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typeof Operator

Introduction

The typeof operator is a built-in JavaScript operator used to determine the data type of a value or variable. It returns the type as a string, making it easy to identify whether a value is a number, string, boolean, object, function, or another JavaScript data type.

The typeof operator is one of the most commonly used debugging and validation tools in JavaScript. It is especially useful in automation testing, where data received from APIs, user input, files, or databases may need to be validated before processing.

What is the typeof Operator?

The typeof operator returns the data type of a variable, value, or expression as a string.

Syntax

typeof value

or

typeof(value)

Both syntaxes produce the same result.

Why Do We Need the typeof Operator?

The typeof operator helps us:

  • Identify the data type of variables.

  • Validate user input.

  • Debug JavaScript programs.

  • Handle different data types correctly.

  • Validate API responses.

  • Prevent runtime errors.

Checking the Type of a Number

let age = 25;

console.log(typeof age);

Output

number

Checking the Type of a String

let language = "JavaScript";

console.log(typeof language);

Output

string

Checking the Type of a Boolean

let isLoggedIn = true;

console.log(typeof isLoggedIn);

Output

boolean

Checking the Type of undefined

let value;

console.log(typeof value);

Output

undefined

Checking the Type of null

let data = null;

console.log(typeof data);

Output

object

Note: This is a well-known historical bug in JavaScript. Although typeof null returns "object", null is a primitive value.

Checking the Type of a BigInt

let bigNumber = 12345678901234567890n;

console.log(typeof bigNumber);

Output

bigint

Checking the Type of a Symbol

let id = Symbol("user");

console.log(typeof id);

Output

symbol

Checking the Type of an Object

const student = {

    name: "John",

    age: 22

};

console.log(typeof student);

Output

object

Checking the Type of an Array

const colors = ["Red", "Blue", "Green"];

console.log(typeof colors);

Output

object

Arrays are objects in JavaScript, so typeof returns "object".

Checking the Type of a Function

function greet() {

    console.log("Welcome");

}

console.log(typeof greet);

Output

function

Functions are a special type of object, and typeof returns "function".

Checking the Type of a Date

const today = new Date();

console.log(typeof today);

Output

object

Checking the Type of NaN

console.log(typeof NaN);

Output

number

Although NaN means Not-a-Number, its type is still "number".

Using typeof with Expressions

console.log(typeof (10 + 20));

Output

number

console.log(typeof ("Hello" + " World"));

Output

string

typeof Return Values

Valuetypeof Result
"Hello""string"
25"number"
true"boolean"
undefined"undefined"
null"object"
123n"bigint"
Symbol()"symbol"
{}"object"
[]"object"
function(){}"function"

Real-World Example

Suppose a user enters their age through a form.

let age = "25";

console.log(typeof age);

Output

string

Before performing calculations, you know that the value should first be converted into a number.

Automation Testing Example

Automation engineers often validate API responses before performing assertions.

const response = {

    status: 200,

    message: "Success"

};

console.log(typeof response.status);

console.log(typeof response.message);

Output

number
string

This confirms that the API returned values with the expected data types.

Another example:

const expectedPrice = 1500;

const actualPrice = "1500";

console.log(typeof expectedPrice);

console.log(typeof actualPrice);

Output

number
string

This helps identify why a comparison might fail and indicates that type conversion may be needed.

Common Mistakes

Assuming Arrays Have Their Own Type

Incorrect assumption:

const numbers = [1, 2, 3];

console.log(typeof numbers);

Output

object

Use Array.isArray() if you need to check whether a value is an array.

console.log(Array.isArray(numbers));

Output

true

Assuming null Returns "null"

Incorrect assumption:

console.log(typeof null);

Output

object

This is a historical behavior of JavaScript.

Confusing NaN

console.log(typeof NaN);

Output

number

Although NaN represents an invalid numeric value, its type is still "number".

Best Practices

Use typeof Before Processing Data

Verify the data type before performing calculations or validations.

Use Array.isArray() for Arrays

Instead of relying on typeof, use:

Array.isArray(value)

to check whether a value is an array.

Use Strict Equality Along with typeof

Example:

if (typeof age === "number") {

    console.log("Valid number");

}

This ensures that only values of the expected type are processed.

Validate API Responses

Use typeof to confirm that API responses contain values of the expected data types before making assertions.

Conclusion

The typeof operator is one of the most useful tools in JavaScript for identifying the data type of variables and values. It is widely used for debugging, input validation, API testing, and writing robust JavaScript applications.

Although there are a few special cases—such as typeof null returning "object" and arrays also returning "object"—understanding these behaviors helps developers avoid common mistakes and write more reliable code.

Frequently Asked Questions (FAQs)

What is the typeof operator in JavaScript?

The typeof operator returns the data type of a value or variable as a string.

What does typeof null return?

It returns:

object

This is a historical behavior of JavaScript.

What does typeof [] return?

It returns:

object

To check whether a value is an array, use:

Array.isArray(value)

What does typeof function(){} return?

It returns:

function

Why is the typeof operator useful in automation testing?

Automation engineers use typeof to validate API responses, user input, JSON data, and other values before performing assertions or calculations.

Key Takeaways

  • The typeof operator returns the data type of a value as a string.

  • It can identify primitive data types such as number, string, boolean, undefined, bigint, and symbol.

  • typeof null returns "object" due to a historical JavaScript behavior.

  • Arrays are also reported as "object" by typeof.

  • Functions return "function" when checked with typeof.

  • typeof NaN returns "number".

  • Use Array.isArray() to check for arrays.

  • Use typeof for debugging and data validation.

  • It is widely used in automation testing to verify API responses and user input.

  • Understanding the typeof operator helps write safer and more reliable JavaScript code.