Svelte101 Workshop Notes

What will I learn?

• Svelte Basics
• How to create a Svelte Component
• How reactivity works
• How to call async functions
• How to create lists in Svelte
• How to dispatch events from one component to another
• How to setup and use single page routing

About this workshop

This workshop is an immersive dive into the Svelte Framework, as a group, we will build a front-end application in Svelte and through the course of building this application, we will learn many of the features and benefits that Svelte has to offer. The notes will reference links to the svelte documentation if you want to dig deeper into any one feature. The purpose of this workshop is to give you a hands on feel of what it is like to work with the framework day in and day out. If you are up to the challenge please code along with the instructor, if the pace is too fast for you, don't worry, after the event the recorded version will be available for your to go at your own pace.

About me

Tom Wilson has been in the software development industry for over 25 years. Continuous learning and teaching have been a part of his journey. Since 2007, Tom has participated in the tech community hosting and running meetups and providing workshops focused on software development. In 2016, Tom launched a coding school in Charleston, SC, JRS Coding School, focused on full-stack javascript. In 2020, Tom founded hyper63; a company focused on leading engineers from beginner to expert.

About the project

In this workshop, we are going to build a weather application, this application, will show the current weather of a given location as well as list our favorite locations and and switch the current weather display from one of the favorite locations, then we will allow the user to add a new favorite city, to their favorites.

Housekeeping

In order to follow along with this workshop you will need an api key from https://weatherbit.io it is free, it takes a little bit of time to provision, so you may want to do this soon.

Setup

Pull workshop template down from github and initialize it and provide link to clone.

npx degit hyper63/weather weather-app
cd $_
git init
git add .
git commit -am "first commit"
gh create s101-2021-7-7 # install gh: https://cli.github.com/manual/installation
# add remote repo
yarn

Open in vscode

code .

Tour Repository

Check out quickstart guide:

https://svelte.dev/blog/the-easiest-way-to-get-started

Quick tour of a common svelte project

• public - contains all public assets
• src - contains your source code

git checkout -b 1-template-basics

Svelte Basics

https://svelte.dev/docs

Lets turn this static html into a template:

src/views/Current.svelte

<script>
  const weather = {
    city: "Charleston, SC",
    temp: "72 &deg; F",
    icon: "a01n",
    description: "clear",
  };
</script>
<nav>
  <div>
    <a href="">F</a>
    |
    <a href="">C</a>
  </div>
  <a href="">Favorites</a>
</nav>
<main>
  <figure>
    <img alt="{weather.description}" src="/icons/{weather.icon}.png" />
  </figure>
  <h3>{weather.city}</h3>
  <h1>{@html weather.temp}</h1>
  <p>{weather.description}</p>
</main>
<style>
  nav {
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
  main {
    display: flex;
    flex-direction: column;
    align-items: center;
  }
</style>

Summary

Svelte is designed to be similar to an old school html page, markup, styles, and scripts in one file. Then {} for dynamic content. Being able to declare a local variable in a script tag and reference the value in the markdup is a clean design. It is very intuitive when you look at the component.

Components - Creating a component

Lets convert the main section of the Current.svelte component into a reusable weather component:

src/components/Weather.svelte

<script>
  // proptypes (we can validate props at runtime using proptypes module)
  export let icon, description, city, temp;
</script>
<figure>
  <img alt="{description}" src="/icons/{icon}.png" />
</figure>
<h3>{city}</h3>
<h1>{@html temp}</h1>
<p>{description}</p>

Now our src/views/Current.svelte component looks like this

<script>
  import Weather from "../components/Weather.svelte";
  const weather = {
    city: "Charleston, SC",
    temp: "72 &deg; F",
    icon: "a01n",
    description: "clear",
  };
</script>
<nav>
  <div>
    <a href="">F</a>
    |
    <a href="">C</a>
  </div>
  <a href="">Favorites</a>
</nav>
<main>
  <Weather {...weather} />
</main>
<style>
  nav {
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
  main {
    display: flex;
    flex-direction: column;
    align-items: center;
  }
</style>

We can pass props by using name={value} or if the prop is the same name as the variable. {value} or we can use the spread operator: {...value} which will take an object and spread each key as a property.

