oxc-project/oxc · warning · OxcDiagnostic

Expected a function {style}.

Error message

Expected a function {style}.

What it means

Oxlint's port of the ESLint rule func-style: it forces one definition style per codebase. The Style enum (camelCase serde over kebab-case values) is `expression` (the default) or `declaration` (crates/oxc_linter/src/rules/eslint/func_style.rs:20). Under `expression`, top-level `function foo() {}` declarations are reported with 'Expected a function expression.'; under `declaration`, `const foo = function () {}` assignments are reported with 'Expected a function declaration.'.

Source

Thrown at crates/oxc_linter/src/rules/eslint/func_style.rs:22

use oxc_ast::{
    AstKind,
    ast::{ArrowFunctionExpression, Function, FunctionType, Super, ThisExpression},
};
use oxc_ast_visit::VisitJs;
use oxc_diagnostics::OxcDiagnostic;
use oxc_macros::declare_oxc_lint;
use oxc_semantic::{AstNode, ScopeFlags};
use oxc_span::Span;

use crate::{
    ast_util::nth_outermost_paren_parent,
    context::LintContext,
    rule::{Rule, TupleRuleConfig},
};

fn func_style_diagnostic(span: Span, style: &str) -> OxcDiagnostic {
    OxcDiagnostic::warn(format!("Expected a function {style}."))
        .with_help("Enforce the consistent use of either `function` declarations or expressions assigned to variables")
        .with_label(span)
}

#[derive(Debug, Default, PartialEq, Clone, Copy, Deserialize, Serialize, JsonSchema)]
#[serde(rename_all = "camelCase")]
enum Style {
    #[default]
    Expression,
    Declaration,
}

impl Style {
    pub fn as_str(&self) -> &str {
        match self {
            Style::Expression => "expression",
            Style::Declaration => "declaration",
        }

View on GitHub (pinned to e1e7af627c)

Solutions

1. Convert the flagged function to the enforced style (expression: const + function/arrow; declaration: `function name()`).
2. Choose the option matching the majority of the codebase to minimize churn.
3. Disable the rule if the team accepts both styles.
4. Watch for hoisting: if the function is used before its definition point, a declaration is required.

Example fix

// before (style: expression)
function helper(x) {
  return x * 2;
}

// after
const helper = (x) => x * 2;

// .oxlintrc.json — pick the dominant style of the codebase
{
  "rules": {
    "func-style": ["warn", "expression"]
  }
}

When it happens

Trigger: Config ["warn", "expression"] plus a `function helper() {}` declaration; config ["warn", "declaration"] plus a `const helper = function () {}` assignment.

Common situations: Standardizing a codebase that grew with mixed styles; hoisting-dependent code that actually requires declarations; formatter rewrites that do not change the underlying style.

Related errors

• Expected method{method_name_str} to have this.
• {prefix} {{ after '{keyword}'{condition_if_needed}.
• Enforce default clauses in switch statements to be last
• Function name `{func_name}` should match property name `{nam
• Function name `{func_name}` should match variable name `{nam

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/846adc849b3924c5. Report an issue: GitHub.