How to Consume and Render Data from a Backend in Svelte

How to Consume and Render Data from a Backend in Svelte

In this Svelte, we'll learn how to consume RESTful APIs with Svelte. Learn how to consume and render data from a backend in Svelte, define and export props, and pass props to components. We'll explore how Svelte consumes and renders data from an API by building a simple app.

Svelte is a modern reactive component framework that runs at build time, converting components into highly efficient imperative code that surgically updates the DOM.

What we’ll be building

In this article, we’ll explore how Svelte consumes and renders data from an API by building a simple app. We’ll first write a simple backend to store our data and then write our Svelte components.


The first thing we’ll do is set up a working directory where we’ll store the code for our application. There are a number of ways to get a Svelte project up and running, and since this isn’t an introductory tutorial on Svelte, we’ll be using degit, a scaffolding tool to clone a Svelte template.

To scaffold our app, which we’ll call continent-app, run the following command in your terminal from your preferred working directory:

npx degit sveltejs/template continent-app

Next is to navigate into the newly created directory and install the dependencies:

cd continent-app && npm install

Once the dependencies installation is complete, we create two component files, Continents and Continent, and then start the app:

touch src/{Continent,Continents}.svelte
npm run dev

You should get the screen below:

Building the API

Now that we have our Svelte app up and running, we are set to build the API before writing the components for the Svelte app. Our API is a simple one that holds hard coded informations about the seven continents that can be retrieved once a call is made to it.

Next, create a new folder api, in the app’s directory and install the following dependencies:

mkdir api && cd api
npm init -y // Quick initialisation of directory
npm install express cors body-parser

After the installation, create a new file, app.js, that will hold the simple backend, and then copy the accompanying code below into it:

touch app.js


We start off by importing the dependencies and initializing them:

const express = require("express");
const bodyParser = require("body-parser");
const cors = require('cors')

const app = express();

Next, we create an array of data in JSON format holding the names, population, number of countries in the continent, and the area in kilometers

const continents = [
    id: 1,
    name: "Asia",
    population: "4,624,520,000",
    no_of_countries: 50,
    area: "44,579,000"
    id: 2,
    name: "Africa",
    population: "1,327,042,300",
    no_of_countries: 54,
    area: "30,370,000"
    id: 3,
    name: "North America",
    population: "590,176,500",
    no_of_countries: 23,
    area: "24,709,000"
    id: 4,
    name: "South America",
    population: "429,276,300",
    no_of_countries: 12,
    area: "17,840,000"
    id: 5,
    name: "Antartica",
    population: "No real data on populants",
    no_of_countries: 0,
    area: "14,000,000"
    id: 6,
    name: "Europe",
    population: "747,447,200",
    no_of_countries: 51,
    area: "10,180,000"
    id: 7,
    name: "Australia",
    population: "42,448,700",
    no_of_countries: 14,
    area: "8,600,000"

Now that we have our continents’ data stored in the continents variable, we will write the handler for the API that allows us retrieve the data as well as start the backend:

app.get("/", (req, res) => {

app.listen(8081, () => {
  console.log("App's running on port 8081");

We have successfully completed the backend app! We can start it with the command:

node app.js

We get a running message, and navigating to the url localhost:8081 returns a list of the continent and its data.

Next we’ll write the Svelte app’s component to retrieve and render data.

Writing the Svelte components

As we have seen above, the Svelte app displays its default landing page, and we have completed the backend. The next step is to write our Svelte components and redesign the app to render our continents data. We’ll be writing two components:

  • Continent: This component renders the data of the continents passed as a prop to it from the Continents component
  • Continents: This component retrieves the list of continents from the backend and renders them through the Continent component

We’ll begin by writing the Continent component that renders the data of continents passed to it from the Continents component.


We’ll begin by creating a prop, continent, in the <script> section of the component.

  // create a prop
  export let continent;

The continent prop will be used to render data, just as in other libraries like React and Vue.

Next, we render the data from the prop. Remember that from our API, we have the following data: name, population, number of countries, and area. We’ll render this just below the script tags:

      Population: <b>{continent.population}</b>   
      Number of countries: <b>{continent.no_of_countries}</b>
      Continent's size: <b>{continent.area}</b>

Great! Next, we’ll add a little styling :

  article {
    margin: 0 0 1em 0;
  h1 {
    font-size: 1.4em;
    margin: 0;
    display: block;

We have successfully completed our Continent component, this is quite straightforward than in other libraries where you have to write plenty code for a component. Next, we write the Continents component.


In this component, we retrieve the list of continents from the backend, iterate over it, and pass each continent as a prop to the Continent component to render it. We’ll begin by importing the onMount() method and the Continent component.

  import { onMount } from "svelte";
  import Continent from "./Continent.svelte";
  // define the data holding variable
  let continents;

Next, we define the onMount method that executes as soon as the Continents component is rendered.

onMount(async () => {
    await fetch(`http://localhost:8081/`)
      .then(r => r.json())
      .then(data => {
        continents = data;


Next thing is to iterate over the continents data retrieved and pass each one as a prop to the Continent. This is done through Svelte’s built-in conditional support.

{#if continents}
  {#each continents as continent }
        <Continent {continent} />
  <p class="loading">loading...</p>

In the code above, we first check if the data has been retrieved. If yes, the data is iterated and rendered through the Continent component, as can be seen in lines 2–8. Otherwise, it displays a loading message.

onMount() component method

Just as we have componentDidMount() in React, we also have the onMount() method in Svelte.
This method is a function that is executed when the component is rendered. It can take a predefined function as an argument, or a function can be defined in it, as seen above.

Next, we add a little styling:

  .loading {
    opacity: 0;
    animation: 0.4s 0.8s forwards fade-in;
  @keyframes fade-in {
    from { opacity: 0; }
    to { opacity: 1; }
  li {
    list-style-type: georgian;
Rendering the app

We’ve successfully written the components, and the next step is to render the data via the app’s main component. We’ll be rewriting the App component:

  import { onMount } from "svelte";
  import Continent from "./Continent.svelte";
  import Continents from "./Continents.svelte";
  let continents;
  let continent;

<h1>The Seven Continents Svelte App</h1>
    <Continents {continents} />

Svelte has a hot-reloading function pre-built, and so if we navigate to our application via [http://localhost:5000](http://localhost:5000), we get a screen as this:

Next we’ll change our app title and style our app a bit (if you’d like to keep it black-and-white, it’s OK to skip this 😊).

  <title>Svelte Continent App</title>

  main {
    background-color: lavenderblush;
    font-size: 15px;
  h1 {
    font-size: 25px;

Once saved, the app reloads and we have this screen:


In this article, we looked at how to consume and render data from a backend in Svelte, define and export props, and pass props to components. We also briefly looked at what the onMount() method is.

The inbuilt template system is also a great advantage to building simple apps since this function removes the need for excessive JavaScript conditionals we’d normally need in, say, React. After reading this tutorial, I believe you should now be able to write components and consume and render consumed data from an API — keep coding, and again, you can find the code used in this article here.

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI