The start value of integer “x” should be how many?

this is a simple function:

function sequence(a,b,c,d){
    var x = 0;
    var y = 0;
    for(var i = a; i<=b; i++){
        y = eval(c);
        x = eval(d);
    }
    return x;
}

a&b must be an integer and c&d must be a string

this code gets the sum of 1 to 5:

sequence(1,5,"i","x+y")

result:

15

this code gets the product of 1 to 5:

sequence(1,5,"i","x*y")

result:

0

the problem is at “var x = 0;“, the value of “x” should be how many?

sequence(1,5,"i","x+y") >>> 15
sequence(1,5,"i","x*y") >>> 0
sequence(1,5,"i","x-y") >>> -15
sequence(1,5,"i","x/y") >>> 0
sequence(1,5,"i","x**y") >>> 0
sequence(1,5,"i","x%y") >>> 0