Summary

Svelte leverages the single file component design, this design keeps your files focused on one specfic unit of work. This allows the scope to be explicit, if I declare a variable in the script tag, it is available to all of the markup. While some may argue that it reduces flexibility, it does lean towards clarity.

Reactivity - Changing display of temp

In the top left corner of our app, we have a couple of links, on F and one C we want to capture the click event of these links and then based on the current temperature unit, we want to convert to the other temperature unit. We can use a module called temperature to help with the conversion. But we need to know the current value of the temperature and the current unit it is in. For example, now we have it listed as Fahrenheit.

In order to listen for a click event, we can use the on directive. The on directive takes an event that we want to listen to and we assign it to a function.

<button on:click="{handleClick}">...</button>
<script>
  function handleClick() {
    console.log("click");
  }
</script>

Then in the function we want to change the temp value and unit based on the click function.

This is where reactivity comes into play. We can listen to the temp and unit variables and create a new derived variable called displayTemp this new variable will be the string we want to present.

<script>
  import { fahrenheitToCelsius, celsiusToFahrenheit } from "temperature";

  import Weather from "../components/Weather.svelte";

  let temp = 21.7;
  let unit = "c";

  $: displayTemp = `${Math.floor(temp)} &deg; ${unit.toUpperCase()}`;

  function convertToF() {
    temp = celsiusToFahrenheit(temp);
    unit = "f";
  }

  function convertToC() {
    temp = fahrenheitToCelsius(temp);
    unit = "c";
  }
</script>
<nav>
  <div>
    <a href="#" on:click|preventDefault="{convertToF}">F</a>
    |
    <a href="#" on:click|preventDefault="{convertToC}">C</a>
  </div>
  <a href="">Favorites</a>
</nav>
<main>{@html displayTemp} ...</main>

Summary

In this lesson, we learned about the on directive and how we can |preventDefault to directives. Also we learned about the $: reactivity command and how it can work like a formula in excel. $: label is valid javascript and is rarely used, in svelte you can think of it as a way to mark a variable or block of code as reactive, which will create watches on every variable in the calculation logic of the expression or block. And when they change, Svelte will run the block or assign the variable.

Async - Calling an API

Now that we have moved our presentation to a component, lets add an api call to our Current view. We want to call the /api/weather endpoint using a GET method call. We need to give the request a querystring of city=${city}&country=${country}.

WEATHER_KEY=(Your key)

Lets use the async library in our lib folder to call the weather api.

src/views/Current.svelte

<script>
  import { getJSON } from "../lib/async.js";

  getJSON("/api/weather?city=charleston,sc&country=us").then(
    console.log.bind(console)
  );
</script>

We should see the weather data come back from the api. We want to use this data to render our weather component.

Using the onMount function from svelte will let us handle async requests and apply the results to local variables.

<script>
  import { getJSON } from "../lib/async.js";
  import Weather from "../components/Weather.svelte";
  import { onMount } from "svelte";

  let weather = {
    city: "Charleston, SC",
    temp: "72 &deg; F",
    icon: "a01n",
    description: "clear",
  };

  onMount(async () => {
    const results = await getJSON("/api/weather?city=charleston,sc&country=us");
    weather = {
      temp: `${results.temp} &deg; C`,
      icon: results.weather.icon,
      description: results.weather.description,
      city: `${results.city_name} ${results.state_code}`,
    };
  });
</script>
<nav>
  <div>
    <a href="">F</a>
    |
    <a href="">C</a>
  </div>
  <a href="">Favorites</a>
</nav>
<main>
  <Weather {...weather} />
</main>
<style>
  nav {
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
  main {
    display: flex;
    flex-direction: column;
    align-items: center;
  }
</style>

Or we could create an async function and use the {#await} command.

