JS typeof

JavaScript typeof Operator

The typeof operator in JavaScript is a unary operator (meaning it operates on a single operand) that is placed before its operand. The operand can be a variable, a literal, or an expression of any type.

Its primary purpose is to evaluate the operand and return a string indicating its underlying data type. This makes typeof incredibly useful for type-checking and validation in your code.

Syntax

typeof operand
// or
typeof(operand)

typeof Return Values

The typeof operator evaluates the operand and returns a lowercase string corresponding to its data type.

Here is a comprehensive list of the return values you can expect from the typeof operator:

Example: Implementing the typeof Operator

The following interactive example demonstrates how to implement the typeof operator to check whether variables are strings or numeric values using a conditional (ternary) operator.

let a = 10;
let b = "String";

// Check if 'b' is a string
let resultB = (typeof b == "string" ? "B is String" : "B is Numeric");
console.log("Result => " + resultB);
// Check if 'a' is a string
let resultA = (typeof a == "string" ? "A is String" : "A is Numeric");
console.log("Result => " + resultA);