Looking for derivative script

Question

I am desperately looking for a JavaScript that can calculate the first derivative of a function. The function always includes only one variable, x.

Answer 1

function slope (f, x, dx) {
    dx = dx || .0000001;
    return (f(x+dx) - f(x)) / dx;
}

var f = function (x) { return Math.pow(x, 2); }

slope(f, 3)