The "typeof" Operator in JavaScript

The "typeof" Operator in JavaScript


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The typeof operator in JavaScript is a powerful tool for identifying the data type of a given value. Understanding how to use typeof is essential for writing robust and error-free JavaScript code. In this blog post, we'll dive deep into the typeof operator, exploring its various use cases, quirks, and best practices.

What is the typeof Operator?

At its core, the typeof operator is a built-in JavaScript operator that allows you to determine the data type of a given value or expression. It returns a string representing the data type of the operand. Here's the basic syntax:

typeof operand

The operand can be a variable, a constant, an expression, or any JavaScript value. The typeof operator will return one of the following string values:

  • "undefined": Indicates that the operand is an undefined value.

  • "boolean": Represents a Boolean data type (true or false).

  • "number": Denotes a numeric value, including integers and floating-point numbers.

  • "string": Represents a sequence of characters enclosed in single or double quotes.

  • "symbol": Introduced in ECMAScript 6 (ES6), symbols are unique and immutable values, often used as object property keys.

  • "object": Signifies an object, including arrays, functions, and objects created with constructors.

  • "function": Denotes a function, including regular functions, arrow functions, and methods.

Let's explore each of these data types and how the typeof operator interacts with them.

Using typeof for Data Type Checking

One of the most common use cases of the typeof operator is checking the data type of a variable or value. You can use it in conditional statements to perform different actions based on the data type. Here's an example:

const value = 42;

if (typeof value === "number") {
  console.log("It's a number!");
} else {
  console.log("It's not a number!");

In this example, we use typeof to check if value is of type "number". If it is, we print "It's a number!"; otherwise, we print "It's not a number!"

Handling undefined Values

The typeof operator is particularly useful for handling potentially undefined variables or properties. Here's how you can use it to avoid runtime errors:

function printName(name) {
  if (typeof name === "string") {
    console.log(`Hello, ${name}!`);
  } else {
    console.log("Name is undefined or not a string.");

printName("Alice"); // Output: Hello, Alice!
printName();         // Output: Name is undefined or not a string.

In this example, we check if name is of type "string" before attempting to print a greeting. This prevents errors when name is undefined.

Dealing with Functions

typeof is a handy tool for distinguishing between regular functions and other types of objects. For instance:

function greet() {

const obj = {};

console.log(typeof greet); // Output: "function"
console.log(typeof obj);   // Output: "object"

Here, typeof greet returns "function", indicating that greet is a function. Meanwhile, typeof obj returns "object", confirming that obj is a regular object.

Caveats and Quirks

While the typeof operator is generally reliable, there are a few quirks to be aware of:

Arrays and Objects

typeof may not be as precise when dealing with arrays and objects. It classifies both arrays and regular objects as "object":

const myArray = [1, 2, 3];
const myObject = { key: "value" };

console.log(typeof myArray);  // Output: "object"
console.log(typeof myObject); // Output: "object"

If you need to differentiate between arrays and objects, you can use other techniques like Array.isArray().


typeof null returns "object", which is a historical quirk in JavaScript. Keep this in mind when checking for null values.

const myNull = null;

console.log(typeof myNull); // Output: "object"


The typeof operator is a valuable tool for data type checking and handling various scenarios in JavaScript. It helps you write more robust and error-resistant code by ensuring that your variables and values are of the expected data types. While it has some quirks, understanding how to use typeof effectively is essential for any JavaScript developer.

As you continue your JavaScript journey, explore more advanced topics like type coercion and type checking