check for a remainder when dividing by 1:

```
function isInt(n) {
return n % 1 === 0;
}
```

If you don’t know that the argument is a number you need two tests:

```
function isInt(n){
return Number(n) === n && n % 1 === 0;
}
function isFloat(n){
return Number(n) === n && n % 1 !== 0;
}
```

**Update 2019**

5 years after this answer was written, a solution was standardized in ECMA Script 2015. That solution is covered in this answer.