how to check for datatype in node js- specifically for integer

I just made some tests in node.js v4.2.4 (but this is true in any javascript implementation):

> typeof NaN
'number'
> isNaN(NaN)
true
> isNaN("hello")
true

the surprise is the first one as type of NaN is "number", but that is how it is defined in javascript.

So the next test brings up unexpected result

> typeof Number("hello")
"number"

because Number("hello") is NaN

The following function makes results as expected:

function isNumeric(n){
  return (typeof n == "number" && !isNaN(n));
}

I think of two ways to test for the type of a value:

Method 1:

You can use the isNaN javascript method, which determines if a value is NaN or not. But because in your case you are testing a numerical value converted to string, Javascript is trying to guess the type of the value and converts it to the number 5 which is not NaN. That's why if you console.log out the result, you will be surprised that the code:

if (isNaN(i)) {
    console.log('This is not number');
}

will not return anything. For this reason a better alternative would be the method 2.

Method 2:

You may use javascript typeof method to test the type of a variable or value

if (typeof i != "number") {
    console.log('This is not number');
}

Notice that i'm using double equal operator, because in this case the type of the value is a string but Javascript internally will convert to Number.

A more robust method to force the value to numerical type is to use Number.isNaN which is part of new Ecmascript 6 (Harmony) proposal, hence not widespread and fully supported by different vendors.

i have used it in this way and its working fine

quantity=prompt("Please enter the quantity","1");
quantity=parseInt(quantity);
if (!isNaN( quantity ))
{
    totalAmount=itemPrice*quantity;

}
return totalAmount;