Consume a JSON REST API in Svelte 3 with fetch(), onMount() and async/await

In this tutorial, we'll see an example of consuming a third-party JSON REST API in Svelte.js 3 using the fetch() method available in modern browsers. We'll also see how to import and use the Svelte's onMount() life-cycle method to run code when the component is mounted and the async/await syntax to await a JavaScript promise. Finally, we'll see how to iterate over the fetched data in the template with the each block.

These are the steps of this example:

• Step 1 - Initializing a Svelte 3 Project
• Step 2 - Fetching the JSON Data
• Step 3 - Displaying Data
• Step 4 - Serving the App

We'll learn:

• How to generate a Svelte 3 app and serve it locally,
• How to use the onMount() life-cycle method to run code when the component is mounted in the DOM
• How to consume a REST API and fetch JSON data from a remote server using the fetch() method
• How to use the async/await syntax with the fetch() and onMount() methods to make asynchronous code look synchronous and avoid using callbacks
• How to iterate over data using the each block

We assume you have Node.js and NPM installed on your local development machine.

Step 1 - Initializing a Svelte 3 Project

Let's start with our first step where we'll initialize our Svelte 3 project using the degit tool.

In your command-line interface, run the following command:

$ npx degit sveltejs/template svelte-fetch-example

Step 2 - Fetching the JSON Data

Next, open the the App.svelte file where the root component of our app exists and add the following code:

<script>
    import { onMount } from "svelte";

    const apiURL = "https://jsonplaceholder.typicode.com/todos";
    let data = [];

</script>

We imported the onMount() life-cycle method from “svelte” and defined a couple of variables. Namly the apiURL and data variables, which hold the URL of the REST API that we're going to consume and the items that will be fetched .

Next, simply call the fetch() method inside the onMount method as follows:

<script>
    // [...]

    onMount(async function() {
        const response = await fetch(apiURL);
        data = await response.json();
    });
</script>    

The fetch() method returns a JavaScript Promise, this means we can use the async/await syntax to avoid using callbacks.

Step 3 - Displaying Data

In the same App.svelte file, add the following HTML markup Next, let’s add the following HTML markup to create the UI of our application and display the news data:

        {#each data as item }
            <div>
                <p> {item.title} </p>
            </div>
        {/each}

</div>

We use the each block to iterate over the fetched data items and we display the title of each item.

Step 4 - Serving the App & Conclusion

After adding the code, let's now serve our app locally to make sure this works. In your terminal, navigate to your project's folder and start the server as follows

$ cd svelte-fetch-example
$ npm start

As a recap, we've seen how to consume a JSON REST API from a third-party server in Svelte 3 using the JavaScript fetch() method, the Svelte' onMount() method and ES7 async/await syntax. We've also seen how to iterate over the fetched data using the each block.

• Author: Ahmed Bouchefra Follow @ahmedbouchefra

• Date: 17 Nov 2019