JavaScript typeof Operator

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.

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

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

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

Valid number provided!
Please provide a number.

Example 2: Dynamic Property Access

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

Dark mode setting is valid.

Primitive Data Types

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

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.”