What causes "Global variable leak." in oxc-project/oxc?

Add a declaration (let isLoading = false) in the intended scope.

Global variable leak.

Error message

Global variable leak.

What it means

The leak diagnostic of oxlint's no-implicit-globals rule. An assignment to a name that is never declared anywhere creates an implicit global at runtime (and throws a ReferenceError under 'use strict' or in modules). The rule flags the assignment with 'Global variable leak.' and suggests declaring the variable if it is intended to be local.

Source

Thrown at crates/oxc_linter/src/rules/eslint/no_implicit_globals.rs:31

    rule::{DefaultRuleConfig, Rule},
};

fn global_non_lexical_binding_diagnostic(kind: &'static str, span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn(format!("Unexpected {kind} declaration in the global scope."))
        .with_help(
            "Wrap it in an IIFE for a local variable, or assign it as a global property for a global variable.",
        )
        .with_label(span)
}

fn global_lexical_binding_diagnostic(kind: &'static str, span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn(format!("Unexpected {kind} declaration in the global scope."))
        .with_help("Wrap it in a block or in an IIFE.")
        .with_label(span)
}

fn global_variable_leak_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Global variable leak.")
        .with_help("Declare the variable if it is intended to be local.")
        .with_label(span)
}

fn assignment_to_readonly_global_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected assignment to read-only global variable.").with_label(span)
}

fn redeclaration_of_readonly_global_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected redeclaration of read-only global variable.").with_label(span)
}

#[derive(Debug, Default, Clone, Serialize, Deserialize, JsonSchema)]
#[serde(rename_all = "camelCase", default, deny_unknown_fields)]
struct NoImplicitGlobalsConfig {
    lexical_bindings: bool,
}

View on GitHub (pinned to e1e7af627c)

Solutions

Add a declaration (let isLoading = false) in the intended scope.
Fix the typo in the identifier name.
If the value really is global, assign it explicitly on globalThis/window.
Enable strict mode or ES modules so the runtime also fails fast.

Example fix

// before
function start() {
  isLoading = true;
}

// after
let isLoading = false;
function start() {
  isLoading = true;
}

Defensive patterns

Strategy: validation

Validate before calling

npx oxlint -A all -D no-undef src/  // no-undef flags implicit-global writes

Prevention

Declare every variable you assign (let/const/var) in the intended scope.
Enable strict mode or ES modules so implicit globals throw at runtime.
Run no-undef in CI to catch typo'd targets before they leak.

When it happens

Trigger: isLoading = true; inside a function where no let isLoading exists anywhere; typo'd targets like resutl = compute(); a declaration removed during refactor while assignments remain.

Common situations: Sloppy-mode legacy code; typos in assignment targets; state declared in one branch and assumed to hoist; merges that drop a declaration but keep its writes.

Related errors

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/d5c2ffa5ed82ebfd. Report an issue: GitHub.