<script>
  import { getJSON } from "../lib/async.js";
  import Weather from "../components/Weather.svelte";

  function getWeather() {
    return getJSON("/api/weather?city=charleston,sc&country=us").then(
      (results) => ({
        temp: `${results.temp} &deg; C`,
        icon: results.weather.icon,
        description: results.weather.description,
        city: `${results.city_name} ${results.state_code}`,
      })
    );
  }
</script>
<nav>
  <div>
    <a href="">F</a>
    |
    <a href="">C</a>
  </div>
  <a href="">Favorites</a>
</nav>
<main>
  {#await getWeather()} Loading... {:then weather}
  <Weather {...weather} />
  {/await}
</main>
<style>
  nav {
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
  main {
    display: flex;
    flex-direction: column;
    align-items: center;
  }
</style>

Summary

In this lesson, we learned about two ways to handle async requests from components at the point of loading.

{#each} and Dispatching Events

Lets start working on the src/views/Favorites.svelte view. This view lists the users favorite cities that they would like to get the weather from. Eventually, we will pull this list from the database, but for now, we will create a local variable.

src/views/Favorites.svelte

We need to handle the onclick event of each item, lets create a Card component then dispatch the click event from the card component.

From the click event, we need to route to the current view with the correct city parameters.

<script>
  import CityCard from "../components/CityCard.svelte";

  let cities = ["Charleston, SC", "New york, NY", "San Francisco, CA"];

  function changeCurrentCity(city) {
    return () => {
      console.log("city", city);
    };
  }
</script>
<nav>
  <div>Favorites</div>
  <a href="">Add</a>
</nav>
<main>
  <section>
    {#each cities as city}
    <CityCard {city} on:click="{changeCurrentCity(city)}" />
    {/each}
  </section>
</main>
<style>
  nav {
    height: 24px;
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
</style>

src/components/CityCard.svelte

<script>
  import { createEventDispatcher } from "svelte";
  export let city;
  const dispatch = createEventDispatcher();

  function handleClick() {
    dispatch("click", { city });
  }
</script>
<aside on:click="{handleClick}">
  <h3>{city}</h3>
</aside>
<style>
  aside:hover {
    background-color: whitesmoke;
  }
</style>

Summary

In this lesson, we created another component to handle each city card and used the {#each} template command to iterate over a list of cities and display the city card. Then we added a custom event to the city card which we handled in the Favorites component. Now we need to navigate from the favorites component to the Current component.

Take Break

Routing

For routing, we are going to use pagejs a framework agnostic routing component modeled after express.

src/App.svelte

<script>
  import {Route} from "tinro";

  import Current from "./views/Current.svelte";
  import Favorites from "./views/Favorites.svelte";

</script>

<Route path="/"><Current /></Route>
<Route path="/favorites/*">
  <Route path="/"><Favorites /></Route>
</Route>

src/views/Current.svelte

Add link to favorites

<a href="/favorites">Favorites</a>

Use page to navigate to Current

<script>
  import CityCard from "../components/CityCard.svelte";
  import { router } from 'tinro';

  let cities = ["Charleston, SC", "New york, NY", "San Francisco, CA"];

  function changeCurrentCity(city) {
    return () => {
      // set current city...then
      router.goto("/");
    };
  }
</script>
<nav>
  <div>Favorites</div>
  <a href="">Add</a>
</nav>
<main>
  <section>
    {#each cities as city}
    <CityCard {city} on:click="{changeCurrentCity(city)}" />
    {/each}
  </section>
</main>
<style>
  nav {
    height: 24px;
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
</style>

Summary

In this section, we learned how to use tinro router to navigate from one component to another component both declaratively and programatically. There are more features to check out with tinro - https://github.com/AlexxNB/tinro

Stores - Sharing data between components

There are several ways to pass data one way is to use svelte stores.

https://svelte.dev/docs#svelte_store

src/store.js

import { writable } from "svelte/store";

export const store = writable({ current: "Charleston, SC" });

export const dispatch = (action) =>
  new Promise((resolve) =>
    store.update((state) => {
      resolve();
      if (action.type === "SET_CURRENT") {
        return { ...state, current: action.payload };
      }
      return state;
    })
  );

src/views/Favorites.svelte

<script>
  import { dispatch } from "../store";
  import CityCard from "../components/CityCard.svelte";
  import page from "page";

  let cities = ["Charleston, SC", "New york, NY", "San Francisco, CA"];

