undefined function parater

[masuhar]

The following code raises an "x is undefined" error at (seemingly) x.car in the else branch.

function append(x, y) {
    if (x===null) {
        return y;
    } else {
        return {car: x.car, cdr: append(x.cdr, y)};
    }
}

var l1 = { car: 123, cdr: null };
var l2 = { car: 456, cdr: null };
var l1 = append(l1,l2);