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JavaScript typeof Operator - GeeksforGeeks

JavaScript typeof Operator

Last Updated : 13 Dec, 2024

The typeof operator in JavaScript is used to determine the data type of a value or variable. It returns a string indicating the type, such as "string", "number", "boolean", "object", etc.

JavaScript


 console.log(typeof "Hello");
console.log(typeof 42);     
console.log(typeof true);
console.log(typeof {});      
console.log(typeof undefined);

Output

string
number
boolean
object
undefined

Key Points:

Always outputs the data type as a string.
Can be used anywhere without imports or dependencies.
Determines the type of literals, variables, and expressions.
Special Cases:
- typeof null returns "object" (a known quirk).
- Functions return "function" instead of "object".
Useful for checking types in dynamic or untyped environments.

Syntax:

typeof operand

operand: The value or variable to check.

Real-World Examples Example 1: Validating User Input JavaScript


 function fun(input) {
    if (typeof input === "number") {
        console.log("Valid number provided!");
    } else {
        console.log("Please provide a number.");
    }
}
fun(42);
fun("Hi");

Output

Valid number provided!
Please provide a number.

Example 2: Dynamic Property Access JavaScript


 const settings = { darkMode: true };
if (typeof settings.darkMode === "boolean") {
    console.log("Dark mode setting is valid.");
}

Output

Dark mode setting is valid.

Primitive Data Types

Primitive data types in JavaScript are basic data types that represent single values. They include:

Data Type Description Number Represents numeric values like integers and floating-point numbers. String Represents textual data enclosed within single quotes ('') or double quotes (""). Boolean Represents true or false values. Undefined Represents a variable that has been declared but has not been assigned a value. Null Represents the intentional absence of any object value. Symbol Represents a unique and immutable data type used as the key of an object's property. BigInt Represents large integers beyond the limit of the Number type. Limitations

typeof null returns "object", which is misleading and a legacy bug in JavaScript.
Cannot distinguish between arrays and objects (typeof [] is "object").
Cannot differentiate between classes or specific object types.
typeof NaN returns "number", though it's technically "not a number."

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