  function changeCurrentCity(city) {
    return () => {
      // set current city...then
      dispatch({ type: "SET_CURRENT", payload: city }).then(() => page("/"));
    };
  }
</script>
<nav>
  <div>Favorites</div>
  <a href="">Add</a>
</nav>
<main>
  <section>
    {#each cities as city}
    <CityCard {city} on:click="{changeCurrentCity(city)}" />
    {/each}
  </section>
</main>
<style>
  nav {
    height: 24px;
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
</style>

src/views/Current.svelte

<script>
  import { getJSON } from "../lib/async.js";
  import { fahrenheitToCelsius, celsiusToFahrenheit } from "temperature";
  import { store } from "../store";

  import Weather from "../components/Weather.svelte";

  let temp = 21.7;
  let unit = "c";

  $: displayTemp = `${Math.floor(temp)} &deg; ${unit.toUpperCase()}`;

  function getWeather() {
    const current = $store.current.toLowerCase().replace(", ", ",");
    return getJSON(`/api/weather?city=${current}&country=us`).then(
      (results) => ({
        temp: `${results.temp} &deg; C`,
        icon: results.weather.icon,
        description: results.weather.description,
        city: `${results.city_name} ${results.state_code}`,
      })
    );
  }

  function convertToF() {
    temp = celsiusToFahrenheit(temp);
    unit = "f";
  }

  function convertToC() {
    temp = fahrenheitToCelsius(temp);
    unit = "c";
  }
</script>
<nav>
  <div>
    <a href="#" on:click="{convertToF}">F</a>
    |
    <a href="#" on:click="{convertToC}">C</a>
  </div>
  <a href="/favorites">Favorites</a>
</nav>
<main>
  {@html displayTemp} {#await getWeather()} Loading... {:then weather}
  <Weather {...weather} />
  {/await}
</main>
<style>
  nav {
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
  main {
    display: flex;
    flex-direction: column;
    align-items: center;
  }
</style>

Summary

A Svelte store is a clear tool to manage state in an application, the API of the Svelte store is so expressive, you can implement single state store patterns like redux or streaming patterns like RxJS and any thing in between. Check out the Svelte Store documentation for a deeper dive into Svelte stores.

Transitions

https://svelte.dev/docs#svelte_transition

src/views/Current.svelte

<script>
  import { getJSON } from "../lib/async.js";
  import { fahrenheitToCelsius, celsiusToFahrenheit } from "temperature";
  import { store } from "../store";
  import { fade, blur } from "svelte/transition";

  import Weather from "../components/Weather.svelte";

  let temp = 21.7;
  let unit = "c";

  $: displayTemp = `${Math.floor(temp)} &deg; ${unit.toUpperCase()}`;

  function getWeather() {
    const current = $store.current.toLowerCase().replace(", ", ",");
    return getJSON(`/api/weather?city=${current}&country=us`).then(
      (results) => ({
        temp: `${results.temp} &deg; C`,
        icon: results.weather.icon,
        description: results.weather.description,
        city: `${results.city_name} ${results.state_code}`,
      })
    );
  }

  function convertToF() {
    temp = celsiusToFahrenheit(temp);
    unit = "f";
  }

  function convertToC() {
    temp = fahrenheitToCelsius(temp);
    unit = "c";
  }
</script>
<nav>
  <div>
    <a href="#" on:click="{convertToF}">F</a>
    |
    <a href="#" on:click="{convertToC}">C</a>
  </div>
  <a href="/favorites">Favorites</a>
</nav>
<main transition:fade="{{duration:" 1000}}>
  {@html displayTemp} {#await getWeather()} Loading... {:then weather}
  <Weather {...weather} />
  {/await}
</main>
<style>
  nav {
    margin-bottom: 0;
    margin-left: 8px;
    margin-right: 8px;
  }
  main {
    display: flex;
    flex-direction: column;
    align-items: center;
  }
</style>

src/views/Favorites.svelte

<script>
  import { blur } from 'svelte/transition'
  import { dispatch } from '../store'
  import CityCard from '../components/CityCard.svelte'
  import { router } from 'tinro'

  let cities = ['Charleston, SC', 'New york, NY', 'San Francisco, CA']

  function changeCurrentCity(city) {
    return () => {
      // set current city...then
      dispatch({type: 'SET_CURRENT', payload: city})
        .then(() => router.goto('/'))
    }
  }

