Bug report: no error when using `@const` variable before initialization

[brandonmcconnell]

Describe the bug

I was trying to include a space after a comma in an if-statement. Consider this repro:

<script>
  const length = 10;
</script>

{#each { length } as _, i}
  {@const nth = i + 1}
  {nth}{#if nth !== length}, {/if}
{/each}

REPL

I did want the space to be a breaking space, so   wouldn't work here. I tried   which I believe is the equivalent code for a traditional breaking space, but that did not work either. (e.g. {nth}{#if nth !== length}, {/if}).

Finally, I did something accidentally and reordered the two lines inside the enclosing {#each} block, only to find that change did achieve what I was trying to do even without a space or special char, but I feel like it shouldn't since the variable is now declared after it's used.

<script>
  const length = 10;
</script>

{#each { length } as _, i}
  {nth}{#if nth !== length},{/if}
  {@const nth = i + 1}
{/each}

REPL

Reproduction

REPL links above

Logs

No response

System Info

macOS v13.1 (22C65)
Svelte v3.58.0 (in REPL)

Severity

annoyance

[sunshine001]

This is whether the check is to be done at compile time or at runtime?

[brandonmcconnell]

This is whether the check is to be done at compile time or at runtime?

The issue itself is not about the compile/run-time timing but rather that (1) the variable can be used before its initialization, which should produce an error, and (2) doing so makes a noticeable difference in terms of whitespace rendered to the DOM.

[kelvinsjk]

Turns out the "hoisting" property of the @const tag was discussed in its RFC and seems to be a feature with its own unit test

The missing space error within the #if tag is probably the same as #6381.

Did a little bit of experiment, and the extra whitespace created in the second REPL is due to the whitespace between ending {/if} tag and the @const tag in the next line. This behavior is observed even when tags other than @const is used.

<!-- no extra whitespace -->
{#if condition},{/if}{@const a=1} 
<!-- extra whitespace -->
{#if condition},{/if}
{@const a=1} 
<!-- extra whitespace -->
{#if condition},{/if} {#if false}never{/if}

Finally, the workaround for your original issue of the space after the comma may be to invoke a JavaScript expression

{#if condition}{", "}{/if}

My experiments in this REPL

[brandonmcconnell]

@kelvinsjk Thanks, I haven't tested it locally, but if the JS expression workaround works, that would fix my original issue!

Aside from that, do you think it would make sense to repurpose this ticket, removing that example, focusing solely on the lack of an error?

[7nik]

So it isn't a bug, and Svelte 5 still has the test for @const hoisting. Should it be documented then?