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JavaScript typeof

James Gallagher - January 04, 2021


Data types are used to store a particular type of data in a programming language. For example, strings can be used to store text-based data in code, whereas numbers can be used to store integers and floating-point numbers.

When you’re working with data types in JavaScript, you may want to find out which type of data a particular value holds. That’s where the typeof method comes in. The JavaScript typeof operator can be used to determine the type of a particular item in your code. This can be useful if you want to check whether data holds the type your program expects, which is a common operation in programming.

In this tutorial, we are going to discuss data types in JavaScript and how you can use the typeof operator to find out the type of data a particular value holds. We will also explore a number of examples of the typeof operator being used in action.

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JavaScript Data Type Refresher

JavaScript features a number of data types which can be used to classify a certain type of data. Each type of data is treated differently by JavaScript, and each data type has its own methods that can only be used with data structured in the appropriate format.

JavaScript uses dynamic data types, which means that the type of data is checked during runtime instead of when the program is being compiled. This means that a variable of the same name can be used to hold different types of data.

In JavaScript, there are six basic data types that can be used to hold particular types of data, which are: string , number , boolean , object , array , and function . Later in this article, we have a reference guide which explains how these values work.

Each of these six data types works in different ways. So, when you’re programming, you may want to determine how a particular value is stored so you know how you should be working with the data. For example, how you work with a string will be different to how you would work with a number.

JavaScript typeof

typeof is a built-in JavaScript operator that returns a string indicating the type of data a particular value holds. The operator can be used to find the type of data held by any value, or it can be used to find the type of a variable.

Here is the syntax for the typeof operator:

typeof data_object

So, let’s say that you have a variable called phone_name and you want to check its data type. You could do so using the following code:

var phone_name = "iPhone 11";
console.log(typeof phone_name);

Our code returns: string . On the first line, we declare a variable called phone_name which holds the string value iPhone 11 . Then, on the next line, we use typeof to check the type of data held in our variable. In this case, the program returns string .

We can check any type of data in JavaScript. Here’s an example of typeof being used to check the value of a variable that has been assigned a Boolean:

var phone_in_stock = true;
console.log(typeof phone_in_stock);

The typeof operator returns: boolean .

The typeof function is often used to compare the data type held by a variable. So, if you wanted to check whether or not phone_name was a string, and run a block of code if phone_name was a string, you could do so using typeof . Here’s an example of an if statement that will execute a block of code if phone_name is a string:

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var phone_name = "iPhone 11";

if (typeof phone_name === "string") {
	console.log("The variable 'phone_name' is a string.");
};

Our code returns:

The variable 'phone_name' is a string.

There’s a lot going on in our code, so let’s break it down. On the first line, we declare a variable called phone_name and assign it the value iPhone 11 . Then, we create an if statement that checks whether or not the typeof phone_name is equal to string .

In this case, the variable phone_name is a string, so the code within our if statement is executed. Hence, the output of our program was The variable phone_name is a string .

JavaScript Typeof Arrays and Objects

When you’re working with JavaScript arrays and object literals, the typeof function will not return the specific type of object with which you are working. Instead, it will return the label object . Here’s an example of typeof being used on both an array and a dictionary:

var sandwich_fillings = ['Ham', 'Cheese', 'Egg Mayo'];
var ham_sandwich = { name: 'Ham', price: 2.50 };
console.log(typeof sandwich_fillings);
console.log(typeof ham_sandwich);

Our code returns:

object
object

As you can see, our program has returned object to describe both the types of data with which we are working, even though our sandwich_fillings variable has been assigned an array, and ham_sandwich has been assigned a dictionary. This is because object is used as a generic value to describe more complex data types in JavaScript.

JavaScript Typeof Undefined and Null

The typeof operator also returns different values for null and undefined. When you’re working with a null value, typeof will return an object , and undefined will return its own undefined value. Here’s an example of these data types in action:

console.log(typeof null);
console.log(typeof undefined);

Our code returns:

object
undefined

As you can see, null is considered an object , and undefined has its own value called undefined . So, if you’re working with these types of data, remember that they will return these values.

JavaScript Data Types

As we discussed earlier, JavaScript offers six main data types which classify particular types of data. These six data types can be divided into three categories: primitive , composite , and special .

Strings, numbers, and booleans are all primitive data types, which means they are primary data types. Objects, arrays, and functions are all considered composite data types, and are all types of objects. Undefined and null are considered special data types.

Let’s break down the main types of data in JavaScript to illustrate what you can expect when you’re working with the typeof operator.

Numbers typeof

JavaScript has one number type which stores both integers and floating-point numbers. As a result, numbers are able to store both decimal and non-decimal numbers. Here’s an example of a number in JavaScript:

var espresso_price = 1.80;

When you use typeof on a number, the following will be returned: number .

Strings typeof

Strings are sequences of one or more characters and can include letters, numbers, symbols, and whitespaces. Strings represent text-based data, and can be declared using either single quotes ( ‘’ ), double quotes ( “” ), or backticks ( `` ).

Here’s an example of a string in JavaScript:

var espresso_name = "Espresso";

"typeof" returns "string" for any string data type.

Booleans typeof

Booleans can store one of two values: either true or false. Booleans are used to represent whether or not a condition has been met, and are often used in mathematical logic. Here’s an example of a boolean in JavaScript:

var espresso_in_stock = true;

"typeof" returns "boolean" for the Boolean data type.

Arrays typeof

Arrays can store multiple values within a single variable, and are defined as a list of comma-separated values enclosed within square brackets ( [] ). Here’s an example of an array in JavaScript:

var coffee_menu = ['Latte', 'Cappuccino', 'Espresso', 'Americano', 'Cortado'];

"typeof" returns "object" for array data types.

Objects typeof

Objects can contain multiple values stores as key/value pairs. These can be used to store and access data in JavaScript. Objects are declared as a list of key/value pairs separated by commas and enclosed within curly braces ( {} ).

Here’s an example of an object in JavaScript:

var latte_information = { name: 'Latte', price: 2.50, milk: true };

"typeof" returns "object" for JavaScript objects.

Undefined and Null typeof

The undefined value is the default value for a variable with no value assigned. Here’s an example of a variable with the type undefined :

var coffee;

This variable has no value, so it is considered undefined by typeof .

null is an object that is used to describe a variable that has no value. Here’s an example of a variable with a null value:

var coffee = null;

Conclusion

The JavaScript typeof operator can be used to determine the type of data a particular value holds. typeof returns the type of an expression or the type of a variable. The typeof operator is useful if you want to figure out how data is stored so you know whether you need to convert data to another type in your program for it to work correctly.

In this tutorial, we explored how to use the typeof operator in JavaScript. We also discussed how typeof responds to arrays, objects, null, and undefined values. In addition, we briefly covered the main types of data in JavaScript and discussed how typeof reacts to those data types.

Now you’re ready to start using the JavaScript typeof operator like a professional!

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James Gallagher

About the author: James Gallagher is a self-taught programmer and the technical content manager at Career Karma. He has experience in range of programming languages and extensive expertise in Python, HTML, CSS, and JavaScript. James has written hundreds of programming tutorials, and he frequently contributes to publications like Codecademy, Treehouse, Repl.it, Afrotech, and others. He also serves as a researcher at Career Karma, publishing comprehensive reports on the bootcamp market.

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