</script>
<div in:blur={{delay: 1000, duration: 1000}}>
<nav>
  <div>Favorites</div>
  <a href="">Add</a>
</nav>
<main>
  <section>
    {#each cities as city}
      <CityCard {city} on:click={changeCurrentCity(city)} />
    {/each}
  </section>
</main>
</div>
<style>
 nav {
   height: 24px;
   margin-bottom: 0;
   margin-left: 8px;
   margin-right: 8px;
 }
</style>

Summary

Having transitions and animations built into Svelte is a huge bonus to the framework, the transitions are declarative and intuitive. With this section we just introduced the concept. For more about transitions checkout the svelte transition section in Svelte Docs: https://svelte.dev/docs#svelte_transition, there are also many talks about svelte transitions on the Svelte Society Youtube channel - https://www.youtube.com/c/sveltesociety

Take Break

Modal and Actions

To handle adding a new city, lets use a modal component.

https://svelte.dev/repl/e94473c00c5c422fa736ba60a2ca0e61?version=3.26.0

<script>
  import { createEventDispatcher } from "svelte";
  import { scale } from "svelte/transition";
  export let open = false;
  const dispatch = createEventDispatcher();

  function handleCloseClick() {
    dispatch("close");
  }

  // action

  function modalAction(node) {
    let fns = [];
    if (document.body.style.overflow !== "hidden") {
      const original = document.body.style.overflow;
      document.body.style.overflow = "hidden";
      fns = [...fns, () => (document.body.style.overflow = original)];
    }
    return {
      destroy: () => fns.map((fn) => fn()),
    };
  }
</script>
{#if open}
<div use:modalAction on:click="{handleCloseClick}">
  <section on:click|stopPropagation>
    <aside in:scale out:scale="{{duration:" 500}}>
      <slot />
      <br />
      <button on:click|preventDefault="{handleCloseClick}">Close</button>
    </aside>
  </section>
</div>
{/if}
<style>
  section {
    height: 100%;
    display: grid;
    place-items: center;
  }
  aside {
    background-color: white;
  }
  div {
    position: absolute;
    background-color: rgba(0, 0, 0, 0.8);
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
  }
</style>

src/views/Add.svelte

<script>
  import { createEventDispatcher } from "svelte";
  const dispatch = createEventDispatcher();
  let city;

  function handleSubmit() {
    dispatch("add", { city });
    city = "";
  }
</script>
<main>
  <section>
    <h3>Add Location</h3>
    <form on:submit|preventDefault="{handleSubmit}">
      <div>
        <label for="city">City</label>
        <input type="text" id="city" bind:value="{city}" />
      </div>
      <!--
      <div>
        <label for="country">Country</label>
        <select id="country">
          <option>-- select --</option>
        </select>
      </div>
      -->
      <div>
        <button id="add-btn">Add</button>
        <a href="">Cancel</a>
      </div>
    </form>
  </section>
</main>

src/views/Favorites.svelte

<script>
  import { blur } from 'svelte/transition'
  import { dispatch } from '../store'
  import CityCard from '../components/CityCard.svelte'
  import { router } from 'tinro'
  import Modal from '../components/Modal.svelte'
  import Add from './Add.svelte'

  let cities = ['Charleston, SC', 'New york, NY', 'San Francisco, CA']
  let open = false

  function changeCurrentCity(city) {
    return () => {
      // set current city...then
      dispatch({type: 'SET_CURRENT', payload: city})
        .then(() => router.goto('/'))
    }
  }

  function openModal() {
    open = true
  }

  function addCity({detail}) {
    cities = [...cities, detail.city]
    open = false
  }

</script>
<div in:blur={{delay: 1000, duration: 1000}}>
<nav>
  <div>Favorites</div>
  <a href="#" on:click={openModal}>Add</a>
</nav>
<main>
  <section>
    {#each cities as city}
      <CityCard {city} on:click={changeCurrentCity(city)} />
    {/each}
  </section>
</main>
</div>
<Modal {open} on:close={_ => open = false}>
  <Add on:add={addCity} />
</Modal>
<style>
 nav {
   height: 24px;
   margin-bottom: 0;
   margin-left: 8px;
   margin-right: 8px;
 }
</style>

Summary

In this section, we introduced slots and actions. With slots you can add content to your component adding markup within the element tags. With actions, you can hook into the browser event system to directly manage elements, as you would with jquery. Actions are great for interacting with third party javascript modules, like d3 and map components.

