oxc-project/oxc · warning · OxcDiagnostic

Parameter '{name}' is declared but never used.{suffix}

Error message

Parameter '{name}' is declared but never used.{suffix}

What it means

Fires when the no_unused_vars rule finds a function parameter binding that is never referenced in the function body (or only inside type positions when only_used_as_type is true). The suffix carries ignore-pattern help text, or is empty for the default '_' parameter under the default ignore pattern.

Source

Thrown at crates/oxc_linter/src/rules/eslint/no_unused_vars/diagnostic.rs:128

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. Rename to _index / _ to match the default underscore ignore convention.
2. Remove the parameter entirely when it is trailing and unneeded: items.map((item) => ...).
3. Tune rule options (args, argsIgnorePattern) in .oxlintrc.json to your team's strictness.
4. Use rest parameters to absorb ignored positionals when arity must be preserved.

Example fix

// before
items.map((item, index) => item * 2);
// after
items.map((item, _index) => item * 2);
// or: items.map((item) => item * 2);

When it happens

Trigger: items.map((item, index) => item * 2) with index unused; function f(a, b) { return a; }; interface method implementations with unused parameters.

Common situations: Framework callbacks with fixed signatures (express handlers, React props, test runners); unused arguments positioned before used ones; refactors that dropped the last use of a parameter.

Related errors

• Parameter '{name}' is declared but only used as a type.{suff
• Default parameters should be last
• Function '{}' has too many parameters ({}). Maximum allowed
• Function has too many parameters ({}). Maximum allowed is {}
• Arrow function has too many parameters ({}). Maximum allowed

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/8460e6db286bf27a. Report an issue: GitHub.