It will automatically come out as 10 if you make sure that both sides of the "+=" are numeric values. If you (for example) read the value of an input box, it will always be a string (even if you entered a number). You'll have to convert it into a numeric value before adding 5 to it
Example
Code:
var myValue = document.getElementById('myinput').value;
myValue = Number(myValue); // convert it to a numeric value if possible, otherwise it will be "NaN"
myValue += 5; // mathematically add 5 to this value