oxc-project/oxc · warning · OxcDiagnostic

' ' is assigned a value but never used.

Error message

{pronoun} '{name}' is assigned a value but never used.{suffix}

What it means

no-unused-vars dead-store message: a value is assigned to an already-declared variable and that particular value is never read afterward (it is overwritten first, or its scope/function ends). Distinct from 'never used': the variable exists, but this stored value is wasted. Two labels point at the declaration and at the dead assignment.

Solutions

Delete the assignment whose stored value is never read.
Hoist to a single const at the point of first use.
If the right-hand side has needed side effects, keep the call but discard its result deliberately and visibly.

Example fix

// before
let total;
total = computeBase();
total = computeBase() + extra;
use(total);
// after
const total = computeBase() + extra;
use(total);

Defensive patterns

Strategy: validation

Prevention

Prefer const at the point of first use over declare-then-assign.
When adding an early return, check whether assignments above it are still read.
Review wholesale reassignments of accumulators; they usually make earlier stores dead.

When it happens

Trigger: let x = 1; x = compute(); use(x); where the first store is dead; an assignment immediately before a return/throw that never reads it; assignments in a branch that then exits without merging.

Common situations: Accumulator variables rebuilt by wholesale reassignment; initialization 'just in case'; assignments left behind after the last consumer was removed.

Related errors

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/8fca36b1d88d3839. Report an issue: GitHub.

Appendix: source

Thrown at crates/oxc_linter/src/rules/eslint/no_unused_vars/diagnostic.rs:101

    })
    .with_label(symbol.span().label(format!("'{name}' is declared here")))
    .with_help(help)
}

/// Variable 'x' is assigned a value but never used.
pub fn assign<R>(
    symbol: &Symbol<'_, '_>,
    assign_span: Span,
    pat: &IgnorePattern<R>,
) -> OxcDiagnostic
where
    R: fmt::Display,
{
    let name = symbol.name();
    let (pronoun, pronoun_plural) = pronoun_for_symbol(symbol.flags());
    let suffix = pat.diagnostic_help(pronoun_plural);

    OxcDiagnostic::warn(format!("{pronoun} '{name}' is assigned a value but never used.{suffix}"))
        .with_labels([
            symbol.span().label(format!("'{name}' is declared here")),
            assign_span.label("it was last assigned here"),
        ])
        .with_help("Did you mean to use this variable?")
}

/// Parameter 'x' is declared but never used.
pub fn param<R>(
    symbol: &Symbol<'_, '_>,
    pat: &IgnorePattern<R>,
    only_used_as_type: bool,
) -> OxcDiagnostic
where
    R: fmt::Display,
{
    let name = symbol.name();
    let suffix = if name == "_" && pat.is_default() {

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