Not a Number in JavaScript

What is a Not a Number

In my last post I was talking on how to use numbers in JavaScript. Now I want to talk about the notorious Not a Number invalid value. Also know as NaN.

A Not a Number indicates that the value of a variable or the result of an arithmetic expression is not a number.

Why I have a NaN?

You can get a NaN in two ways: by declaration or by an arithmetic expression:

var x = NaN; // use the keyword NaN for the declaration
var y = 10 / "cars"; // returns NaN

Any operation between numbers and strings will result in a NaN. The only exception is when the string is made only of numbers.

var invalid = 5 * "cars"; // returns NaN
var valid = 5 * "7"; // returns 35

Bare in mind, that any arithmetic operation involving a variable with a value of NaN will result also in NaN. This also includes when working with concatenation:

var n = NaN;
var y = n * 10; // returns NaN
var s = "MyString" + n; // returns NaN

How to test for NaN

There is a global function in JavaScript to test any value for possible NaN. The function isNaN() will return true if the value is Not a Number, or false, if not.

var x = 16;
var n = NaN;
var y = n * 10; // returns NaN

var xIsNotValid = isNaN(x); // returns false
var nIsNotValid = isValid(n); // returns true
var yIsNotValid = isValid(y); // returns true

Comparing numbers and NaN

When comparing a number value with a NaN using the == or the === operators the result will always be false.

var n = NaN;
var y = 7;

var r = n == y;
alert(r); // will display false

var r2 = n === y;
alert(r2); // will display false

If we want to be sure that the NaN and a number compare as true, we must rely on the typeof keyword. The typeof of any NaN, will always be number.

var n = NaN;
var y = 7;

var tOfn = typeof n;
var tOfy = typeof y;

var r = tOfn == tOfy;
alert(r); // will display true

var r2 = tOfn === tOfy;
alert(r2); // will display true

Dealing with a lot of arithmetic expressions involving numbers and strings, could result in NaN values. Knowing about how they behave, can save us a lot of debugging time and a lot of headaches.

About the Author


I am a “multi hat” software developer with more than 18 years in experience. I had worked from government agencies, insurance companies, ticketing system and educational business using a lot of technologies like SQL Server, Oracle, ASP.NET, MVC, HTML, CSS, Javascript, Linq, etc.

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