How do you check that a number is NaN in JavaScript?

Try this code:

isNaN(parseFloat("geoff"))

For checking whether any value is NaN, instead of just numbers, see here: How do you test for NaN in Javascript?

I just came across this technique in the book Effective JavaScript that is pretty simple:

Since NaN is the only JavaScript value that is treated as unequal to itself, you can always test if a value is NaN by checking it for equality to itself:

var a = NaN;
a !== a; // true 

var b = "foo";
b !== b; // false 

var c = undefined; 
c !== c; // false

var d = {};
d !== d; // false

var e = { valueOf: "foo" }; 
e !== e; // false

Didn't realize this until @allsyed commented, but this is in the ECMA spec: https://tc39.github.io/ecma262/#sec-isnan-number

Use this code:

isNaN('geoff');

See isNaN() docs on MDN.

alert ( isNaN('abcd'));  // alerts true
alert ( isNaN('2.0'));  // alerts false
alert ( isNaN(2.0));  // alerts false

As far as a value of type Number is to be tested whether it is a NaN or not, the global function isNaN will do the work

isNaN(any-Number);

For a generic approach which works for all the types in JS, we can use any of the following:

For ECMAScript-5 Users:

#1
if(x !== x) {
    console.info('x is NaN.');
}
else {
    console.info('x is NOT a NaN.');
}

For people using ECMAScript-6:

#2
Number.isNaN(x);

And For consistency purpose across ECMAScript 5 & 6 both, we can also use this polyfill for Number.isNan

#3
//Polyfill from MDN
Number.isNaN = Number.isNaN || function(value) {
    return typeof value === "number" && isNaN(value);
}
// Or
Number.isNaN = Number.isNaN || function(value) {     
    return value !== value;
}

please check This Answer for more details.

As of ES6, Object.is(..) is a new utility that can be used to test two values for absolute equality:

var a = 3 / 'bar';
Object.is(a, NaN); // true