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

Question

I tried the following to check for the datatype (specifically for integer), but not working.

var i = "5";

if(Number(i) = 'NaN')
{
 console.log('This is not number'));
}

Answer 1

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.

Answer 2

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;

Answer 3

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));
}

Answer 4

Your logic is correct but you have 2 mistakes apparently everyone missed:

just change if(Number(i) = 'NaN') to if(Number(i) == NaN)

NaN is a constant and you should use double equality signs to compare, a single one is used to assign values to variables.

Answer 5

you can try this one isNaN(Number(x)) where x is any thing like string or number

Answer 6

You can check your numbers by checking their constructor.

var i = "5";

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

Answer 7

If you want to know if "1" ou 1 can be casted to a number, you can use this code :

if (isNaN(i*1)) {
     console.log('i is not a number'); 
}

Answer 8

var val = ... //the value you want to check
if(Number.isNaN(Number(val))){
  console.log("This is NOT a number!");
}else{
  console.log("This IS a number!");
}

Answer 9

If you use node 12, you can use

Number.isInteger(..)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isInteger