Testing

Install cypress

The first thing we need to do is install cypress and cypress-svelte-unit-test:

yarn add -D [email protected] cypress-svelte-unit-test

Initialize Cypress

yarn run cypress open

Once you see the modal, you click ok and then close the cypress window.

Tell cypress to use rollup when working with svelte components

Open an index.js file in the cypress/plugins directory and edit the following function:

module.exports = (on) => {
  const filePreprocessor = require("@bahmutov/cy-rollup");
  on("file:preprocessor", filePreprocessor());
};

This code will give cypress the information it needs to compile the svelte component

Turn on component support for cypress

When you installed cypress, the install created a cypress.json file, in this file we need to add the following entries:

{
  "experimentalComponentTesting": true,
  "componentFolder": "src",
  "testFiles": "**/*spec.js"
}

The first entry is a flag to enable component testing, the second is the location of the components, finally the third entry is a pattern matcher for the test files.

Write a test

Now, we have our project configured we can write a test.

In our src folder, lets create a test for the App component.

create a new file src/views/Add.spec.js

import Add from "./Add.svelte";
import { mount } from "cypress-svelte-unit-test";

it("add city using form", () => {
  mount(Add, {
    callbacks: {
      add: cy.stub().as("add"),
    },
  });
  cy.get("input#city").type("Boston, MA");
  cy.get("button#add-btn").click();
  cy.get("@add")
    .should("be.called")
    .its("firstCall.args.0.detail")
    .should("deep.equal", { city: "Boston, MA" });
});

src/views/Add.svelte

<script>
  import { createEventDispatcher } from "svelte";
  const dispatch = createEventDispatcher();

  let city = "";
  let selected = "";

  function submitCity() {
    dispatch("add", { city });
    selected = city;
    city = "";
  }
</script>
<main>
  <section>
    <h3>Add Location</h3>
    {#if selected !== ''}
    <div id="selected">selected: {selected}</div>
    {/if}
    <form on:submit|preventDefault="{submitCity}">
      <div>
        <label for="city">City</label>
        <input type="text" id="city" bind:value="{city}" />
      </div>
      <!--
      <div>
        <label for="country">Country</label>
        <select id="country">
          <option>-- select --</option>
        </select>
      </div>
      -->
      <div>
        <button id="add-btn">Add</button>
        <a href="">Cancel</a>
      </div>
    </form>
  </section>
</main>

The it function takes a description and a callback function, in the callback function, we use the imported mount function to mount the App component passing the name prop. Then we use cypress contains function to find the dom element h1 and validate if it contains the following text 'Hello World!'

Run the Test

Now that we have our test, lets run it and see if it passes.

yarn run cypress run

This command will run cypress in the console.

If everything went as planned you should see a print out showing App.spec.js passed and all specs passed!

https://docs.cypress.io/guides/overview/why-cypress.html#In-a-nutshell

Summary

This is a very short introduction to cypress, cypress is a powerful tool for testing Javascript using different styles and patterns. Cypress gives you a clean way to TDD with Svelte.

Fin

This ends the workshop, but you do not have to end the journey here, you can take this application to greater lengths:

• Add a dynamic background, based on the weather
• Store your favorite cities on localStorage
• Show wind and tidal information from weatherbit
• Convert web app to mobile app using capacitorjs
• What else?

This workshop is designed to continue to practice and re-enforce component architecture concepts. So feel free to go through the workshop as a tutorial a couple of times to get some refinement. Then, try to create the app with no direction from the guide, see what you remember, and see what you don't, then refer to complete the parts you are not familar with, then repeat.

Thank you!

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