typeofComparisons

Reports typeof expressions that compare impossible string literals.

✅ This rule is included in the ts javascript presets.

The typeof operator in JavaScript returns one of a specific set of string values: "bigint", "boolean", "function", "number", "object", "string", "symbol", or "undefined". Comparing the result of typeof to any other string literal is usually a typo and the comparison will never be true. This rule enforces that typeof expressions are only compared to valid string literals.

Examples

❌ Incorrect

if (typeof value === "strnig") {
  processString(value);
}

if (typeof callback !== "fucntion") {
  throw new Error("Invalid callback");
}

if (typeof data == "undefimed") {
  initialize();
}

if (typeof count === "nunber") {
  calculate(count);
}

✅ Correct

if (typeof value === "string") {
  processString(value);
}

if (typeof callback !== "function") {
  throw new Error("Invalid callback");
}

if (typeof data == "undefined") {
  initialize();
}

if (typeof count === "number") {
  calculate(count);
}

Options

This rule is not configurable.

When Not To Use It

If your project is a rare one that operates on non-standard objects, such as ancient versions of obscure browsers, this rule might not benefit you. For all other projects, using either this rule or a full type-checker should always be done.

Further Reading

• MDN: typeof operator
• MDN: JavaScript data types

Equivalents in Other Linters

• Biome: useValidTypeof
• Deno: valid-typeof
• ESLint: valid-typeof
• Oxlint: eslint/valid-typeof