Reactive Statements in Svelte

How to work with Reactive Statements in Svelte

In Svelte you can listen for changes in the component state, and update other variables.

For example if you have a count variable:

<script>
let count = 0
</script>

and you update it by clicking a button:

<script>
let count = 0

const incrementCount = () => {
  count = count + 1
}
</script>

{count} <button on:click={incrementCount}>+1</button>

You can listen for changes on count using the special syntax $: which defines a new block that Svelte will re-run when any variable referenced into it changes.

Here’s an example:

<script>
let count = 0

const incrementCount = () => {
  count = count + 1
}

$: console.log(`${count}`)
</script>

{count} <button on:click={incrementCount}>+1</button>

I used the block:

$: console.log(`${count}`)

You can write more than one of them:

<script>
$: console.log(`the count is ${count}`)
$: console.log(`double the count is ${count * 2}`)
</script>

And you can also add a block to group more than one statement:

<script>
$: {
  console.log(`the count is ${count}`)
  console.log(`double the count is ${count * 2}`)
}
</script>

I used a console.log() call in there, but you can update other variables too:

<script>
let count = 0
let double = 0

$: {
  console.log(`the count is ${count}`)
  double = count * 2
  console.log(`double the count is ${double}`)
}
</script>