Resolving Promises in Svelte Templates

In this blog post, we will explore how to effectively work with promises in Svelte templates and leverage their power in managing asynchronous events in JavaScript.

Promises have become a valuable tool in handling asynchronous operations, and with the introduction of the await syntax in ES2017, working with promises has become even easier.

Svelte provides the {#await} syntax in templates, which allows us to directly incorporate promises at the template level. This functionality allows us to wait for promises to resolve and define different user interface states for the different stages of a promise: unresolved, resolved, and rejected.

Let’s take a closer look at how it works. First, we define a promise, and using the {#await} block, we await its resolution. Once the promise resolves, we can access the result in the {:then} block. Here’s an example:

<script>
  const fetchImage = (async () => {
    const response = await fetch('https://dog.ceo/api/breeds/image/random');
    return await response.json();
  })();
</script>

{#await fetchImage}
  <p>...waiting</p>
{:then data}
  <img src={data.message} alt="Dog image" />
{/await}

In this code snippet, we define a fetchImage promise that asynchronously fetches a random dog image from a web API. Within the template, the {#await} block is used to display a loading message while waiting for the promise to resolve. Once the promise resolves, the dog image URL is accessible in the {:then} block, and we can render the image accordingly.

If the promise encounters an error and gets rejected, we can handle it using the {:catch} block. Here’s an example:

{#await fetchImage}
  <p>...waiting</p>
{:then data}
  <img src={data.message} alt="Dog image" />
{:catch error}
  <p>An error occurred!</p>
{/await}

In this case, if the promise gets rejected, the {:catch} block will catch the error and display an appropriate message.

You can try out the example and see the code in action in the Svelte REPL.

By utilizing the {#await} syntax in Svelte templates, we can seamlessly incorporate promises into our UI and create interactive experiences that effectively handle asynchronous events.