JS TypeOf

JavaScript typeof: Inspecting Value Types and Edge Cases

The JavaScript typeof operator returns a lowercase string describing the data type of an expression or variable. It helps you guard function inputs and check whether a global API exists without throwing a ReferenceError.

Complete typeof Return Values

Know these eight possible string results from typeof:

• "string", "number", "boolean", "bigint", "symbol", "undefined": Returned for those six primitive types.
• "function": Returned for functions and ES6 class constructors.
• "object": Returned for plain objects, arrays, Date, RegExp, Map, Set, and notoriously null.

Safe Type Checking Example

Because typeof null returns "object" and typeof [1, 2] also returns "object", pair typeof with explicit === null and Array.isArray() checks.

function describeValue(value) {
  if (value === null) return "null";
  if (Array.isArray(value)) return "array";
  if (typeof value === "number" && Number.isNaN(value)) return "NaN";
  return typeof value;
}

console.log(describeValue(null));           // "null"
console.log(describeValue([1, 2, 3]));      // "array"
console.log(describeValue(Number("abc")));  // "NaN"
console.log(describeValue(() => {}));       // "function"
console.log(describeValue(42n));            // "bigint"

Common Mistakes to Avoid

• Checking typeof val === "object" without excluding null: Passing null passes that check and crashes when you read val.name. Always write val !== null && typeof val === "object".
• Using typeof to tell arrays apart from objects: typeof [] is "object"; always call Array.isArray(val).
• Using typeof on an uninitialized let variable before its line: Calling typeof x on an undeclared global returns "undefined", yet calling typeof x inside a block above let x = 1 throws a ReferenceError due to the Temporal Dead Zone.

Frequently Asked Questions

Why does typeof null return "object" in JavaScript?

It is a famous backward-compatibility bug from the very first 1995 JavaScript engine, where both objects and null shared a zero type tag. Changing it today would break millions of legacy websites.

What does typeof NaN return?

typeof NaN returns "number" because NaN belongs to the IEEE 754 floating-point number specification. Check Number.isNaN(val) to catch NaN.

What does typeof return for an ES6 class?

Classes in JavaScript are syntactic sugar over constructor functions, so typeof MyClass returns "function".