I have the following code which interprets the numbers as strings and adds them like 100300 instead of 400:
var rate = document.getElementById(\'pur_price\')
As @sergio said, you can use JavaScript's built in parseInt function. Just for reference, you can multiply it by one. JavaScript always converts a string to a number if you multiply it. So you could do something like this:
var num1 = "300";
var num2 = "100";
var num1 = num1 * 1; // Converts string to the number 300
var num2 = num2 * 1;
document.getElementById('number').value = num1+num2;
This is just an alternate way to do the same thing. Use whatever is easiest for you. Hope that helps!