How to not evaluate an expression until it is used? : I have a global variable that references local variables within a function, but I don't want the global variable to be evaluated until it's used.

    var a = (c > d)
    funciton b() {
        var c = 5;
        var d = 3;
        if (a) {
            return true;
        }
        else {
            return false;
        }
    }

The condition in global variable `a` will immediately be evaluated and it'll throw an error that the variables in the condition are not defined. `a` is only used in function `b`, so how can I make it so `a` doesn't evaluate until it's used in the if statement?