Javascript variables not adding, only concatenating as strings

Question

I have some simple code that should add two numbers and it seems to keep adding them, i.e. if I have a variable that is initialized as 0 , I add 100 once, and then I add 200, it seems to add it as follows 0 + 100 + 100 + 200, returning 400 NOT 300. How can I get this to add correctly?

function calcScores(amount) {
    amount = parseInt(amount,10);
    for(var key in teamSelection) {
        if (teamSelection[key]['Selected']) {
            score = parseInt(teamSelection[key]['Score'],10);
            var total = +score + +amount;
            teamSelection[key]['Score'] = total;
        }
    }
}

Answer 1

var total = +score + +amount;

This line is confusing to me. If you want to add score and amount to total, use

var total += score + amount;