How to Check if a String is not a Number in Javascript
In this tutorial, you will learn how to check if a string is not 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 not 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 if the string is not a number 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
, andh1
). Thediv
element is just a wrapper for the rest of the elements. - The
innerText
for thebutton
element is“Check”
and for theh1
element, it is“Result”
. - We have done some basic styling using CSS and added the link to our
style.css
stylesheet inside thehead
element. - We have also included our javascript file
script.js
with ascript
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 andh1
element using thedocument.querySelector()
method and stored them inbtnCheck
andoutput
variables respectively. - We have attached a
click
event listener to thebutton
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 passingmyVar
as a parameter. - In the
isNumber()
method, we are using thetypeof
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()
andparseFloat()
methods to make sure we have a valid number. - After completing all the checks inside
isNumber()
method, we will return either true or false. - Depending upon the result of the check, we will assign
“Not Number”
or“Number”
to theresult
variable. - We are displaying the
result
in theh1
element using theinnerText
property.
let btnCheck = document.querySelector("button"); let output = document.querySelector("h1"); let myVar = "1.5"; btnCheck.addEventListener("click", () => { let result = !isNumber(myVar) ? "Not Number" : "Number"; output.innerText = result; }); function isNumber(num) { if (typeof num != "string") return false; return !isNaN(num) && !isNaN(parseFloat(num)); }