Attempt at Circular Buffer - Javascript

Right! Here is my attempt at a circular buffer (For use in a graphing program, using the canvas element). Have yet to get round to testing it out.

Question is - Can anyone see any flaws in my logic? Or bottlenecks?

/**
*   A circular buffer class.
*   @To add value -> bufferObject.addValue(xValue, yValue);
*   @To get the First-in value use -> bufferObject.getValue(0);
*   @To get the Last-in value use -> bufferObject.getValue(bufferObject.length);
**/

var circularBuffer = function (bufferSize) {

    this.bufferSize = bufferSize;
    this.buffer = new Array(this.bufferSize); // After testing on jPerf -> 2 x 1D array seems fastest solution.

    this.end = 0;
    this.start = 0;

    // Adds values to array in circular.
    this.addValue = function(xValue, yValue) {
        this.buffer[this.end] = {x : xValue, y: yValue};
        if (this.end != this.bufferSize) this.end++;
        else this.end = 0;
        if(this.end == this.start) this.start ++;
    };

    // Returns a value from the buffer
    this.getValue = function(index) {

        var i = index+this.start;

        if(i >= this.bufferSize) i -= this.bufferSize; //Check here.

        return this.buffer[i]
    };

    // Returns the length of the buffer
    this.getLength = function() {
        if(this.start > this.end || this.start == this.bufferSize) {
            return this.xBuffer.length;
        } else {
            return this.end - this.start;
        }
    };

    // Returns true if the buffer has been initialized.
    this.isInitialized = function() {
        if(this.end != this.start) return true;
        else return false;
    };
}

Please feel free to reuse this code.

Updated twice (and tested!).

Solution

Update: Found another implementation Circular buffer in JavaScript

Made class variables private, corrected old xBuffer reference. Will do more edits tonight.

/**
*   A circular buffer class.
*   @To add value -> bufferObject.addValue(xValue, yValue);
*   @To get the First-in value use -> bufferObject.getValue(0);
*   @To get the Last-in value use -> bufferObject.getValue(bufferObject.length);
**/

var circularBuffer = function (buffer_size) {

    var bufferSize = buffer_size;
    var buffer = new Array(bufferSize); // After testing on jPerf -> 2 x 1D array seems fastest solution.

    var end = 0;
    var start = 0;

    // Adds values to array in circular.
    this.addValue = function(xValue, yValue) {
        buffer[end] = {x : xValue, y: yValue};
        if (end != bufferSize) end++;
        else end = 0;
        if(end == start) start++;
    };

    // Returns a value from the buffer
    this.getValue = function(index) {

        var i = index+start;

        if(i >= bufferSize) i -= bufferSize; //Check here.

        return buffer[i];
    };

    // Returns the length of the buffer
    this.getLength = function() {
        if(start > end || start == bufferSize) {
            return buffer.length;
        } else {
            return end - start;
        }
    };

    // Returns true if the buffer has been initialized.
    this.isInitialized = function() {
        return (end != start) ? true : false;
    };
}