How to check if a value is a number or not a number using JavaScript

Written by Ian Carnaghan · 11 sec read >

By using the isNaN() JavaScript function. isNaN = Is Not A Number


document.write(isNaN(“Hello World!”)); //Will print true since Hello World! is not a number
document.write(isNaN(123456)); //Will print false, since 123456 is a number.

var phoneNumber = 0724956785234;
if(isNaN(phoneNumber) == true){
alert(“Please enter a valid phone number”);
return false;

Last Updated On March 20, 2018
Written by Ian Carnaghan
I am a software developer and online educator who likes to keep up with all the latest in technology. I also manage cloud infrastructure, continuous monitoring, DevOps processes, security, and continuous integration and deployment. Profile

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