unassignedVariables

Reports variables that are declared but never assigned a value.

✅ This rule is included in the ts untyped preset.

Variables declared without an initializer and never assigned a value will always be undefined. This is often a mistake, such as forgetting to initialize a variable or assign to it before use.

Examples

❌ Incorrect

let value;
console.log(value);

function calculate() {
  let result;
  return result;
}

var unassigned: number;
processValue(unassigned);

✅ Correct

let value = 42;
console.log(value);

function calculate() {
  let result = 0;
  return result;
}

let assigned: number;
assigned = 10;
processValue(assigned);

Options

This rule is not configurable.

When Not To Use It

If you intentionally want to work with unassigned variables that default to undefined, you might want to disable this rule. However, it’s generally better to explicitly assign undefined to make the intent clear.

Further Reading

• MDN documentation on variable declarations
• MDN documentation on undefined

Equivalents in Other Linters

• Biome: noUnassignedVariables
• ESLint: no-unassigned-vars
• Oxlint: eslint/no-unassigned-vars