oxc-project/oxc · error · OxcDiagnostic

Compare with `undefined` directly instead of using `typeof`.

Error message

Compare with `undefined` directly instead of using `typeof`.

What it means

Raised by unicorn/no-typeof-undefined when code tests for undefined with `typeof x === 'undefined'`. Except for potentially undeclared variables, a direct comparison is clearer; the faulting input is the typeof-based comparison at the labeled span (an autofix usually rewrites it).

Source

Thrown at crates/oxc_linter/src/rules/unicorn/no_typeof_undefined.rs:18

use oxc_ast::{AstKind, ast::Expression};
use oxc_diagnostics::OxcDiagnostic;
use oxc_macros::declare_oxc_lint;
use oxc_span::{GetSpan, Span};
use oxc_syntax::operator::{BinaryOperator, UnaryOperator};
use schemars::JsonSchema;
use serde::Deserialize;

use crate::{
    AstNode,
    ast_util::get_declaration_of_variable,
    context::LintContext,
    fixer::{RuleFix, RuleFixer},
    rule::{DefaultRuleConfig, Rule},
};

fn no_typeof_undefined_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Compare with `undefined` directly instead of using `typeof`.")
        .with_label(span)
}

#[derive(Debug, Default, Clone, JsonSchema, Deserialize)]
#[serde(rename_all = "camelCase", default, deny_unknown_fields)]
pub struct NoTypeofUndefined {
    /// If set to `true`, also report `typeof x === "undefined"` when `x` may be a global
    /// variable that is not declared (commonly checked via `typeof foo === "undefined"`).
    check_global_variables: bool,
}

declare_oxc_lint!(
    /// ### What it does
    ///
    /// Disallow `typeof` comparisons with `undefined`.
    ///
    /// ### Why is this bad?
    ///

View on GitHub (pinned to e1e7af627c)

Solutions

1. Compare directly: `x === undefined` or `x === void 0`
2. Keep `typeof` only when the variable may be genuinely undeclared and that must be tolerated

When it happens

Trigger: Thrown at crates/oxc_linter/src/rules/unicorn/no_typeof_undefined.rs:18 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/d36a1b9620ce476a. Report an issue: GitHub.