oxc-project/oxc · warning · OxcDiagnostic

' ' is already defined.

Error message

'{name}' is already defined.

What it means

Diagnostic from oxlint's port of ESLint `no-redeclare`. It fires when the same binding name is declared more than once within one scope. JS only permits this for `var`/`function` (and TS redeclare forms), and it usually means a bug or a merge leftover. The primary label points at the original declaration, the secondary label at the redeclaration.

Solutions

1. Delete or rename the duplicate so each name is declared once per scope.
2. Replace the second declaration with a plain assignment (`a = 10` instead of `var a = 10`).
3. For intended overloading in TS, use one function with union parameter types, or separate the variants behind `declare function` in a namespace.
4. If the duplication is intentional, add an `oxlint-ignore no-redeclare` comment or disable the rule in .oxlintrc.

Example fix

// before
var a = 3;
var a = 10;

// after
var a = 3;
a = 10;

// Pre-commit check: catch duplicate var/function names per top-level scope
const { execSync } = require('node:child_process');
const src = require('node:fs').readFileSync(process.argv[2], 'utf8');
const seen = new Set();
for (const m of src.matchAll(/(?:var|let|const|function)\s+([A-Za-z_$][\w$]*)/g)) {
  if (seen.has(m[1])) {
    console.error(`duplicate declaration: ${m[1]}`);
    process.exitCode = 1;
  }
  seen.add(m[1]);
}

When it happens

Trigger: `run_once` walks every symbol in the semantic scoping data and reports each adjacent pair in `symbol_redeclarations`: `var a = 1; var a = 2;`, two non-`declare` function declarations of the same name in one scope, `var f; function f(){}`. In TypeScript sources, TS redeclaration entries are compared too (`class C {} var C;`).

Common situations: Copy-paste or git-merge leftovers; migrating loose scripts into a linted repo; TypeScript code that tries to overload functions by declaring them twice instead of using a single signature with union types.

Related errors

• ' ' is already declared in the upper scope.
• Unexpected var, use let or const instead.
• All 'var' declarations must be at the top of the function…
• ' ' is already defined as a built-in global variable.
• Variable declarations should be sorted

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/ad3808d9a9193d31. Report an issue: GitHub.

Appendix: source

Thrown at crates/oxc_linter/src/rules/eslint/no_redeclare.rs:15

use javascript_globals::GLOBALS_BUILTIN;

use oxc_diagnostics::OxcDiagnostic;
use oxc_macros::declare_oxc_lint;
use oxc_span::Span;
use schemars::JsonSchema;
use serde::Deserialize;

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

fn no_redeclare_diagnostic(name: &str, decl_span: Span, re_decl_span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn(format!("'{name}' is already defined."))
        .with_help("Use a different variable name or remove the duplicate declaration.")
        .with_labels([
            decl_span.label(format!("'{name}' is already defined.")),
            re_decl_span.label("It can not be redeclared here."),
        ])
}

fn no_redeclare_as_builtin_in_diagnostic(name: &str, span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn(format!("'{name}' is already defined as a built-in global variable."))
        .with_help("Use a different variable name to avoid shadowing the built-in global.")
        .with_label(span)
}

#[derive(Debug, Clone, JsonSchema, Deserialize)]
#[serde(rename_all = "camelCase", default, deny_unknown_fields)]
pub struct NoRedeclare {
    /// When set `true`, it flags redeclaring built-in globals (e.g., `let Object = 1;`).
    builtin_globals: bool,

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