javascript - Check if string contains only digits

#short
#javascript
#numbers
#digits
I want to check if a string contains only digits. I used this:
var isANumber = isNaN(theValue) === false;

if (isANumber){
..
}

But realized that it also allows + and -. Basically, I want to make sure an input contains ONLY digits and no other characters. Since +100 and -5 are both numbers, isNaN() is not the right way to go.
Perhaps a regexp is what I need? Any tips?