How to check if Variable is a number in JavaScript

It is a common occurrence to need to check if the value of a variable is a number or not in JavaScript. Because JavaScript is not a statically typed language, it is sometimes not clear what type of value a variable is.

In this post, we'll learn the different ways to check if a value is a number.

isNaN()

The best way to check if a value is a number is to use the isNaN() method.

The isNaN() is a built-in function that returns true if the value passed to it is not a number.

Let's look at an example of isNaN():

JAVASCRIPT
const variable = 3;

isNaN(variable); // false

isNaN(3.5); // false

isNaN("hello world"); // true

isNaN(undefined); // true

Because isNaN() returns true if the value passed to it is not a number, if it returns false, then it means that the value is a number.

You can encapsulate this into a function to make it more readable:

JAVASCRIPT
const isANumber = (value) => !isNaN(value);

isANumber(3); // true

isANumber(3.5); // true

isANumber("hello world"); // false

isANumber(undefined); // false

typeof

Another way to check if a value is a number is to use the typeof operator.

The typeof operator returns a string that represents the type of the value passed to it. Therefore, if the variable is a number, the typeof operator will return number.

JAVASCRIPT
const variable = 3;

typeof variable; // number

typeof 3.5; // number

typeof "hello world"; // string

typeof undefined; // undefined

Using that information, you can just do a simple string comparison to check if the value is a number.

JAVASCRIPT
const variable = 3;

typeof variable === "number"; // true

typeof 3.5 === "number"; // true

typeof "hello world" === "number"; // false

typeof undefined === "number"; // false

You can also encapsulate this into a function to make it more readable:

JAVASCRIPT
const variable = 3;

const isNumber = (value) => typeof value === "number";

isNumber(variable); // true

isNumber(3.5); // true

isNumber("hello world"); // false

isNumber(undefined); // false

Conclusion

There you have it, the two best ways to check if a value is a number in JavaScript.

Hopefully, you've found the examples and functions useful to you.

Happy coding!

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