Svelte:Conditional Logic in Templates

In a Svelte component, when it comes to rendering HTML you can work with some specific syntax to craft the UI you need at every stage of the application lifecycle.

In particular, we’ll now explore conditional structures.

The problem is this: you want to be able to look at a value/expression, and if that points to a true value do something, if that points to a false value then do something else.

Svelte provides us a very powerful set of control structures.

The first is if:

{#if isRed}
  <p>Red</p>
{/if}

There is an opening {#if} and an ending {/if}. The opening markup checks for a value or statement to be truthy. In this case isRed can be a boolean with a true value:

<script>
let isRed = true
</script>

An empty string is falsy, but a string with some content is truthy.

0 is falsy, but a number > 0 is truthy.

The boolean value true is truthy, of course, and false is falsy.

If the opening markup is not satisfied (a falsy value is provided), then nothing happens.

To do something else if that’s not satisfied, we use the appropriately called else statement:

{#if isRed}
  <p>Red</p>
{:else}
  <p>Not red</p>
{/if}

Either the first block is rendered in the template or the second one. There’s no other option.

You can use any JavaScript expression into the if block condition, so you can negate an option using the ! operator:

{#if !isRed}
  <p>Not red</p>
{:else}
  <p>Red</p>
{/if}

Now, inside the else you might want to check for an additional condition. That’s where the {:else if somethingElse} syntax comes along:

{#if isRed}
  <p>Red</p>
{:else if isGreen}
  <p>Green</p>
{:else}
  <p>Not red nor green</p>
{/if}

You can have many of these blocks, not just one, and you can nest them. Here’s a more complex example:

{#if isRed}
  <p>Red</p>
{:else if isGreen}
  <p>Green</p>
{:else if isBlue}
  <p>It is blue</p>
{:else}
  {#if isDog}
    <p>It is a dog</p>
  {/if}
{/if}