…#23540)

## What

A `var` initialized to a falsy constant and never reassigned is always falsy. oxc already folds such a `let`/`const` everywhere, but a `var` is held back by a hoisting check (a read before the declarator runs sees the hoisted `undefined`), so a multi-read `var` flag past a non-trivial declarative prelude was never folded.

In **boolean context** that check is unnecessary: a pre-init `var` read yields `undefined`, which is indistinguishable from the falsy init inside `if (x)` / `x ? … : …` / `!x`. So fold such reads to `false` there; the existing `if (false)` dead-code pass removes the branch.

This is the bundled-flag shape behind this issue. Svelte's `var hydrating = false` is read by `if (hydrating)` throughout the runtime; once a client-only bundle tree-shakes the `set_hydrating` setter, `hydrating` is a write-once falsy `var` and the whole hydration path becomes dead and is eliminated.

## Why it is sound

- `SymbolValue::boolean_falsy` is set only for a write-once binding with a falsy constant initializer.
- It folds to `false` **only** in boolean context; value-context reads keep their hoisting-correct behavior (never folded here).
- Gated off for a script's top-level `var` (a global another script can reassign, so an in-module write count of 0 doesn't prove write-once) and for `eval` scopes.

## Verification

- `oxc_minifier` tests pass; idempotent.
- On a real client-only Svelte (Vite/Rollup) bundle, every `if (hydrating)` check is eliminated.
- `minsize`: react 23.14 → 23.12 kB, lodash 70.96 → 70.92 kB (gzip down too). `allocs_minifier.snap`: +20 sys allocs on antd (from evaluating `var` initializers; arena allocs unchanged).
- Cross-validated for soundness: on the Svelte bundle, terser `--module` and esbuild `--format=esm` independently fold the same write-once flag (16 → 0), agreeing with oxc. On a Vue 3 client bundle the runtime's flags are multi-write (e.g. `isInSSRComponentSetup` has two assignments, so it can be truthy) and oxc correctly leaves them untouched — the write-once gate is the right line.

## Scope (partial fix)

This handles the real-world **bundled** shape — a write-once falsy `var` flag read in boolean context (the Svelte/Vue hydration case, where the bundler has already tree-shaken the setter). It does **not** fold the issue's literal source snippet:

```js
let foo = false;
function bar(val) { foo = val; }
if (foo) { bar(true) }
console.log(foo);
```

There `foo` is still reassigned via `bar` (so it isn't write-once), and proving the write dead requires self-referential / flow-sensitive analysis — out of scope here (see #22753 for that direction). So this is a partial step.

Refs #14001.

Prepared with AI assistance.

…#23540)

## What

A `var` initialized to a falsy constant and never reassigned is always falsy. oxc already folds such a `let`/`const` everywhere, but a `var` is held back by a hoisting check (a read before the declarator runs sees the hoisted `undefined`), so a multi-read `var` flag past a non-trivial declarative prelude was never folded.

In **boolean context** that check is unnecessary: a pre-init `var` read yields `undefined`, which is indistinguishable from the falsy init inside `if (x)` / `x ? … : …` / `!x`. So fold such reads to `false` there; the existing `if (false)` dead-code pass removes the branch.

This is the bundled-flag shape behind this issue. Svelte's `var hydrating = false` is read by `if (hydrating)` throughout the runtime; once a client-only bundle tree-shakes the `set_hydrating` setter, `hydrating` is a write-once falsy `var` and the whole hydration path becomes dead and is eliminated.

## Why it is sound

- `SymbolValue::boolean_falsy` is set only for a write-once binding with a falsy constant initializer.
- It folds to `false` **only** in boolean context; value-context reads keep their hoisting-correct behavior (never folded here).
- Gated off for a script's top-level `var` (a global another script can reassign, so an in-module write count of 0 doesn't prove write-once) and for `eval` scopes.

## Verification

- `oxc_minifier` tests pass; idempotent.
- On a real client-only Svelte (Vite/Rollup) bundle, every `if (hydrating)` check is eliminated.
- `minsize`: react 23.14 → 23.12 kB, lodash 70.96 → 70.92 kB (gzip down too). `allocs_minifier.snap`: +20 sys allocs on antd (from evaluating `var` initializers; arena allocs unchanged).
- Cross-validated for soundness: on the Svelte bundle, terser `--module` and esbuild `--format=esm` independently fold the same write-once flag (16 → 0), agreeing with oxc. On a Vue 3 client bundle the runtime's flags are multi-write (e.g. `isInSSRComponentSetup` has two assignments, so it can be truthy) and oxc correctly leaves them untouched — the write-once gate is the right line.

## Scope (partial fix)

This handles the real-world **bundled** shape — a write-once falsy `var` flag read in boolean context (the Svelte/Vue hydration case, where the bundler has already tree-shaken the setter). It does **not** fold the issue's literal source snippet:

```js
let foo = false;
function bar(val) { foo = val; }
if (foo) { bar(true) }
console.log(foo);
```

There `foo` is still reassigned via `bar` (so it isn't write-once), and proving the write dead requires self-referential / flow-sensitive analysis — out of scope here (see #22753 for that direction). So this is a partial step.

Refs #14001.

Prepared with AI assistance.