Basic implementation of derivatives and limits in JavaScript Extends the Function object

gistfile1.js

console.clear();

Function.prototype.limit = function (x, accuracy) {
    var f = this; // for clarity
    var C; // constant

    if (accuracy === undefined) {
        accuracy = 6; // if no accuracy is passed, check to the sixth decimal
    }

    function compareRSLwithLSL(i) {
        C = 1 / i;
        /*
         * Numerically analyze the limit by comparing the RSL with the LSL
         * The accuracy of the comparison will be determined by the accuracy parameter
         * (default to the 1000th decimal place)
         */
        if (i >= Math.pow(10, accuracy)) {
            return (f(x + C)).toFixed(accuracy - 1);
        } else {
            if (Math.abs(f(x + C) - f(x - C)) <= (1 / (i / 100))) {
                return compareRSLwithLSL(i * 10);
            } else {
                return NaN;
            }
        }
    }

    return compareRSLwithLSL(Math.pow(10, accuracy - 2));
};

Function.prototype.msec = function (x1, x2) {
    /* q
     * Return the slope of the secant line
     * Uses the basic slope formula
     * y2 - y1 / x2 - x1
     */
    var f = this; // for clarity
    return (f(x1) - f(x2)) / (x1 - x2);
}

Function.prototype.derivative = function (accuracy) {
    /*
     * Functional that returns the derivative
     * Based on a secant line taken at two points that are extremely
     * close to one another
     */
    var f = this; // for clarity

    if (accuracy === undefined) {
        accuracy = 6; // if no accuracy is passed, check to the sixth decimal
    }

    return function (x) {
        if (isNaN(f.limit(x, accuracy))) {
            return NaN;

        } else {
            return f.msec(x, x + (1 / Math.pow(10, accuracy))).toFixed(accuracy - 1);
        }
    };
};

function quadratic(x) {
    // f(x) = x^2
    return x * x;
};

function peacewise(x) {
    //f(x) = { 5  if x > 2
    //         10 if x <= 2
    if (x > 2) {
        return 5;
    } else {
        return 10;
    }
};

var C = 2; // constant

console.log(quadratic.limit(C)); // evaluates to 4
console.log(peacewise.limit(C)); // evaluates to NaN, the JavaScript equivelent of DNE
console.log(quadratic.derivative()(C)); // evaluates to 4
console.log(peacewise.derivative()(C)); // evalutaes to NaN, the peacewise defined function is
// not differntiable at 2