav>
+    <Link to="/">Home</Link>
+    <Link to="about">About</Link>
+    <Link to="blog">Blog</Link>
+  </nav>
+  <div>
+    <Route path="blog/:id" component="{BlogPost}" />
+    <Route path="blog" component="{Blog}" />
+    <Route path="about" component="{About}" />
+    <Route path="/"><Home /></Route>
+  </div>
+</Router>
+```
+
+## Data Stores
+
+```js
+// stores.js
+import { writable } from "svelte/store";
+
+export const count = writable(0);
+```
+
+```html
+<script>
+  import { onDestroy } from "svelte";
+  import { count } from ".path/to/stores.js";
+
+  const unsubscriber = count.subscribe((value) => {
+    // do stuff on load or value change
+  });
+
+  count.update((n) => n + 1);
+
+  count.set(1);
+  // or
+  $count = 1;
+
+  onDestroy(unsubscriber);
+</script>
+
+<!-- use $ to reference a store value -->
+<p>{$count}</p>
+```