March 28, 2022

How to Check if a String is a Number in Javascript

By Krishnaa JavaScript Questions 0 Comments

In this tutorial, you will learn how to check if a string is a number in javascript. A variable that holds a plain text as its value is considered to be of string type. There are certain primitive types in javascript and number is one of them. As the name suggests, any variable that holds a number as its value is considered to be of number type.

There are numerous ways to check if a string is a number. We are going to use one of the easiest solutions which involves the usage of the typeof operator, isNaN() and parseFloat() methods. The typeof operator returns the type of the variable. The isNaN() method checks whether a given string is a number or not. The parseFloat() method takes a string as a parameter and returns a floating-point number.

In the following example, we have one global variable and upon click of a button, we will check whether it is a number or not and display the result on the screen. Please have a look over the code example and the steps given below.

HTML & CSS

We have 3 elements in the HTML file (div, button, and h1). The div element is just a wrapper for the rest of the elements.
The innerText for the button element is “Check” and for the h1 element, it is “Result”.
We have done some basic styling using CSS and added the link to our style.css stylesheet inside the head element.
We have also included our javascript file script.js with a script tag at the bottom.

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" href="style.css">
  <title>Document</title>
</head>

<body>

  <div class="container">    
    <button>Check</button>
    <h1>Result</h1>
  </div>

  <script src="script.js"></script>
</body>

</html>

.container {        
    text-align: center;
}

button {
  margin-top: 10px;
  padding: 10px 20px;
}

Javascript

We have selected the button element and h1 element using the document.querySelector() method and stored them in btnCheck and output variables respectively.
We have attached a click event listener to the button element.
We have a global variable myVar which holds a string as its value.
In the event handler function, we are calling the isNumber() method and passing myVar as a parameter.
In the isNumber() method, we are using the typeof operator to make sure the variable is of type string. If it is not of type string, we will return false.
In the next step, we are using isNaN() and parseFloat() methods to make sure we have a valid number.
Depending upon the result of the check, we will assign “Yes” or “No” to the result variable.
We are displaying the result in the h1 element using the innerText property.

let btnCheck = document.querySelector("button");
let output = document.querySelector("h1");

let myVar = "1.5";

btnCheck.addEventListener("click", () => {
  let result = isNumber(myVar) ? "Yes" : "No";
  output.innerText = result;
});

function isNumber(num) {
  if (typeof num != "string") return false;
  return !isNaN(num) && !isNaN(parseFloat(num));
}

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