undefinedVariables

Reports using variables that are not defined.

✅ This rule is included in the ts untyped preset.

Variables must be declared before they can be used in JavaScript. Using undefined variables will cause a ReferenceError at runtime. This rule helps catch typos and missing declarations early in development.

Examples

❌ Incorrect

console.log(undefinedVar);

function calculate() {
  return result;
}

const value = first + second;

✅ Correct

const value = 5;
console.log(value);

function calculate(input: number) {
  const result = input * 2;
  return result;
}

const first = 1;
const second = 2;
const value = first + second;

Options

This rule is not configurable.

When Not To Use It

This rule can be safely disabled if you’re using TypeScript’s type checking, which will catch undefined variables through its own error reporting.

Further Reading

• MDN: ReferenceError
• MDN: Grammar and types - Declarations

Equivalents in Other Linters

• Biome: noUndeclaredVariables
• ESLint: no-undef
• Oxlint: eslint/no-undef