#no-undef

#Configuration

import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-undef': 'error',
    },
  },
]);

Disallow the use of undeclared variables.

This rule reports identifiers that reference variables which have not been declared via var, let, const, function, class, import, or as a parameter.

Resolution is lexical scope analysis: declared bindings, standard ECMAScript built-ins (Array, Promise, JSON, undefined, and similar), and anything declared through languageOptions.globals or a /* global */ comment. On a file with type information available, names known only to the type checker — DOM/Node globals such as console or window, or anything declared in a .d.ts file — are recognized too.

Enable this rule for files that aren't type-checked. On a type-checked file, tsc already reports references to undeclared names. On files without type information, declare the host globals your code relies on (console, process, setTimeout, and similar) through languageOptions.globals or a /* global */ comment, the same way ESLint's own no-undef expects.

#Options

#typeof

Type: boolean Default: false

When set to true, typeof expressions will be checked for undeclared variables. By default, typeof of an undeclared variable does not trigger a warning, since typeof returns "undefined" for undeclared variables without throwing a ReferenceError.

#Examples

#Invalid

a = 1; // 'a' is not defined.
var x = b; // 'b' is not defined.
undeclaredFunc(); // 'undeclaredFunc' is not defined.

With { "typeof": true }:

typeof x === 'string'; // 'x' is not defined.

#Valid

var a = 1;
a;

function f() {}
f();

typeof maybeUndefined === 'string';