oxc-project/oxc · warning · OxcDiagnostic

'{name}' was used before it was defined.

Error message

'{name}' was used before it was defined.

What it means

Diagnostic from oxlint's no-use-before-define rule. An identifier is referenced at a point where its declaration has not been reached in source order; for let/const/class/enum bindings this is a runtime TDZ ReferenceError, for hoisted var/function it merely hurts readability. Config flags control which kinds are checked (defaults: classes, enums, functions, typedefs, variables all true; ignoreTypeReferences true; allowNamedExports false).

Source

Thrown at crates/oxc_linter/src/rules/eslint/no_use_before_define.rs:27

    reference::Reference,
    scope::ScopeId,
    symbol::{SymbolFlags, SymbolId},
};
use schemars::JsonSchema;
use serde::{Deserialize, Serialize};

use crate::{
    AstNode,
    context::LintContext,
    rule::{DefaultRuleConfig, Rule},
};

fn no_use_before_define_diagnostic(
    name: &str,
    usage_span: Span,
    declaration_span: Option<Span>,
) -> OxcDiagnostic {
    let diagnostic = OxcDiagnostic::warn(format!("'{name}' was used before it was defined."))
        .with_label(usage_span.primary_label("used here"))
        .with_help("Move the declaration before any references to it, or remove the reference if it is not needed.");
    if let Some(declaration_span) = declaration_span.filter(|span| !span.is_unspanned()) {
        diagnostic.and_label(declaration_span.label("defined here"))
    } else {
        diagnostic
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, JsonSchema)]
#[serde(rename_all = "camelCase", default, deny_unknown_fields)]
pub struct NoUseBeforeDefineConfig {
    /// Allow named exports that appear before declaration.
    allow_named_exports: bool,
    /// Check class declarations.
    classes: bool,
    /// Check enum declarations.
    enums: bool,

View on GitHub (pinned to a3d33dda7c)

Solutions

1. Move the declaration above the first use.
2. Convert const fn = () => {} to a hoisted function fn() {} when early calls are intended.
3. Relax specific kinds in .oxlintrc.json, e.g. { "functions": false } (same for classes/variables/enums/typedefs).
4. For type-only forward references, keep "ignoreTypeReferences": true (default) instead of disabling the rule.

Example fix

// before
const main = () => helper();
const helper = () => 1;
// after
const helper = () => 1;
const main = () => helper();

When it happens

Trigger: Calling a const arrow function before its declaration; new Foo() above class Foo {}; referencing an enum member before the enum; using a variable declared with let later in the same scope.

Common situations: Mutually recursive helpers written bottom-up; hoisting-reliant code migrated to const arrow functions; circular imports; named exports referenced before declaration in module scope.

Related errors

• Variable or `function` declarations are not allowed in neste
• All 'var' declarations must be at the top of the function sc
• TS5042
• Empty array binding pattern
• Empty object binding pattern

AI-assisted analysis of oxc-project/oxc@a3d33dda7c (2026-08-20). Data as JSON: /api/errors/b67d248d9ccb5c0a. Report an issue: GitHub.