Svelte templates: conditional logic

Any good templating language for the Web provides you at least 2 things: a conditional structure, and a loop.

Svelte is no exception, and in this post I’ll look into conditional structures.

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}