Add, subtract, multiply, divide - practical JS closure/currying

maths.js

maths = function(opp, a) {
    checkNum = function(num) {
        if (typeof num !== 'number') {
            throw "Error: Quiting because '" + num + "' isn't a number";
        }
    }
    checkNum(a);
    return function(b) {
        [a, b].forEach(function(n) {
            checkNum(n);
        });
        return eval([b, opp, a].join(""));
    }
}

//TODO: ES?