void <identifier> statements are removed

[rChaoz]

Cause

void <identifier> statements are removed, you can test by running await transform("<input>"):

// input:
let a = 123; void a
// output:
let a = 123;

Why does this matter?

In Svelte, runes are used to implement reactivity. For example, let a = $state(123) is transformed into let a = $.state(123), and all accesses of the variable are transformed from a to $.get(a). Most importantly, $.get() inside special sections called effects, will be tracked, causing the effect code to re-run whenever the variable changes. Svelte's transformation happens after esbuild.

Sample transformation:

// input:
let a = $state(0)
$effect(() => {
    void a;
    console.log("effect runs")
})
// esbuild:
let a = $state(0)
$effect(() => {
    console.log("effect runs")
})
// svelte:
import * as $ from "svelte/internal"
let a = $.state(0)
$.user_effect(() => {
    console.log("effect runs")
})

The correct final output should be:

import * as $ from "svelte/internal"
let a = $.state(0)
$.user_effect(() => {
    void $.get(a)
    console.log("effect runs")
})

If esbuild were to run on this code, the void statement won't get removed, as it knows side-effects are possible.

Problem

I couldn't find a combination of options to prevent the void statement from getting removed. Is there an option to stop this from happening? If not, then I think either esbuild shouldn't remove such statements by default, especially when tree-shaking is not enabled, or at least this should be configurable somehow.

[hyrious]

It seems you can remove the void and esbuild will preserve the single identifider statement (see it online):

let a = $state(0)
$effect(() => { a; console.log(1) })
let f = () => a++

[threepointone]

why are you running esbuild before the svelte pass? esbuild can't give guarantees over custom syntax assumptions

[rChaoz]

Because of many reasons: this is the Vite pipeline, which uses esbuild before any other plugin, among other things, to transform Typescript into Javascript.