oxc-project/oxc · warning · OxcDiagnostic

Parameter '{name}' is declared but only used as a type.{suff

Error message

Parameter '{name}' is declared but only used as a type.{suffix}

What it means

no-unused-vars parameter variant: a function parameter's only references are in type positions, so it is unused as a value. The suffix is suppressed for the exact name '_' under the default pattern; otherwise ignore-pattern guidance is appended.

Source

Thrown at crates/oxc_linter/src/rules/eslint/no_unused_vars/diagnostic.rs:126

/// 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() {
        std::borrow::Cow::Borrowed("")
    } else {
        pat.diagnostic_help("parameters")
    };

    OxcDiagnostic::warn(if only_used_as_type {
        format!("Parameter '{name}' is declared but only used as a type.{suffix}")
    } else {
        format!("Parameter '{name}' is declared but never used.{suffix}")
    })
    .with_label(symbol.span().label(format!("'{name}' is declared here")))
    .with_help("Consider removing this parameter.")
}

/// Identifier 'x' imported but never used.
pub fn imported(symbol: &Symbol<'_, '_>) -> OxcDiagnostic {
    let (pronoun, _) = pronoun_for_symbol(symbol.flags());
    let name = symbol.name();

    OxcDiagnostic::warn(format!("{pronoun} '{name}' is imported but never used."))
        .with_label(symbol.span().label(format!("'{name}' is imported here")))
        .with_help("Consider removing this import.")
}

View on GitHub (pinned to e1e7af627c)

Solutions

1. Remove the parameter (and update call sites) when it serves no purpose.
2. Rename to _name or _ so it matches the default argsIgnorePattern convention.
3. Re-derive the type from the annotation instead of the parameter: type E = Event.
4. Adjust argsIgnorePattern in the rule options for project-wide conventions.

Example fix

// before
function handler(event: Event) {
  type E = typeof event;
  register<E>();
}
// after
type E = Event;
function handler() { register<E>(); }

When it happens

Trigger: function handler(event: Event) where the body only uses it as type E = typeof event; callback parameters kept for the signature but only referenced in annotations.

Common situations: TypeScript inference helpers; callbacks whose parameters exist to satisfy a signature; refactors that moved value uses out but left type aliases behind.

Related errors

• {pronoun} is {verb} but only used as a type.{suffix}
• Parameter '{name}' is declared but never used.{suffix}
• Default parameters should be last
• Function '{}' has too many parameters ({}). Maximum allowed
• Function has too many parameters ({}). Maximum allowed is {}

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/21e99b61a405e248. Report an issue: GitHub.