# 2010 07 15 typeof number is not number

How do you determine if a number is an integer in JavaScript?

```
x = 1;
x === Math.floor(x);
// returns true
```

But what happens if we try to add a method for this to the Number prototype?

```
Number.prototype.isInteger = function() {
return this === Math.floor(this);
}
x = 1;
x.isInteger();
// returns false!
```

Why? It turns out that when you add methods to Number, the type of the number inside the method becomes "object" rather than "number", but Math.floor returns a result of type "number". If you use the === operator, the two values are no longer equal because they're different types. So the method can be fixed two ways.

Solution 1 is to avoid comparing types:

```
Number.prototype.isInteger = function() {
return this == Math.floor(this);
// works but breaks if you care about 0 vs other falsy values
}
```

Solution 2 is better; cast "this" to the Number type and then the types are equal.

```
Number.prototype.isInteger = function() {
return Number(this) === Math.floor(this);
}
```

--- @attaboy