How to Check if a String is a Number in JavaScript?

Most web developers face the problem to check if a string is a number in JavaScript. If you are one of them, you are in the right place. Keep reading.📖

If you’re working with strings in JavaScript, you may need to determine whether a particular string represents a number. For instance, if you are designing a website and want to check different fields, whether entered values are numbers or not, then you are in the situation of what to do.

In this article, first of all, we’ll understand what strings and numbers in JavaScript are. Furthermore, we will discuss different ways to check whether a String is a number. What is NaN, and how isNaN, isInteger(), Regular Expressions, typeof, parseInt(), or parseFloat(), help us check if a string is a number or not. Now, let’s start giving answers to these questions one by one. 👇

What Are String and Number in JavaScript?

In JavaScript, strings are used to store and manipulate text. We can write zero or any number of characters inside single or double quotes:

Code

let str = 'example';

In JavaScript, we use the let keyword to declare the variable. In JavaScript, we can write a number with or without decimals, but it is considered one type:

Code

let n= 20;

5 Ways to Check if String is a Number in JavaScript

We can check in JavaScript wheater a string is a number or not with the help of these three ways:

1. The isNaN() method
2. The isInteger() method
3. The Regular Expressions
4. Use the typeof operator
5. Use the parseInt() or parseFloat() functions

Method 1: Using isNaN()

Before we discuss isNaN, let’s understand what NaN property is. NaN stands for Not a Number. NaN is used to checking whether a value entered is a number. For example, 

Code

console.log(Math.sqrt(-256));

Output

NaN

The isNaN() method checks the type of a value. It returns true if the value passed to it is Not a Number. Let’s understand isNaN() method with the help of the following example.

Code

// Declare a variable with a string

var str = "Text"

// Check whether str string is a number or not

        if(isNaN(str))

{            console.log(str + " is not a number ");

     }

else

{

            console.log(str + " is a number ");

 }

// Declare a variable with string

var num = 20;

        if(isNaN(num))

{

            console.log(num + " is not a number ");
}

else

{
            console.log(num + " is a number ");
}

Output

Text is not a number

20 is a number

Method 2: Using isInteger()

In JavaScript, the Number.isInteger() method determines whether the passed value is an integer. It returns true if the value is a number. Otherwise, it returns false.

General Syntax

Number.isInteger(number)

Let’s understand this method with the help of an example.

Code

function chkInteger(str) {

  if (typeof str !== 'string') {

return false;

  }

 

  const num = Number(str);

 // Check the string if it contains a number or not

  if (Number.isInteger(num)) {

return true;

  }

 

  return false;

}

// Return true  

console.log(chkInteger('1')); 

// Return false

console.log(chkInteger('test'));

Output

true

false

Method 3: Using Regular Expression(RegEx)

RegEx stands for Regular Expression in JavaScript. It is an object that describes a pattern of characters enclosed between slashes (/) to define a search. It performs pattern matching, search and replace functions on String.

A regular expression pattern contains simple characters or a combination of simple and special characters:

General regular expression

/^[0-9]+$/ 

In the above example, we used metacharacters, such as ^  and $. The ^ character marks represent the beginning of the string input, and the $ character marks represent the end of the string. 

We use the plus symbol +, which matches one or more pattern occurrences and the brackets ([]) to find a range of characters. We can use \d in place of [0-9].

The following are the two methods that we can use with Regular Expressions.

1. test() 
2. exec() 

We can also use some of the String methods with it, such as:

1. match()
2. replace()
3. search()
4. split()

We can create Regular Expressions by calling the RegExp() constructor function. For example:

Code

const myregex = new RegExp(/^[0-9]+$/);

// pass string so it will test

console.log(myregex.test('123'));

Output

True

In this example, we pass 123 as a string, and regex compares it with the patterns(only decimal) we write here. It will return true in case of a number and false in case of no number. Here in the pattern, we can replace [0-9] with \d. 

We can also write a function to check whether a string contains a number:

Code

console.log("Check whether a string is number");

function chkInteger(str) {

return /^\d+$/.test(str);

}

console.log(chkInteger('Example'));      

console.log(chkInteger('20202'));

Output

Check whether a string is a number

false

true

Method 4: Use the typeof Operator

One simple way to check if a string is a number is to use the typeof operator. This operator returns a string indicating the type of the operand. For example:

let str = '123';
console.log(typeof str); // Output: string

let num = 123;
console.log(typeof num); // Output: number

As you can see, the typeof operator returns ‘string’ for a string value and ‘number’ for a numeric value. So, you can use this operator to check if a string is a number by checking if the typeof the result is ‘number’.

Method 5: Use the parseInt() or parseFloat() Functions

A third option for checking if a string is a number is to use the parseInt() or parseFloat() function. These functions attempt to parse a string and return a numeric value. If the string cannot be parsed as a number, they return NaN. For example:

let str = '123';
console.log(parseInt(str)); // Output: 123

let str2 = 'abc';
console.log(parseInt(str2)); // Output: NaN

As you can see, the parseInt() function was able to parse the string ‘123’ and return a numeric value, but it was unable to parse the string ‘abc’ and returned NaN instead.

Conclusion

To summarize the article, on how to check if a string is a number in JavaScript, we’ve discussed the five different ways, including isNaN() method, isInteger() method, regular expression, typeof operator, parseInt() or parseFloat() functions.

Let’s have a quick review of the topics discussed in this article.

1. What are Strings and Numbers?
2. Ways to check if String is a number
3. Use of isNaN() method
4. Use of isInteger() method
5. Use of Regular Expressions(RegEx)
6. Use of typeof operator
7. Use of parseInt() or parseFloat() functions

If this article is beneficial and solves your problem, don’t forget to share it with your coding partners—also, comment below 👇 which solution is more suitable in your case.

Happy coding!🥳