Resolve promises in Svelte templates

Promises are an awesome tool we have at our disposal to work with asynchronous events in JavaScript.

The relatively recent introduction of the await syntax in ES2017 made using promises even simpler.

Svelte provides us the {#await} syntax in templates to directly work with promises at the template level.

We can wait for promises to resolve, and define a different UI for the various states of a promise: unresolved, resolved and rejected.

Here’s how it works. We define a promise, and using the {#await} block we wait for it to resolve.

Once the promise resolves, the result is passed to the {:then} block:

<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}

You can detect a promise rejection by adding a {:catch} block:

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

Run the example: https://svelte.dev/repl/70e61d6cc91345cdaca2db9b7077a941