Check if input is number or letter javascript

Question

I'm using forms in HTML and javascript. I would like an alert to pop up only if the user inputs a LETTER and clicks submit.

So I have the HTML code:

<form name="myForm" action="" onsubmit="return checkInp()" method="post">
    First name: <input type="text" name="age">
<input type="submit" value="Submit">   

And the javascript code:

function checkInp()
{
var x=document.forms["myForm"]["age"].value;
if (x consists of any letters) // this is the code I need to change
{
alert("Must input numbers");
return false;
}
}

Answer 1

You could use the isNaN Function. It returns true if the data is not a number. That would be something like that:

function checkInp()
{
    var x=document.forms["myForm"]["age"].value;
    if (isNaN(x)) // this is the code I need to change
    {
        alert("Must input numbers");
        return false;
    }
}

Note: isNan considers 10.2 as a valid number.

Answer 2

I think the easiest would be to create a Number object with the string and check if with the help of isInteger function provided by Number class itself.

Number.isInteger(Number('1')) //true
Number.isInteger(Number('1 mango')) //false
Number.isInteger(Number(1)) //true
Number.isInteger(Number(1.9)) //false

Answer 3

The best and modern way is typeof (variable) if you care about real number not number from string. For example:

var a = 1;
var b = '1';

typeof a: // Output: "number"
typeof b: // Output: "string

Answer 4

you can use isNaN(). it returns true when data is not number.

var data = 'hello there';
if(isNaN(data)){
  alert("it is not number");
}else {
  alert("its a valid number");
}

Answer 5

You can use the isNaN function to determine if a value does not convert to a number. Example as below:

function checkInp()
{
  var x=document.forms["myForm"]["age"].value;
  if (isNaN(x)) 
  {
    alert("Must input numbers");
    return false;
  }
}