using an if statement to check if NaN

Question

I'm getting a numeric value from a form. Then I check to see if it's NaN. If it is a number I want to set that value to a variable. The problem is that when I enter a valid number I still get an alert and the number isn't passed to the variable "date". How should I modify my statement so that when it is a valid number I can assign it to the variable date?

var adate = document.getElementById("dueDate").value;    

    if ( adate == NaN || " ") {
    alert("Please enter a due date");
    return;
    }

    else {
    var date = (new Date()).setDate(adate);
    }

    processDate(date);

Answer 1

Use Javascript's isNaN() function.

Checking equality with NaN is always false, as per IEEE's standards. Stephen Canon, a member of the IEEE-754 committee that decided this, has an excellent answer explaining this here.

Answer 2

As strange as it seems, NaN !== NaN.

if (adate !== adate || adate !== " ") {
  //...
}

The isNaN function would work in a lot of cases. There is a good case to be made that it is broken, though.

One nice way of getting around this is:

MyNamespace.isNaN = function (x) {
  return x !== x;
}